2. Material and methods
Field trials were conducted on potato crops grown on fine silt in Nodebais Belgium in 1995
and 1996, full details of which are given in De- blonde et al. 1999. Briefly, the trials comprised
six contrasting European cultivars five of which were common reported to differ in drought resis-
tance and earliness. We used two early cultivars Eersteling, sensitive; and Jaerla, tolerant, two
mid-early cultivars Krostar Eersteling, sensitive, both years; Claustar, intermediate, in 1995; and
Bintje, tolerant, in 1996 and two mid-late culti- vars Nicola and De´sire´e, both tolerant. A strip-
plot design with two factors cultivar and water status and four replicates was used in both years.
Plot size was 3 × 9 m and three water supply treatments were applied irrigated, rainfed and
drought. Row distance was 75 cm, and the dis- tance between plants was 32 cm. Field trials were
planted on the 15th and the 12th of April in 1995 and 1996, respectively. The irrigated treatment
received water from rainfall plus a limited amount of irrigation 37 and 35 mm, respectively, in 1995
and 1996. The rainfed treatment received water from rainfall only. The drought treatment was
imposed by placing a strong plastic sheet on the soil surface of the plots from 50 emergence to
8 weeks after thereby excluding 85 mm 1995 and 57 mm 1996 of rainfall. The irrigated, rain-
fed and drought correspond to treatments 1, 2 and 3 in Deblonde et al. 1999.
3. Measurements
The number of green leaves was measured at 108, 124 and 157 days after planting in 1996 on
three consecutive plants in each plot for Krostar and De´sire´e. When a leaf could not be considered
as completely green, visual estimates of green leaf area were made on a scale of 25, 50, 75 and 100.
A yellow leaf was counted as fully senescent. Only leaves on the main stem were recorded.
Leaf lengths, from the 14th to the 19th leaf on the main axis, were measured in 1996 on two
cultivars Krostar and De´sire´e 133 days after planting. According to Vos and Biemond 1992
leaf numbers 10 – 18 on the main stem correspond to the leaves of larger size.
Stem height distance from the soil surface up to the insertion of the petiole of ‘uppermost’ leaf
was measured twice in 1995 101 and 115 days after planting and once in 1996 109 days after
planting on all cultivars. We define the ‘upper- most’ leaf as the leaf attached on the top of the
uppermost internode characterised by a length superior to 3 mm. Stems were upheld vertically
during the measurements. Statistical analysis was based on F-test and contrast tested according to
Scheffe’s test.
Tuber dry weight was determined on two plants lifted fortnightly from the central ridges in each
plot with the aid of a spade. The first harvest started 60 and 67 days after planting in 1995 and
1996, respectively. Thus a total of 14 early culti- vars or 16 late cultivars plants altogether were
harvested to assess the evolution of tuber weight. Four plants per plot were lifted at final harvest.
Tubers were separated from the rest of the plant and weighed in the field. The number of tubers
per plot was determined tubers smaller than 10 mm were not considered. Other details of tuber
sampling and harvest are given in Deblonde et al. 1999.
We investigated if a relationship could be estab- lished between stem height and final tuber dry
weight across cultivars.
4. Results
4
.
1
. Stem height Statistical analysis for stem height within years
showed significant differences between cultivars and treatments Table 1. Earlier cultivars were in
general shorter than later cultivars. The drought treatment significantly reduced stem height in
comparison with the irrigated treatment whereas the rainfed treatment did not. There was a signifi-
cant cultivars-by-treatment interaction in 1996. The interactions observed in 1996 were explained
by the behaviour of four cultivars: two early cultivars Jaerla and Eersteling reduced less stem
height than the other cultivars in the drought
P .M
.K .
Deblonde ,
J .F
. Ledent
Europ .
J .
Agronomy
14 2001
31 –
41
Table 1 Stem height in cm and final tuber dry weight in kg m
− 2
measured in 1995 and 1996 for three water treatments n = 8 for stem height and n = 16 for tuber dry weight.
1995 1996
Tuber dry weight Stem height 109 DAP
a
Stem height 115 DAP Tuber dry weight
Irrigated Rainfed
Drought Irrigated
Rainfed Drought
Irrigated Rainfed
Drought Irrigated
Rainfed Drought
0.84 52.2
56.7 45.4
1.08 1.05
0.83 1.09
0.78 53.5
60.0 62.9
Eersteling 1.03
0.83 0.87
43.6 43.1
37.6 1.18
0.93 0.82
Jaerla 53.9
54.8 48.3
0.89 –
– –
– –
0.95 –
0.93 Claustar
62.5 57.6
57.5 –
65.3 62.8
49.6 1.22
0.99 Bintje
0.96 –
– –
– –
0.83 66.5
60.4 51.9
1.35 1.05
1.09 0.99
Krostar 1.17
67.6 74.8
85.5 0.98
65.8 59.6
46.6 Nicola
1.08 73.5
0.88 0.97
73.4 65.4
1.14 1.13
1.09 65.1
66.1 48.8
1.14 1.16
1.14 0.98
73.5 De´sire´e
1.17 58.6
72.1 1.04a
68.6a 0.92a
59.7a 58.1a
46.6b 1.18a
0.97a 0.92a
65.4a 58.5b
1.08a Mean
Analysis within years Tuber dry weight
Stem height 109 DAP Tuber dry weight
Stem height 115 DAP C
a
T NS
C×T NS
NS 0.08
S.E.
b
0.05 4.18
1.54 Analysis over years
Tuber dry weight Stem height
Y
a
NS C
T NS
C×T NS
NS Y×C
Y×T NS
NS NS
Y×C×T NS
a
C, cultivar; T, treatment; Y, year; DAP, days after planting.
b
S.E., standard error of cultivar×treatment interactions. Significant at the PB0.05 level.
Significant only at PB0.1.
Fig. 1. Evolution of the soil water potential absolute values at the depth of 20 – 40 cm under the mother tuber, Treatment 1, 2 and 3 respectively: irrigated, rainfed and drought.
treatment P B 0.01 and two mid-late cultivars Nicola and De´sire´e showed a stronger stem
height reduction in the drought treatment than the other cultivars P B 0.01. Analysis over years
indicated that stems were significantly shorter in 1996 than in 1995 and confirmed the presence of
main effects but the presence of cultivar by treat- ment interactions was not established across
years. This may be due to the drier season start of 1996 in comparison with 1995. In the drought
treatment the growth of the plants depended on the water stored in the soil and was related to
rainfall before plastic placing and this was much higher in 1995 than in 1996 111 and 61 mm,
respectively. The reduction of stem heights in the mid-late cultivars was therefore smaller in that
treatment in 1995. The lowest soil water potential values measured at the depth of 20 – 40 cm
reached − 0.3 and − 0.5 MPa in 1995 and 1996, respectively Fig. 1. Lower soil water potential
values were undoubtedly reached in the drought treatment.
4
.
2
. Relationship between stem height ratios and TDWS
Stem remained until 136 days after planting in 1995 and 122 days after planting in 1996. The
relationship between stem height ratios and TDWS was positive in 1995 R
2
= 0.21 and nega-
tive in 1996 R
2
= 0.80; Fig. 2. The lower deter-
mination coefficient in 1995 was essentially due to cultivar De´sire´e which is at the upper left of the
graph in Fig. 2 R
2
rose to 0.90 when cultivar De´sire´e was omitted in the regression. This culti-
var reduced strongly 20 its stem height in the drought treatment in comparison with the irri-
gated treatment but the reduction in tuber dry
Fig. 2. Relationship between stem height ratio and TDWS defined respectively as stem height and tuber dry weight in the drought treatment relative to the irrigated treatment for all cultivars in 1995 and 1996. The first letter of the cultivar Section 2 is given
at the right side of each data point.
Table 2 Results of the linear regression for the relationship between
stem height cm measured in the middle of the season and tuber dry weight g m
− 2
at final harvest Slope
R
2
Intercept n
1995
Irrigated 630.7
6.57 0.58
6 11.26
0.68 Rainfed
6 300.5
2.26 0.03
a
782.7 6
Drought
1996
2.89 0.07
Irrigated 6
1003.3 7.88
0.27 513.8
6 Rainfed
Drought 13.43
291.4 0.53
6
a
The determination coefficient rose to 0.33 when cultivar Krostar was omitted in the regression but the slope remained
non-significant. Significant at the PB0.05 level.
Significant only at PB0.1.
tionship observed in 1996. Indeed, the drought stress started earlier that year which explains the
lower stem height ratios in comparison with 1995. Also the drought stress ended earlier which al-
lowed cultivars which had not finished their life cycle to recuperate a part of their tuber produc-
tion at that time. This might explain the presence of early cultivars Eersteling and Jaerla at the
right bottom side of the graph slight stem height reduction of 13 – 14 with strong tuber yield
reduction 28 – 30 whereas the mid-late cultivars De´sire´e and Nicola were at the opposite left
corner strong stem height reduction 21 – 24 with only a slight tuber dry weight reduction
10 – 14.
4
.
3
. Relationship between stem height and tuber dry weight
In the irrigated treatment, the slope of the linear relation between stem height and final tuber
dry weight was not significantly different from zero Table 2 although early cultivars had gener-
ally lower stem height and yields than late culti- vars. We obtained a significant relationship
between stem height and tuber yield for the rain- fed treatment in 1995. For the drought treatment
in 1996 the relation was significant only at P B 0.1. In the other cases the drought treatment,
1995; the rainfed treatment, 1996 the relationship between stem height and final tuber dry weight
was not significant.
4
.
4
. Leaf length Leaf length for each leaf number separately;
data not presented did not significantly differ among cultivars absence of cultivar effects and
cultivar by treatment interactions. Fig. 3 presents the leaf lengths mean over cultivars for the three
water regimes for the leaf numbers 14 – 19. Al- though a tendency toward shorter leaf length was
observed for the drought treatment for each leaf number it became only significant above the 16th
leaf. Leaf lengths in the rainfed treatment were never significantly different from those in the
irrigated treatment. The statistical analysis of the weight 7 was slighter. All other cultivars
showed a good agreement between these two criteria that year. A first possible explanation is
that stem height is not related to tuber yield but this does not explain the good relationship found
for the other cultivars. A second more likely explanation is that drought was not terminal and
De´sire´e, the latest cultivar of our set of cultivars, could recuperate a part of the tuber production at
the end of the season. This was not or less possi- ble for earlier cultivars because they had finished
or were ending their life cycle. This second expla- nation allows understanding of the negative rela-
Fig. 3. Leaf lengths means over cultivars measured at 133 DAP days after planting in 1996 standard error of treat-
ments is given by
T
.
leaf lengths of different leaf numbers within a cultivar showed that leaves inserted higher had
shorter lengths than leaves of lower ranks whatever the water regime. This reduction of lengths of the
upper leaves was higher in the case of the drought treatment significant leaf number by treatment
interactions; F-test. This was observed in cultivar Krostar as well as in cultivar De´sire´e.
4
.
5
. Number of green lea6es The number of green leaves present on the main
stem decreased through dates of measurements from 108 days after planting to 157 days after
planting; Table 3. Cultivar Krostar showed a significantly higher number of green leaves than
De´sire´e only for one date 108 days after planting; P B 0.01, afterwards differences between the two
cultivars were not significant. The drought treat- ment reduced significantly the number of green
leaves at two dates 108 and 124 days after planting; P B 0.01 as did also the rainfed treatment for one
date 124 days after planting in comparison with the irrigated treatment. However, at the end of the
season, differences between the three treatments levelled off with a tendency of the drought treat-
ment to show a higher number of green leaves. This was especially so in cultivar Krostar. Cultivars
behaved similarly across treatments cultivar by treatment interactions were not significant.
4
.
6
. Tuber number The statistical analysis of tuber number mea-
sured at final harvest and the corresponding aver- age tuber dry weight are shown in Table 4. Cultivar
differences were observed for both parameters both years. Treatments differences were observed in
1995 for tuber number and for average tuber dry weight in 1996. In 1996, the average tuber number
was significantly P B 0.05 lower than in 1995 14.1 instead of 15.3, analysis over years whereas the
average tuber dry weight was significantly P B 0.01 higher 20.5 instead of 17.2 g. The higher
average tuber dry weight compensated largely the tuber number reduction observed that year. Fi-
nally, the average total dry weights per plant were similar 264.1 and 288.4 g per plant in 1995 and
1996, respectively. The statistical analysis indi- cated for both parameters that one or several
cultivars behaved differently across years. More- over, the effects of treatments on tuber number
were also different between years. Differences in the fore-season may account for these results as indi-
cated above.
5. Discussion
Kategori