Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol15.1995:
Tree Physiology 15, 499--505
© 1995 Heron Publishing----Victoria, Canada
Phenological measurements of microsporogenesis in trees
ALPO J. LUOMAJOKI
The Finnish Forest Research Institute, Kolari Research Station, FIN-95900 Kolari, Finland
Received January 21, 1994
Summary The value of two heat sum methods, one linear
(degree days > 5 °C) and the other curvilinear (period units),
were assessed together with calendar days as predictors of the
duration of microsporogenesis in seven natural stands of Norway spruce (Picea abies (L.) Karst.) and eleven natural stands
of Scots pine (Pinus sylvestris L.). Microsporogenesis was
divided into two subperiods: March 19 to tetrads (i.e., the end
of meiosis) and tetrads to anthesis. The total period from March
19 to anthesis was also assessed.
The methods were compared on a calendar day basis. When
annual deviations between the predicted (stand means) and the
observed annual heat sums were converted to days, the period
unit method outperformed the other methods for both subphases and for the total period of microsporogenesis. The
degree day parameter was more variable but a better predictor
of the duration of the initial phase up to tetrads and of the total
period than the calendar day parameter, but the calendar day
parameter more accurately predicted the duration of the subphase from tetrads to anthesis. The heat sum methods were
better predictors of the duration of development of microsporogenesis in exceptionally cold or warm years than the
calendar day method.
arbitrary. The use of a biofix is unavoidable if both ends of a
process are not limited by observable events. The first phase of
microsporogenesis up to tetrad formation and the total period
of microsporogenesis up to anthesis both suffer from the unavoidable use of a biofix.
The aim of this study was to appraise the advantages of using
a curvilinear heat sum method rather than a linear heat sum
method for estimating developmental intervals during microsporogenesis. In particular, degree days (> 5 °C) and the
period unit system of Sarvas (1972) were compared for forecasting the progress of microsporogenesis in Norway spruce
(Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.).
Both heat sum methods were compared to calendar days as
timing parameters. The postmeiotic phase of microsporogenesis was given special attention because of the advantage of
dispensing with a biofix. The consequences of using a biofix
to mark the onset of microsporogenesis were also evaluated.
Physiological reasons were sought to explain differences in the
predictive accuracy of the parameters.
Keywords: anthesis, heat sum, phenology, Picea abies, Pinus
sylvestris, tetrad phase.
Ten to 30 male buds were excised either from each tree or from
a population of 10 trees daily, one to six times a day, depending
on air temperatures (Luomajoki 1977, 1984). Each sample was
handled separately. The buds were bisected longitudinally (one
half being discarded), and each half bud was put in fresh
fixative containing 9/11 (v/v) glacial acetic acid/absolute ethanol. A pooled squashed sample from 10 to 30 fixed bud halves
was subsequently prepared in acetic (or formic) orcein on 4--6
slides. Between 400 and 600 pollen mother cells (PMC) from
each pooled squashed sample were inspected under the microscope. The onset of each phase of meiosis was considered a
point event.
Because meiotic materials were more scarce than those of
anthesis, the study was limited by the meiosis data. The meiosis data had to be adjusted according to the biofix, which was
set at March 19 (i.e., the first day of the year with 12 h of
sunlight) for the anthesis study (Luomajoki 1993a, 1993b).
Consequently, the period unit heat sum data given by Luomajoki (1984) are not fully comparable with the data used here.
For this study, the tetrad stage data were reprocessed (see
Figures 2 and 3). The Scots pine data differed from the earlier
published data (n = 43; Luomajoki 1984) by the addition of one
data set (Rovaniemi XXIX in 1971; locality 8 in Figure 1)
Introduction
The progress of many developmental phases in organisms is
strongly temperature dependent. Various temperature sum
methods have been developed to predict events such as flowering, fruit ripening and the emergence of pests. Simple calendar time has also been used for this purpose.
Although linear heat sum systems, like degree days, have
been criticized because they are nonphysiological (for a historical review see Sarvas 1972, cf. Wang 1960), these methods
are frequently used in botanical studies because of their simplicity. On the other hand, curvilinear regressions of development rate on temperature have been used occasionally in
entomological studies (Shelford 1927, Messenger and Flitters
1958), but only rarely in botanical studies (Sarvas 1972).
Postmeiotic microsporogenesis from tetrads to anthesis offers a unique opportunity to measure precisely the length of a
generative process without the complications caused by a
biofix (zero point). In practice, the biofix is more or less
Materials and methods
500
LUOMAJOKI
consisting of two observations (each based on a sample of 20
male buds) of the occurrence of the tetrad stage (Table 1).
The stands for the study of early microsporogenesis were the
same as used in earlier anthesis studies (Luomajoki 1993a,
1993b). Anthesis was measured on site, but male buds were
sampled for the assessment of pollen mother cells.
Mature, thinned stands of several hectares were classified as
normal stands for pollination (see Sarvas 1962). Antheses were
measured in each stand at tree-top height with one to three
self-recording pollen samplers (model Sarvas-Vilska 1963, see
Sarvas 1968). The mean of daily pollen catches was used when
more than one sampler was used. A Fuess (Berlin Steglitz)
thermograph was also placed at tree-top height in each stand.
The pollen catch was counted with the aid of a microscope
from the recording bands, and the results presented in terms of
daily catches of the recorders, catch averages, cumulative sums
and cumulative percentages of the pollen catch (Sarvas 1972).
The cumulative percentages were plotted with the Systat/Sygraph computer program (Wilkinson 1990). The ordinate scale
is a Gauss integral with a linear abscissa scale. The abscissa
showed the cumulative temperature sum at the end of each day
(corresponding to the measurement of the cumulative pollen
catch). Because of the effects of secondary pollen on anthesis
data (see Luomajoki 1993b), exclusion of points outside the
zone from −2 to +1.2 standard deviations was necessary to
position the regression line (Figures 2 and 3). The point of 50%
completion of each stage was used as a criterion for attaining
a given stage. The 50% point is unbiased by standard deviation.
One series of observations of anthesis in Norway spruce was
Figure 1. Localities of stands studied: (1) Bromarv, (2) Eckerö,
(3) Tuusula, (4) Jokioinen, (5) Heinola, (6) Punkaharju, (7) Vilppula,
(8) Rovaniemi, and (9) Kittilä.
Table 1. Stand characteristics and years of study; PMC = pollen mother cell.
Origin (locality)
Lat., long.
Elevation
Age in 1970
(years)
Anthesis
(years)
Tetrads of PMC
(years)
Remarks on stand
Picea abies (n = 17)
Bromarv I1 (1)
Heinola 566 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju LII (6)
60°02′, 23°05′
61°08′, 26°02′
60°50′, 23°30′
68°02′, 24°09′
61°49′, 29°20′
27
113
106
275
92
126
120
51
172
96
1963--73
1966--71
1966--73
1963, 65--73
1964--74
1966--67
1966
1969
1967--68, 73
1966--71, 73
Clear cut, 1976
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
66°21′, 26°40′
60°22′, 24°59′
182
50
127
67
1963--73
1967--73
1970, 73
1968
Pinus sylvestris (n = 44)
Bromarv II (1)
Bromarv III (1)
Eckerö I (2)
Heinola 566 (5)
Kittilä I (9)
Kittilä II (9)
Punkaharju XLV (6)
Rovaniemi XXVII (8)
Rovaniemi XXIX (8)
Tuusula XXXII (3)
Vilppula 2a (7)
60°02′, 23°03′
60°03′, 23°03′
60°11′, 19°34′
61°07′, 26°01′
68°02′, 24°09′
68°02′, 24°08′
61°48′, 29°19′
66°21′, 26°44′
66°21′, 26°38′
60°21′, 25°01′
62°04′, 24°29′
41
35
33
133
280
330
106
118
165
70
130
84
63
140
127
220
190
119
112
106
144
190
1964--69
1965--73
1966--69
1964--71
1963, 65--69, 71--73
1965--69, 71--73
1964--74
1963--73
1965--73
1964--69
1965--69
1967--69
1966--69, 71, 73
1969
1966--68
1967--69, 73
1967--69
1965--72
1967--70
1967--73
1967--68
1967--69
1
Origin Finland, Lammi
Clear cut, 1975--76
400 kg urea ha −1 given
in winter 1967--68
Clear cut, 1985
Plot numbers were assigned by the former Department of Silviculture of the Finnish Forest Research Institute. All other stand origins are local
except for Punkaharju LII.
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
rejected because of a minimal pollen catch (Bromarv I in 1968)
to avoid bias by secondary pollen from neighboring stands.
The nine localities involved in the study are shown in Figure 1. In all, 61 microsporogeneses studied from the biofix
(March 19) to 50% anthesis completion were available, and 57
of them were used for a comparison by means of the annual
deviations (between predicted and observed day) and by coefficients of variation.
The methods were compared on the basis of days (see Hari
and Häkkinen 1991), but the means of the basic data of the heat
sum methods are also shown (see Tables 2--4). The observations made in calendar days were used directly, but for the heat
sum methods, the expected heat sum (stand mean) and each
observed annual value were compared. For each year of study,
the expected heat sum (stand mean) was scanned on the particular stand-specific annual heat sum scale in either direction
from the heat sum actually observed to obtain the predicted
day. It was necessary to convert the basic data obtained by the
heat sum methods to a day basis, because the accumulation of
daily heat sums accelerated toward the end of the study period
(Figure 4).
501
Results
Based on estimates of period unit heat sums, the first phase of
microsporogenesis in Scots pine (Table 2) from March 19 to
50% tetrad phase completion (Figure 5) was similar in length
to the second part from tetrads to anthesis (Table 3), whereas
in Norway spruce, the first phase was only about half the
length of the second phase (Tables 2 and 3). However, when
based on observed duration in calendar days, the second phase
was considerably shorter than the first phase in both species
(Tables 2 and 3). The difference between the two methods was
a result of low ambient temperatures during the first phase.
Low temperatures during the first phase also explain why the
Figure 3. Tetrad stage of microsporogenesis (left) and anthesis (right)
in Scots pine at Rovaniemi, Stand XXIX. The method described in
Figure 2 was applied. Fifty percent of the pollen mother cells were in
tetrad stage at 2963 period units, and 50% of pollen had reached
anthesis at 6878 period units. The standard deviation (SD) of tetrad
phase was 209 period units, and the SD of anthesis was 618 period
units. The dates of 50% completion of the tetrad and anthesis phases
were on June 10 and 28, respectively.
Figure 2. Tetrad stage of microsporogenesis (left) and anthesis (right)
in Norway spruce at Punkaharju, Stand LII. The lines give the 50%
completion points (at 0 of ordinate) of the tetrad stage and of anthesis:
50% of the pollen mother cells were in tetrad stage at 1577 period units
and 50% of pollen in anthesis at 5244 period units. For anthesis, the
central larger squares were used to position the line whereas the
smaller ones beyond the limits of −2 to +1.2 standard deviations were
excluded (see Luomajoki 1993b for details of the method). The slopes
of the lines give the standard deviations (SD) of the two processes. The
SD can be read along the line from the abscissa between the 0 and +1
points of the ordinate. As percentages, these points correspond to the
50 and 84.13% points of the scale. The SD of tetrad stage was 204
period units and the SD of anthesis was 414 period units. The dates of
50% completion of the tetrad and anthesis phases were May 10 and
June 2, respectively.
Figure 4. A schematic pattern of accumulation of various units for
measuring the timing of anthesis. The calendar day scale is a uniform
variable, unchanged from day to day. Period units usually start to
accumulate before the biofix (March 19), whereas degree days usually
start to accumulate after the biofix. Both heat sum types accumulate at
an increasing rate so that larger daily heat sums occurr near the end of
the study. The onset of the accumulation of neither period units nor
degree days was a useful point to place the biofix.
502
LUOMAJOKI
Table 2. Comparison of timing parameters in measuring microsporogenesis from March 19 to50% completion of tetrad phase.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
1214 (9.5)2
1775
1615
1785 (8.2)
1661 (11.9)
1745 (2.1)
1639
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
3300 (5.1)
3163 (4.3)
3279
3280 (14.7)
2852 (7.7)
3029 (15.9)
3363 (6.3)
3146 (7.0)
3199 (8.5)
3155 (14.2)
3497 (13.5)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since March 19
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
Mean
Mean of
annual
deviations1
45.5 (17.1)
--75.3 (4.7)
51.7 (6.1)
67.5 (5.2)
--
5.50
--2.44
2.12
2.50
--
7.0 (40.4)
29.0
19.0
24.3 (22.6)
26.4 (37.4)
26.0 (10.9)
27.0
49.0 (8.7)
--75.7 (3.3)
53.1 (6.0)
67.5 (3.1)
--
3.00
--1.78
2.20
1.50
--
45.5 (10.9)
56.0
49.0
75.3 (4.1)
51.1 (8.3)
67.5 (5.2)
39.0
3.50
--2.22
3.27
2.50
--
67.0 (1.5)
64.7 (2.5)
-64.3 (3.2)
84.0 (7.2)
87.3 (9.3)
68.4 (4.1)
79.0 (5.8)
78.7 (4.1)
62.0 (4.6)
69.7 (2.2)
0.67
1.33
-1.56
5.00
6.22
2.29
3.00
2.33
2.00
1.11
63.3 (16.6)
65.2 (24.7)
64.0
80.3 (12.5)
60.8 (19.7)
68.0 (21.4)
83.8 (14.6)
70.5 (13.2)
75.9 (22.0)
58.0 (14.6)
77.3 (22.8)
67.7 (2.3)
65.7 (6.0)
-66.7 (6.8)
84.5 (7.6)
89.3 (4.5)
69.4 (5.3)
79.5 (5.3)
79.4 (4.8)
60.0 (9.4)
70.3 (3.6)
1.11
3.00
-3.11
5.00
2.89
2.88
3.00
2.78
4.00
1.78
67.3 (1.7)
65.2 (3.3)
72.0
66.3 (7.7)
84.5 (7.0)
86.7 (11.2)
68.4 (4.7)
79.0 (6.3)
79.0 (5.3)
64.5 (7.7)
69.7 (3.6)
0.89
1.56
-3.78
4.25
7.11
2.79
3.50
3.43
3.50
1.78
Between observed day and predicted day.
Coefficient of variation in brackets.
Figure 5. A diagrammatic presentation of the timing of microsporogenesis in Norway spruce and Scots pine in southern and northern
Finland. Distances between the solid points (d) were measured in this
study; open points (s) mark the estimated time when 50% of the
pollen mother cells were in leptotene. S = southern Finland, 60--62° N
lat.; N = northern Finland, 68° N lat.
predictions based on period units and degree days differed (cf.
Figure 6).
All three periods of microsporogenesis (i.e., first phase,
second phase and total period) in Scots pine were shorter in
terms of period units in the north than in the south of Finland.
A similar correlation with latitude was also found when the
prediction was based on the degree day method, even though
the change in the length of the first phase of microsporogenesis
from March 19 to tetrads was minimal (Figure 6). When the
prediction was based on calendar days, both the first part and
the total period of microsporogenesis were considerably
longer in the north than in the south of Finland; however, the
duration of the second phase of microsporogenesis in calendar
days was not correlated with latitude.
A comparison of the performance of the three methods was
only possible on a day basis. For both the heat sum methods,
the predicted day was used as the criterion, whereas no conversion was necessary for the calendar day method. Based on the
coefficient of variation, the period unit method was the best
predictor for both subphases of microsporogenesis as well as
for the total period from biofix to tetrads (Tables 2--4). The
degree day method produced less consistent results than the
period unit method, performing second best for the initial
phase from biofix to tetrads and for the total period. The
calendar day method was second best in performance for the
phase from tetrads to anthesis, but it was the worst parameter
for the initial phase and the total period (Tables 2--4).
The mean of annual deviations between predicted and observed day is given in Tables 2--4. Ranking the parameters by
the mean of annual deviations rather than by the coefficient of
variation gave similar results with one exception. For the total
period, considering only the accuracy of the forecast and the
number of stands, the degree day method outperformed the
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
503
Table 3. Comparison of timing parameters in measuring microsporogenesis from 50% completion of tetrad phase to 50% completion of anthesis.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
3277 (5.7)2
3040
3225
3072 (9.4)
3488 (6.3)
3032 (3.3)
4223
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
3432 (1.4)
3586 (3.0)
3311
3655 (10.4)
3556 (5.9)
3184 (8.2)
3685 (3.8)
3391 (5.3)
3368 (9.6)
3590 (5.8)
3642 (2.8)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since 50% tetrad phase
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
Mean
Mean of
annual
deviations1
30.5 (7.0)
--22.3 (5.2)
25.9 (12.1)
17.0 (33.3)
--
1.50
--0.89
2.45
4.00
--
116.5 (5.5)
116.0
108.0
114.3 (12.7)
131.1 (7.0)
117.5 (5.4)
149.0
30.0 (9.4)
--22.7 (6.7)
26.6 (13.0)
17.5 (36.4)
--
2.00
--1.11
2.78
4.50
_
30.0 (4.7)
18.0
25.0
22.3 (6.8)
25.7 (16.1)
17.0 (33.3)
39.0
1.00
--1.11
3.18
4.00
--
19.0 (5.3)
19.3 (9.1)
-18.0 (24.2)
21.0 (26.1)
20.3 (28.8)
18.1 (14.0)
18.0 (16.4)
18.1 (17.3)
18.0 (0.0)
17.7 (13.1)
0.67
1.33
-3.33
4.00
4.44
2.13
2.00
2.69
0.00
1.78
141.3 (1.8)
143.2 (3.6)
136.0
146.0 (11.6)
133.3 (6.5)
121.3 (8.3)
149.4 (6.7)
133.8 (5.4)
132.1 (10.9)
144.0 (3.9)
144.0 (0.0)
19.7 (7.8)
19.8 (8.7)
-18.7 (20.3)
21.0 (29.6)
20.3 (37.2)
19.3 (14.6)
18.3 (18.1)
18.6 (19.1)
18.5 (3.8)
17.7 (13.1)
1.11
1.22
-2.89
4.50
5.78
2.25
2.25
2.78
0.50
1.78
19.0 (5.3)
19.3 (9.1)
20.0
18.3 (28.0)
20.5 (21.3)
20.0 (32.8)
18.1 (14.3)
17.8 (16.8)
17.9 (19.2)
18.0 (7.9)
17.7 (13.1)
0.67
1.33
-3.78
3.25
4.67
1.94
2.25
3.02
1.00
1.78
Between observed day and predicted day.
Coefficient of variation in brackets.
other methods in terms of mean of annual deviations. However,
when the total number of years of study in the stands was
considered, the period unit method outperformed the other
methods. The mean of annual deviations is a simple unsquared
value in contrast to the coefficient of variation, which involves
squared terms that tend to amplify the occasional large differences between estimates.
Tables 2--4 also demonstrate that the coefficient of variation
changed considerably when the units of the heat sum methods
were converted to a predicted day basis. The performance of
the heat sum methods was apparently inferior when based on
the coefficient of variation of the basic data than after conversion to a calendar day basis. Nevertheless, even before conversion of the data, the period unit method was the best measure
of the duration of the second phase of microsporogenesis.
Discussion
Figure 6. Lengths of periods of microsporogenesis in Scots pine (n =
44) in degree days as annual figures with reference to latitude. The
period from March 19 to tetrads is shown by h (y = 86.247 − 0.226x,
R2 = 0.002), tetrads to anthesis by n (y = 273.736 − 2.119x, R2 =
0.310), and March 19 to anthesis by s (y = 359.983 − 2.345x, R2 =
0.135). Stages of microsporogenesis were measured at 50% completion. The latitudinal spread from Locality 1 to 9 is visible along the
abscissa, whereas the variation between years is shown vertically. The
regression coefficient was not significant for the first (bottom line) or
third equation (top line), but it was significant for the second equation
(middle line).
Placing the biofix on March 19 was justified by the metabolic
activity observed in microsporangiate strobilus primordia of
Scots pine in Finland (Kupila-Ahvenniemi et al. 1978, Hohtola
et al. 1984, Häggman 1987, Häggman 1991), but it did not
account for the inevitable annual variations.
The effects of irradiance were ignored. It is commonly
assumed that irradiance effects are not significant for conifers
in spring (Mirov 1956); however, light is known to affect
microsporogenesis in other species (Alnus, Betula) toward the
504
LUOMAJOKI
Table 4. Comparison of timing parameters in measuring microsporogenesis from March 19 to50% completion of anthesis.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
4491 (1.6)2
4815
4840
4857 (7.4)
5149 (3.6)
4778 (1.3)
5862
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
6732 (2.6)
6749 (2.1)
6590
6935 (2.7)
6407 (6.5)
6212 (4.1)
7049 (3.9)
6538 (1.7)
6567 (3.7)
6745 (3.6)
7139 (5.8)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since March 19
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
77.0 (5.5)
--97.3 (3.3)
76.7 (3.5)
84.5 (2.5)
--
3.00
--2.44
2.04
1.50
--
123.5 (7.4)
145.0
127.0
138.7 (14.3)
157.6 (5.6)
143.5 (6.4)
176.0
75.5 (2.8
--98.0 (2.7)
77.3 (3.7)
85.5 (4.1)
--
1.50
--2.00
2.33
2.50
--
75.5 (4.7)
74.0
74.0
97.7 (4.1)
76.9 (3.8)
84.5 (2.5)
78.0
2.50
--2.89
2.41
1.50
--
86.7 (1.8)
84.8 (4.0)
-85.0 (1.2)
106.3 (2.9)
106.7 (1.4)
86.9 (2.0)
96.8 (4.7)
97.0 (3.0)
83.0 (1.7)
88.0 (1.1)
1.11
2.89
-0.67
2.25
1.11
1.41
3.75
2.29
1.00
0.67
204.7 (5.7)
208.3 (6.9)
200.0
226.3 (6.8)
194.0 (8.6)
189.3 (9.3)
233.1 (8.4)
204.3 (7.5)
208.0 (7.8)
202.0 (1.4)
221.3 (8.0)
87.0 (1.1)
85.3 (4.5)
-85.7 (1.3)
106.0 (3.2)
107.7 (0.5)
87.8 (2.6)
97.3 (5.2)
97.0 (3.5)
83.0 (3.4)
88.0 (1.1)
0.67
3.22
-0.89
2.50
0.44
1.63
3.75
2.29
2.00
0.67
86.3 (1.8)
84.5 (3.9)
92.0
84.7 (2.7)
105.0 (2.2)
106.7 (3.0)
86.5 (2.1)
96.8 (4.7)
96.9 (3.6)
82.5 (4.3)
87.3 (1.7)
1.11
2.67
-1.78
2.00
2.44
1.38
3.75
2.73
2.50
1.11
Mean
Mean of
annual
deviations1
Between observed day and predicted day.
Coefficient of variation in brackets.
end of the growing season (Luomajoki 1986). The thermal
effects of direct sunlight (Luomajoki 1977, Pukacki 1980)
were also ignored.
The basic period unit method was a better predictor of both
the first and second phases of microsporogenesis than degree
days. The success of the curvilinear heat sum method for the
second phase can be accounted for by the existence of real
phenological observations that precisely define both the onset
and the end of the period. The use of the basic heat sum data
can be defended on account of its simplicity. Conversion to a
predicted day basis compensates for the accelerating heat sum
accumulation toward the end of the period under study, thereby
making the comparison of parameters more objective. The
coefficient of variation can only be used for assessing the
parameters when the biofix is the same for all parameters and
when the parameters can be converted to a common scale (days
in the present case).
The period unit regression (Sarvas 1972) was developed
under an assumption of invariability in the rate of development
in a range of different tree species in the boreal zone. The shape
of the regression was derived from forced experiments with
meiosis in aspen (Populus tremula L.) and the opening rate of
birch catkins (Betula pendula Roth, B. pubescens Ehrh.). Sarvas (1972) also applied period units to the measurement of
development rate in conifers, and tested the model with two
species of Larix for the temperature range from 0.3 to 9.6 °C.
I compared the regression developed by Sarvas (1972) with
Wilson’s (1959) early results and found close similarities in the
lower parts of the two curves (Figure 7). Wilson’s (1959)
method was inaccurate for the shortest durations of meiosis, so
the upper part of the curve is the least reliable. Generally, the
largest differences among curvilinear regressions of this kind
occur at the upper inflection of the curve.
The climate in Finland is variable, and long cold or warm
periods occur. In an exceptionally cold or warm year, heat sum
methods will be better predictors of a developmental process
than calendar days. Nevertheless, even sophisticated heat sum
methods cannot fully compensate for the effects of extremes in
weather (cf. Luomajoki 1984, 1993a).
The results indicate that the performance of the different
methods depended on the stage of development under study.
This confirms Wang’s (1960) conclusion that the thermal reactions of plants vary with phase of development and indicates
that the homogeneity condition for successful simulation of
development set by Sarvas (1977) is not met. Although temperature dependency of a developmental process may change
with time, curvilinear heat sums have a potential advantage
over linear heat sums if properly used. Furthermore, curvilinear heat sums can now easily be computerized. Although the
degree day heat sum method was a poor predictor of the
duration of microsporogenesis, it is better to apply the degree
day method for forecasts rather than to depend on calendar
time only, because the degree day method accounts for climatic extremes.
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
505
References
Figure 7. The rate of development on temperature during the ‘active
period’ (Sarvas 1972) compared with Wilson’s (1959) results from the
progress of the meiosis in Endymion. The regressions were made to
unite at 10 °C after calculating the relative rates of development from
Wilson’s data (circles; curve-fitting by the author). The regression of
Sarvas (line of dots) is shown without observed data. Note the lower
inflection (toe or foot) and the upper inflection (shoulder). The ordinate gives the rate of development in period units (Sarvas 1972).
Because variation of point events tends to increase with time
(Sarvas 1972), the variation at meiosis should be less than at
anthesis. Figures 2 and 3 demonstrate that this is so, although
atmospheric factors can slow down anthesis and thus amplify
the apparent variation. Sarvas (1967, 1972) concluded that
variation during a developmental process remains at 6% of the
coefficient of variation as a result of the simultaneous growth
of the standard deviation and the mean. For the four point
events shown in Figures 2 and 3, the CVs ranged from 7.1 to
12.9%.
Acknowledgments
The materials for this study were collected in the former Department
of Silviculture of the Finnish Forest Research Institute. Professor
Risto Sarvas (deceased in 1974) initiated extensive studies on the
flowering of forest trees and thereby made this study possible.
Häggman, H. 1987. Seasonal variations in the ribosome assemblies
and in in vitro translations in the buds of Scots pine. Acta Univ.
Ouluensis Ser. A, Sci. Rerum Nat. 193:1--39.
Häggman, J. 1991. Cytokinins in developing buds and tissue cultures
of Scots pine and their role in xylem production of loblolly pine.
Acta Univ. Ouluensis Ser. A, Sci. Rerum Nat. 220:1--68.
Hari, P. and R. Häkkinen 1991. The utilization of old phenological
time series of bud burst to compare models describing annual cycles
of plants. Tree Physiol. 8:281--287.
Hohtola, A., S. Kupila-Ahvenniemi and R. Ohtonen. 1984. Seasonal
changes in the cytoplasmic structures of sporogenous cells of the
Scots pine. Ann. Bot. Fenn. 21:143--149.
Kupila-Ahvenniemi, S., S. Pihakaski and K. Pihakaski. 1978. Wintertime changes in the ultrastructure and metabolism of the microsporangiate strobili of the Scots pine. Planta 144:19--29.
Luomajoki, A. 1977. Effects of temperature on spermatophyte male
meiosis. Hereditas 85:33--48.
Luomajoki, A. 1984. The tetrad phase of microsporogenesis in trees
with reference to the annual cycle. Hereditas 101:179--197.
Luomajoki, A. 1986. The latitudinal and yearly variation in the timing
of microsporogenesis in Alnus, Betula and Corylus. Hereditas
104:231--243.
Luomajoki, A. 1993a. Climatic adaptation of Scots pine (Pinus
sylvestris L.) in Finland based on male flowering phenology. Acta
For. Fenn. 237:1--27.
Luomajoki, A. 1993b. Climatic adaptation of Norway spruce (Picea
abies (L.) Karst.) in Finland based on male flowering phenology.
Acta For. Fenn. 242:1--28.
Messenger, P.S. and N.E. Flitters. 1958. Effect of constant temperature
environments on the egg stage of three species of Hawaiian fruit
flies. Ann. Entomol. Soc. Am. 51:109--119.
Mirov, N. 1956. Photoperiod and flowering of pines. For. Sci. 2:328-332.
Pukacki, P. 1980. Temperature of Norway spruce and Scots pine buds.
Arbor. Kórnickie 25:277--286.
Sarvas, J. 1977. Mathematical model for the physiological clock and
growth. Acta For. Fenn. 156:1--25.
Sarvas, R. 1962. Investigations on the flowering and seed crop of
Pinus sylvestris. Commun. Inst. For. Fenn. 53:1--198.
Sarvas, R. 1967. The annual period of development of forest trees.
Proc. Finn. Acad. Sci. Lett. 1965:211--231.
Sarvas, R. 1968. Investigations on the flowering and seed crop of
Picea abies. Commun. Inst. For. Fenn. 67:1--84.
Sarvas, R. 1972. Investigations on the annual cycle of development of
forest trees. Active period. Commun. Inst. For. Fenn. 76:1--110.
Shelford, V.E. 1927. An experimental investigation of the relations of
the codling moth to weather and climate. Ill. Nat. Hist. Surv. Bull.
16:307--440.
Wang, J. 1960. A critique of the heat unit approach to plant response
studies. Ecology 41:785--790.
Wilkinson, L. 1990. Sygraph: the system for graphics. Systat Inc.,
Evanston, 547 p.
Wilson, J.Y. 1959. Duration of meiosis in relation to temperature.
Heredity 13:263--267.
© 1995 Heron Publishing----Victoria, Canada
Phenological measurements of microsporogenesis in trees
ALPO J. LUOMAJOKI
The Finnish Forest Research Institute, Kolari Research Station, FIN-95900 Kolari, Finland
Received January 21, 1994
Summary The value of two heat sum methods, one linear
(degree days > 5 °C) and the other curvilinear (period units),
were assessed together with calendar days as predictors of the
duration of microsporogenesis in seven natural stands of Norway spruce (Picea abies (L.) Karst.) and eleven natural stands
of Scots pine (Pinus sylvestris L.). Microsporogenesis was
divided into two subperiods: March 19 to tetrads (i.e., the end
of meiosis) and tetrads to anthesis. The total period from March
19 to anthesis was also assessed.
The methods were compared on a calendar day basis. When
annual deviations between the predicted (stand means) and the
observed annual heat sums were converted to days, the period
unit method outperformed the other methods for both subphases and for the total period of microsporogenesis. The
degree day parameter was more variable but a better predictor
of the duration of the initial phase up to tetrads and of the total
period than the calendar day parameter, but the calendar day
parameter more accurately predicted the duration of the subphase from tetrads to anthesis. The heat sum methods were
better predictors of the duration of development of microsporogenesis in exceptionally cold or warm years than the
calendar day method.
arbitrary. The use of a biofix is unavoidable if both ends of a
process are not limited by observable events. The first phase of
microsporogenesis up to tetrad formation and the total period
of microsporogenesis up to anthesis both suffer from the unavoidable use of a biofix.
The aim of this study was to appraise the advantages of using
a curvilinear heat sum method rather than a linear heat sum
method for estimating developmental intervals during microsporogenesis. In particular, degree days (> 5 °C) and the
period unit system of Sarvas (1972) were compared for forecasting the progress of microsporogenesis in Norway spruce
(Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.).
Both heat sum methods were compared to calendar days as
timing parameters. The postmeiotic phase of microsporogenesis was given special attention because of the advantage of
dispensing with a biofix. The consequences of using a biofix
to mark the onset of microsporogenesis were also evaluated.
Physiological reasons were sought to explain differences in the
predictive accuracy of the parameters.
Keywords: anthesis, heat sum, phenology, Picea abies, Pinus
sylvestris, tetrad phase.
Ten to 30 male buds were excised either from each tree or from
a population of 10 trees daily, one to six times a day, depending
on air temperatures (Luomajoki 1977, 1984). Each sample was
handled separately. The buds were bisected longitudinally (one
half being discarded), and each half bud was put in fresh
fixative containing 9/11 (v/v) glacial acetic acid/absolute ethanol. A pooled squashed sample from 10 to 30 fixed bud halves
was subsequently prepared in acetic (or formic) orcein on 4--6
slides. Between 400 and 600 pollen mother cells (PMC) from
each pooled squashed sample were inspected under the microscope. The onset of each phase of meiosis was considered a
point event.
Because meiotic materials were more scarce than those of
anthesis, the study was limited by the meiosis data. The meiosis data had to be adjusted according to the biofix, which was
set at March 19 (i.e., the first day of the year with 12 h of
sunlight) for the anthesis study (Luomajoki 1993a, 1993b).
Consequently, the period unit heat sum data given by Luomajoki (1984) are not fully comparable with the data used here.
For this study, the tetrad stage data were reprocessed (see
Figures 2 and 3). The Scots pine data differed from the earlier
published data (n = 43; Luomajoki 1984) by the addition of one
data set (Rovaniemi XXIX in 1971; locality 8 in Figure 1)
Introduction
The progress of many developmental phases in organisms is
strongly temperature dependent. Various temperature sum
methods have been developed to predict events such as flowering, fruit ripening and the emergence of pests. Simple calendar time has also been used for this purpose.
Although linear heat sum systems, like degree days, have
been criticized because they are nonphysiological (for a historical review see Sarvas 1972, cf. Wang 1960), these methods
are frequently used in botanical studies because of their simplicity. On the other hand, curvilinear regressions of development rate on temperature have been used occasionally in
entomological studies (Shelford 1927, Messenger and Flitters
1958), but only rarely in botanical studies (Sarvas 1972).
Postmeiotic microsporogenesis from tetrads to anthesis offers a unique opportunity to measure precisely the length of a
generative process without the complications caused by a
biofix (zero point). In practice, the biofix is more or less
Materials and methods
500
LUOMAJOKI
consisting of two observations (each based on a sample of 20
male buds) of the occurrence of the tetrad stage (Table 1).
The stands for the study of early microsporogenesis were the
same as used in earlier anthesis studies (Luomajoki 1993a,
1993b). Anthesis was measured on site, but male buds were
sampled for the assessment of pollen mother cells.
Mature, thinned stands of several hectares were classified as
normal stands for pollination (see Sarvas 1962). Antheses were
measured in each stand at tree-top height with one to three
self-recording pollen samplers (model Sarvas-Vilska 1963, see
Sarvas 1968). The mean of daily pollen catches was used when
more than one sampler was used. A Fuess (Berlin Steglitz)
thermograph was also placed at tree-top height in each stand.
The pollen catch was counted with the aid of a microscope
from the recording bands, and the results presented in terms of
daily catches of the recorders, catch averages, cumulative sums
and cumulative percentages of the pollen catch (Sarvas 1972).
The cumulative percentages were plotted with the Systat/Sygraph computer program (Wilkinson 1990). The ordinate scale
is a Gauss integral with a linear abscissa scale. The abscissa
showed the cumulative temperature sum at the end of each day
(corresponding to the measurement of the cumulative pollen
catch). Because of the effects of secondary pollen on anthesis
data (see Luomajoki 1993b), exclusion of points outside the
zone from −2 to +1.2 standard deviations was necessary to
position the regression line (Figures 2 and 3). The point of 50%
completion of each stage was used as a criterion for attaining
a given stage. The 50% point is unbiased by standard deviation.
One series of observations of anthesis in Norway spruce was
Figure 1. Localities of stands studied: (1) Bromarv, (2) Eckerö,
(3) Tuusula, (4) Jokioinen, (5) Heinola, (6) Punkaharju, (7) Vilppula,
(8) Rovaniemi, and (9) Kittilä.
Table 1. Stand characteristics and years of study; PMC = pollen mother cell.
Origin (locality)
Lat., long.
Elevation
Age in 1970
(years)
Anthesis
(years)
Tetrads of PMC
(years)
Remarks on stand
Picea abies (n = 17)
Bromarv I1 (1)
Heinola 566 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju LII (6)
60°02′, 23°05′
61°08′, 26°02′
60°50′, 23°30′
68°02′, 24°09′
61°49′, 29°20′
27
113
106
275
92
126
120
51
172
96
1963--73
1966--71
1966--73
1963, 65--73
1964--74
1966--67
1966
1969
1967--68, 73
1966--71, 73
Clear cut, 1976
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
66°21′, 26°40′
60°22′, 24°59′
182
50
127
67
1963--73
1967--73
1970, 73
1968
Pinus sylvestris (n = 44)
Bromarv II (1)
Bromarv III (1)
Eckerö I (2)
Heinola 566 (5)
Kittilä I (9)
Kittilä II (9)
Punkaharju XLV (6)
Rovaniemi XXVII (8)
Rovaniemi XXIX (8)
Tuusula XXXII (3)
Vilppula 2a (7)
60°02′, 23°03′
60°03′, 23°03′
60°11′, 19°34′
61°07′, 26°01′
68°02′, 24°09′
68°02′, 24°08′
61°48′, 29°19′
66°21′, 26°44′
66°21′, 26°38′
60°21′, 25°01′
62°04′, 24°29′
41
35
33
133
280
330
106
118
165
70
130
84
63
140
127
220
190
119
112
106
144
190
1964--69
1965--73
1966--69
1964--71
1963, 65--69, 71--73
1965--69, 71--73
1964--74
1963--73
1965--73
1964--69
1965--69
1967--69
1966--69, 71, 73
1969
1966--68
1967--69, 73
1967--69
1965--72
1967--70
1967--73
1967--68
1967--69
1
Origin Finland, Lammi
Clear cut, 1975--76
400 kg urea ha −1 given
in winter 1967--68
Clear cut, 1985
Plot numbers were assigned by the former Department of Silviculture of the Finnish Forest Research Institute. All other stand origins are local
except for Punkaharju LII.
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
rejected because of a minimal pollen catch (Bromarv I in 1968)
to avoid bias by secondary pollen from neighboring stands.
The nine localities involved in the study are shown in Figure 1. In all, 61 microsporogeneses studied from the biofix
(March 19) to 50% anthesis completion were available, and 57
of them were used for a comparison by means of the annual
deviations (between predicted and observed day) and by coefficients of variation.
The methods were compared on the basis of days (see Hari
and Häkkinen 1991), but the means of the basic data of the heat
sum methods are also shown (see Tables 2--4). The observations made in calendar days were used directly, but for the heat
sum methods, the expected heat sum (stand mean) and each
observed annual value were compared. For each year of study,
the expected heat sum (stand mean) was scanned on the particular stand-specific annual heat sum scale in either direction
from the heat sum actually observed to obtain the predicted
day. It was necessary to convert the basic data obtained by the
heat sum methods to a day basis, because the accumulation of
daily heat sums accelerated toward the end of the study period
(Figure 4).
501
Results
Based on estimates of period unit heat sums, the first phase of
microsporogenesis in Scots pine (Table 2) from March 19 to
50% tetrad phase completion (Figure 5) was similar in length
to the second part from tetrads to anthesis (Table 3), whereas
in Norway spruce, the first phase was only about half the
length of the second phase (Tables 2 and 3). However, when
based on observed duration in calendar days, the second phase
was considerably shorter than the first phase in both species
(Tables 2 and 3). The difference between the two methods was
a result of low ambient temperatures during the first phase.
Low temperatures during the first phase also explain why the
Figure 3. Tetrad stage of microsporogenesis (left) and anthesis (right)
in Scots pine at Rovaniemi, Stand XXIX. The method described in
Figure 2 was applied. Fifty percent of the pollen mother cells were in
tetrad stage at 2963 period units, and 50% of pollen had reached
anthesis at 6878 period units. The standard deviation (SD) of tetrad
phase was 209 period units, and the SD of anthesis was 618 period
units. The dates of 50% completion of the tetrad and anthesis phases
were on June 10 and 28, respectively.
Figure 2. Tetrad stage of microsporogenesis (left) and anthesis (right)
in Norway spruce at Punkaharju, Stand LII. The lines give the 50%
completion points (at 0 of ordinate) of the tetrad stage and of anthesis:
50% of the pollen mother cells were in tetrad stage at 1577 period units
and 50% of pollen in anthesis at 5244 period units. For anthesis, the
central larger squares were used to position the line whereas the
smaller ones beyond the limits of −2 to +1.2 standard deviations were
excluded (see Luomajoki 1993b for details of the method). The slopes
of the lines give the standard deviations (SD) of the two processes. The
SD can be read along the line from the abscissa between the 0 and +1
points of the ordinate. As percentages, these points correspond to the
50 and 84.13% points of the scale. The SD of tetrad stage was 204
period units and the SD of anthesis was 414 period units. The dates of
50% completion of the tetrad and anthesis phases were May 10 and
June 2, respectively.
Figure 4. A schematic pattern of accumulation of various units for
measuring the timing of anthesis. The calendar day scale is a uniform
variable, unchanged from day to day. Period units usually start to
accumulate before the biofix (March 19), whereas degree days usually
start to accumulate after the biofix. Both heat sum types accumulate at
an increasing rate so that larger daily heat sums occurr near the end of
the study. The onset of the accumulation of neither period units nor
degree days was a useful point to place the biofix.
502
LUOMAJOKI
Table 2. Comparison of timing parameters in measuring microsporogenesis from March 19 to50% completion of tetrad phase.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
1214 (9.5)2
1775
1615
1785 (8.2)
1661 (11.9)
1745 (2.1)
1639
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
3300 (5.1)
3163 (4.3)
3279
3280 (14.7)
2852 (7.7)
3029 (15.9)
3363 (6.3)
3146 (7.0)
3199 (8.5)
3155 (14.2)
3497 (13.5)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since March 19
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
Mean
Mean of
annual
deviations1
45.5 (17.1)
--75.3 (4.7)
51.7 (6.1)
67.5 (5.2)
--
5.50
--2.44
2.12
2.50
--
7.0 (40.4)
29.0
19.0
24.3 (22.6)
26.4 (37.4)
26.0 (10.9)
27.0
49.0 (8.7)
--75.7 (3.3)
53.1 (6.0)
67.5 (3.1)
--
3.00
--1.78
2.20
1.50
--
45.5 (10.9)
56.0
49.0
75.3 (4.1)
51.1 (8.3)
67.5 (5.2)
39.0
3.50
--2.22
3.27
2.50
--
67.0 (1.5)
64.7 (2.5)
-64.3 (3.2)
84.0 (7.2)
87.3 (9.3)
68.4 (4.1)
79.0 (5.8)
78.7 (4.1)
62.0 (4.6)
69.7 (2.2)
0.67
1.33
-1.56
5.00
6.22
2.29
3.00
2.33
2.00
1.11
63.3 (16.6)
65.2 (24.7)
64.0
80.3 (12.5)
60.8 (19.7)
68.0 (21.4)
83.8 (14.6)
70.5 (13.2)
75.9 (22.0)
58.0 (14.6)
77.3 (22.8)
67.7 (2.3)
65.7 (6.0)
-66.7 (6.8)
84.5 (7.6)
89.3 (4.5)
69.4 (5.3)
79.5 (5.3)
79.4 (4.8)
60.0 (9.4)
70.3 (3.6)
1.11
3.00
-3.11
5.00
2.89
2.88
3.00
2.78
4.00
1.78
67.3 (1.7)
65.2 (3.3)
72.0
66.3 (7.7)
84.5 (7.0)
86.7 (11.2)
68.4 (4.7)
79.0 (6.3)
79.0 (5.3)
64.5 (7.7)
69.7 (3.6)
0.89
1.56
-3.78
4.25
7.11
2.79
3.50
3.43
3.50
1.78
Between observed day and predicted day.
Coefficient of variation in brackets.
Figure 5. A diagrammatic presentation of the timing of microsporogenesis in Norway spruce and Scots pine in southern and northern
Finland. Distances between the solid points (d) were measured in this
study; open points (s) mark the estimated time when 50% of the
pollen mother cells were in leptotene. S = southern Finland, 60--62° N
lat.; N = northern Finland, 68° N lat.
predictions based on period units and degree days differed (cf.
Figure 6).
All three periods of microsporogenesis (i.e., first phase,
second phase and total period) in Scots pine were shorter in
terms of period units in the north than in the south of Finland.
A similar correlation with latitude was also found when the
prediction was based on the degree day method, even though
the change in the length of the first phase of microsporogenesis
from March 19 to tetrads was minimal (Figure 6). When the
prediction was based on calendar days, both the first part and
the total period of microsporogenesis were considerably
longer in the north than in the south of Finland; however, the
duration of the second phase of microsporogenesis in calendar
days was not correlated with latitude.
A comparison of the performance of the three methods was
only possible on a day basis. For both the heat sum methods,
the predicted day was used as the criterion, whereas no conversion was necessary for the calendar day method. Based on the
coefficient of variation, the period unit method was the best
predictor for both subphases of microsporogenesis as well as
for the total period from biofix to tetrads (Tables 2--4). The
degree day method produced less consistent results than the
period unit method, performing second best for the initial
phase from biofix to tetrads and for the total period. The
calendar day method was second best in performance for the
phase from tetrads to anthesis, but it was the worst parameter
for the initial phase and the total period (Tables 2--4).
The mean of annual deviations between predicted and observed day is given in Tables 2--4. Ranking the parameters by
the mean of annual deviations rather than by the coefficient of
variation gave similar results with one exception. For the total
period, considering only the accuracy of the forecast and the
number of stands, the degree day method outperformed the
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
503
Table 3. Comparison of timing parameters in measuring microsporogenesis from 50% completion of tetrad phase to 50% completion of anthesis.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
3277 (5.7)2
3040
3225
3072 (9.4)
3488 (6.3)
3032 (3.3)
4223
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
3432 (1.4)
3586 (3.0)
3311
3655 (10.4)
3556 (5.9)
3184 (8.2)
3685 (3.8)
3391 (5.3)
3368 (9.6)
3590 (5.8)
3642 (2.8)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since 50% tetrad phase
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
Mean
Mean of
annual
deviations1
30.5 (7.0)
--22.3 (5.2)
25.9 (12.1)
17.0 (33.3)
--
1.50
--0.89
2.45
4.00
--
116.5 (5.5)
116.0
108.0
114.3 (12.7)
131.1 (7.0)
117.5 (5.4)
149.0
30.0 (9.4)
--22.7 (6.7)
26.6 (13.0)
17.5 (36.4)
--
2.00
--1.11
2.78
4.50
_
30.0 (4.7)
18.0
25.0
22.3 (6.8)
25.7 (16.1)
17.0 (33.3)
39.0
1.00
--1.11
3.18
4.00
--
19.0 (5.3)
19.3 (9.1)
-18.0 (24.2)
21.0 (26.1)
20.3 (28.8)
18.1 (14.0)
18.0 (16.4)
18.1 (17.3)
18.0 (0.0)
17.7 (13.1)
0.67
1.33
-3.33
4.00
4.44
2.13
2.00
2.69
0.00
1.78
141.3 (1.8)
143.2 (3.6)
136.0
146.0 (11.6)
133.3 (6.5)
121.3 (8.3)
149.4 (6.7)
133.8 (5.4)
132.1 (10.9)
144.0 (3.9)
144.0 (0.0)
19.7 (7.8)
19.8 (8.7)
-18.7 (20.3)
21.0 (29.6)
20.3 (37.2)
19.3 (14.6)
18.3 (18.1)
18.6 (19.1)
18.5 (3.8)
17.7 (13.1)
1.11
1.22
-2.89
4.50
5.78
2.25
2.25
2.78
0.50
1.78
19.0 (5.3)
19.3 (9.1)
20.0
18.3 (28.0)
20.5 (21.3)
20.0 (32.8)
18.1 (14.3)
17.8 (16.8)
17.9 (19.2)
18.0 (7.9)
17.7 (13.1)
0.67
1.33
-3.78
3.25
4.67
1.94
2.25
3.02
1.00
1.78
Between observed day and predicted day.
Coefficient of variation in brackets.
other methods in terms of mean of annual deviations. However,
when the total number of years of study in the stands was
considered, the period unit method outperformed the other
methods. The mean of annual deviations is a simple unsquared
value in contrast to the coefficient of variation, which involves
squared terms that tend to amplify the occasional large differences between estimates.
Tables 2--4 also demonstrate that the coefficient of variation
changed considerably when the units of the heat sum methods
were converted to a predicted day basis. The performance of
the heat sum methods was apparently inferior when based on
the coefficient of variation of the basic data than after conversion to a calendar day basis. Nevertheless, even before conversion of the data, the period unit method was the best measure
of the duration of the second phase of microsporogenesis.
Discussion
Figure 6. Lengths of periods of microsporogenesis in Scots pine (n =
44) in degree days as annual figures with reference to latitude. The
period from March 19 to tetrads is shown by h (y = 86.247 − 0.226x,
R2 = 0.002), tetrads to anthesis by n (y = 273.736 − 2.119x, R2 =
0.310), and March 19 to anthesis by s (y = 359.983 − 2.345x, R2 =
0.135). Stages of microsporogenesis were measured at 50% completion. The latitudinal spread from Locality 1 to 9 is visible along the
abscissa, whereas the variation between years is shown vertically. The
regression coefficient was not significant for the first (bottom line) or
third equation (top line), but it was significant for the second equation
(middle line).
Placing the biofix on March 19 was justified by the metabolic
activity observed in microsporangiate strobilus primordia of
Scots pine in Finland (Kupila-Ahvenniemi et al. 1978, Hohtola
et al. 1984, Häggman 1987, Häggman 1991), but it did not
account for the inevitable annual variations.
The effects of irradiance were ignored. It is commonly
assumed that irradiance effects are not significant for conifers
in spring (Mirov 1956); however, light is known to affect
microsporogenesis in other species (Alnus, Betula) toward the
504
LUOMAJOKI
Table 4. Comparison of timing parameters in measuring microsporogenesis from March 19 to50% completion of anthesis.
Origin (locality)
Years
Degree days > 5 °C
Period units
Observed
Predicted day
Period
units
Mean of
predicted
days
2
1
1
3
7
2
1
4491 (1.6)2
4815
4840
4857 (7.4)
5149 (3.6)
4778 (1.3)
5862
Pinus sylvestris (n = 44)
Bromarv II (1)
3
Bromarv III (1)
6
Eckerö I (2)
1
Heinola 566 (5)
3
Kittilä, Pallas I (9)
4
Kittilä, Pallas II (9)
3
Punkaharju XLV (6)
8
Rovaniemi XXVII (8) 4
Rovaniemi XXIX (8) 7
Tuusula XXXII (3)
2
Vilppula 2a (7)
3
6732 (2.6)
6749 (2.1)
6590
6935 (2.7)
6407 (6.5)
6212 (4.1)
7049 (3.9)
6538 (1.7)
6567 (3.7)
6745 (3.6)
7139 (5.8)
Picea abies (n = 17)
Bromarv I (1)
Heinola 565 (5)
Jokioinen I (4)
Kittilä, Pallas IV (9)
Punkaharju L II (6)
Rovaniemi XVIII (8)
Tuusula XXXIV (3)
1
2
Days since March 19
Observed
Predicted day
Mean of
annual
deviations1
Degree
days
> 5 °C
Mean of
predicted
days
Mean of
annual
deviations1
77.0 (5.5)
--97.3 (3.3)
76.7 (3.5)
84.5 (2.5)
--
3.00
--2.44
2.04
1.50
--
123.5 (7.4)
145.0
127.0
138.7 (14.3)
157.6 (5.6)
143.5 (6.4)
176.0
75.5 (2.8
--98.0 (2.7)
77.3 (3.7)
85.5 (4.1)
--
1.50
--2.00
2.33
2.50
--
75.5 (4.7)
74.0
74.0
97.7 (4.1)
76.9 (3.8)
84.5 (2.5)
78.0
2.50
--2.89
2.41
1.50
--
86.7 (1.8)
84.8 (4.0)
-85.0 (1.2)
106.3 (2.9)
106.7 (1.4)
86.9 (2.0)
96.8 (4.7)
97.0 (3.0)
83.0 (1.7)
88.0 (1.1)
1.11
2.89
-0.67
2.25
1.11
1.41
3.75
2.29
1.00
0.67
204.7 (5.7)
208.3 (6.9)
200.0
226.3 (6.8)
194.0 (8.6)
189.3 (9.3)
233.1 (8.4)
204.3 (7.5)
208.0 (7.8)
202.0 (1.4)
221.3 (8.0)
87.0 (1.1)
85.3 (4.5)
-85.7 (1.3)
106.0 (3.2)
107.7 (0.5)
87.8 (2.6)
97.3 (5.2)
97.0 (3.5)
83.0 (3.4)
88.0 (1.1)
0.67
3.22
-0.89
2.50
0.44
1.63
3.75
2.29
2.00
0.67
86.3 (1.8)
84.5 (3.9)
92.0
84.7 (2.7)
105.0 (2.2)
106.7 (3.0)
86.5 (2.1)
96.8 (4.7)
96.9 (3.6)
82.5 (4.3)
87.3 (1.7)
1.11
2.67
-1.78
2.00
2.44
1.38
3.75
2.73
2.50
1.11
Mean
Mean of
annual
deviations1
Between observed day and predicted day.
Coefficient of variation in brackets.
end of the growing season (Luomajoki 1986). The thermal
effects of direct sunlight (Luomajoki 1977, Pukacki 1980)
were also ignored.
The basic period unit method was a better predictor of both
the first and second phases of microsporogenesis than degree
days. The success of the curvilinear heat sum method for the
second phase can be accounted for by the existence of real
phenological observations that precisely define both the onset
and the end of the period. The use of the basic heat sum data
can be defended on account of its simplicity. Conversion to a
predicted day basis compensates for the accelerating heat sum
accumulation toward the end of the period under study, thereby
making the comparison of parameters more objective. The
coefficient of variation can only be used for assessing the
parameters when the biofix is the same for all parameters and
when the parameters can be converted to a common scale (days
in the present case).
The period unit regression (Sarvas 1972) was developed
under an assumption of invariability in the rate of development
in a range of different tree species in the boreal zone. The shape
of the regression was derived from forced experiments with
meiosis in aspen (Populus tremula L.) and the opening rate of
birch catkins (Betula pendula Roth, B. pubescens Ehrh.). Sarvas (1972) also applied period units to the measurement of
development rate in conifers, and tested the model with two
species of Larix for the temperature range from 0.3 to 9.6 °C.
I compared the regression developed by Sarvas (1972) with
Wilson’s (1959) early results and found close similarities in the
lower parts of the two curves (Figure 7). Wilson’s (1959)
method was inaccurate for the shortest durations of meiosis, so
the upper part of the curve is the least reliable. Generally, the
largest differences among curvilinear regressions of this kind
occur at the upper inflection of the curve.
The climate in Finland is variable, and long cold or warm
periods occur. In an exceptionally cold or warm year, heat sum
methods will be better predictors of a developmental process
than calendar days. Nevertheless, even sophisticated heat sum
methods cannot fully compensate for the effects of extremes in
weather (cf. Luomajoki 1984, 1993a).
The results indicate that the performance of the different
methods depended on the stage of development under study.
This confirms Wang’s (1960) conclusion that the thermal reactions of plants vary with phase of development and indicates
that the homogeneity condition for successful simulation of
development set by Sarvas (1977) is not met. Although temperature dependency of a developmental process may change
with time, curvilinear heat sums have a potential advantage
over linear heat sums if properly used. Furthermore, curvilinear heat sums can now easily be computerized. Although the
degree day heat sum method was a poor predictor of the
duration of microsporogenesis, it is better to apply the degree
day method for forecasts rather than to depend on calendar
time only, because the degree day method accounts for climatic extremes.
PHENOLOGICAL MEASURMENTS OF MICROSPOROGENESIS
505
References
Figure 7. The rate of development on temperature during the ‘active
period’ (Sarvas 1972) compared with Wilson’s (1959) results from the
progress of the meiosis in Endymion. The regressions were made to
unite at 10 °C after calculating the relative rates of development from
Wilson’s data (circles; curve-fitting by the author). The regression of
Sarvas (line of dots) is shown without observed data. Note the lower
inflection (toe or foot) and the upper inflection (shoulder). The ordinate gives the rate of development in period units (Sarvas 1972).
Because variation of point events tends to increase with time
(Sarvas 1972), the variation at meiosis should be less than at
anthesis. Figures 2 and 3 demonstrate that this is so, although
atmospheric factors can slow down anthesis and thus amplify
the apparent variation. Sarvas (1967, 1972) concluded that
variation during a developmental process remains at 6% of the
coefficient of variation as a result of the simultaneous growth
of the standard deviation and the mean. For the four point
events shown in Figures 2 and 3, the CVs ranged from 7.1 to
12.9%.
Acknowledgments
The materials for this study were collected in the former Department
of Silviculture of the Finnish Forest Research Institute. Professor
Risto Sarvas (deceased in 1974) initiated extensive studies on the
flowering of forest trees and thereby made this study possible.
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