and ML. Nitrogen fertiliser favoured an earlier start to tillering at all sites: there are more TC +
plants at ME, and there are more T1 + plants at LE and in the dense stands of SGE with nitrogen
supply Fig. 5. Nevertheless, differences between sparse stands at LE and ML are not explainable
by daylength, density or nitrogen.
3
.
3
. Tillering dynamics Some plants which were already tillering were
able to continue tillering after the sampling dates in May-sown situations, especially at LE and
unlike ME and SGE. Many WT plants are still able to bear T3 in dense stands at LE and ML
Table 2. The rate of the tillering cessation was different: two phyllochrons were enough to stop
the tillering of 88 – 97 of the bearing-tiller plants in all conditions of ME and SGE, but in LE
sparse stands only 45 – 65 of these plants stopped their tillering in two phyllochrons. In the
LE sparse stands, some plants started to tiller after others had stopped in a same sample plot
data not shown.
The differences in shoot numbers between the ‘pasty-ripe’ samples and the ‘tillering architecture’
samples Table 2 showed that the dynamics of the stand differed between situation treatments.
At ME, a general and quick cessation of tiller- ing occurred inside the first quarter of the dura-
tion of stem elongation. No more tillers appeared afterwards and the usual disappearance of dead
tillers was observed in dense stands.
In SGE sparse stands, 130 additional shoots per square meter must come from a late resump-
tion of tillering after a general cessation which was observed at the beginning of stem elongation.
In dense stands, some plants were still tillering at the ‘tillering architecture’ sampling date, but they
stopped immediately afterwards.
At LE, further tillering must have also occurred to account for the observed final numbers of
shoots, even in dense stands. Many of the plants were still able to tiller in accordance with the
model after the ‘tillering architecture’ sampling date. Except in SN conditions, a short time of
further tillering is enough to produce the addi- tional number of shoots which was observed at
the ‘pasty-ripe’ stage, if all plants capable did, actually tiller Table 2. In SN conditions, a quar-
ter of the plants had to double the shoot number of the stand; with at least three phyllochrons
required to end tillering Table 2, the last tillering cessation occurred towards the ‘heading’ stage at
the earliest. In dense stands, almost all plants that were able to tiller in the future were young WT
plants. It is unlikely that most of these plants were beginning to tiller at this time. Therefore, more
tillering time than one phyllochron would be needed on the other plants.
At ML, many plants were capable of tillering after the ‘tillering architecture’ sampling date, but
in reality no more shoots emerged afterwards Table 2. The last tillering cessation occurred at
the ‘tillering architecture’ sampling date.
4. Discussion
4
.
1
. Canopy closure The observed stand biomasses Fig. 4 are com-
pared to the biomass range at canopy closure found by Biscoe et al. 1975. All canopies at ME
and dense stands at SGE and ML are likely to have been closed before the ‘heading’ sampling.
At LE, dense stands may have also been barely closed at the ‘heading’ stage, while the sparse
stands remained open at this stage. The significant disappearance of many dead tillers in dense stands
at ME and in DN conditions at SGE Table 2 is consistent with a canopy closure earlier in these
stands than in the others.
4
.
2
. The beginning of the tillering
:
the conditioning of the phenotypes
A tiller is already growing before it emerges. According to Malvoisin 1984, Skinner and Nel-
son 1994 the tiller bud begins to grow at the end of the elongation of the sheath of its subtending
leaf, that is at the emergence of the next leaf. Thus, the TC bud begins to grow at the emer-
gence of the first leaf, the T1 bud at the emer- gence of the second leaf, and so on. Two
phyllochrons of growth within the subtending
M .
Lafarge Europ
. J
. Agronomy
12 2000
211 –
223
Table 2 Percentage of plants which are still able to tiller within the model after the ‘tillering architecture’ sampling, changes in shoot number between ‘tillering architecture’
and ‘pasty-ripe’ samples, and additional tillering events when significantly more shoots were counted on the ‘pasty-ripe’ samples
a
Additional tillering Percent of plants that are able to
Number of shoot axes per square meter at Number of plants
Situation treatment and cropping
per square meter tiller within the model
b
after the required by the
‘tillering architecture’ sampling final number of
condition shoots: minimum
Already tillering length of the
WT pl. bearing ‘Tillering
‘Pasty-ripe’ Significant
plants sampling
Architecture’ differences
five leaves continuation within
sampling the model
c
or late resumption of
tillering 712
ME–SO 124
801 n.s.
795 n.s.
909 –
ME–SN 128
901 –
ME–DO 287
1 1082
\ 979
– \
ME–DN 269
1251 118
B 486
Till. resumption SGE–SO
360 SGE–SN
596 120
Till. resumption 464
B 691
– n.s.
369 5
SGE–DO 746
\ 402
705 –
1 974
SGE–DN 333
B 534
One phyllochron LE–SO
129 13
24 128
B 770
Three phyllochrons 10
16 LE–SN
363 934
One phyllochron B
LE–DO 582
694 45
3 B
637 1210
One phyllochron 4
48 944
LE–DN 545
– n.s.
1 3
563 ML–SO
127 n.s.
126 559
– 3
652 ML–SN
833 n.s.
811 –
ML–DO 540
1 33
929 n.s.
896 –
ML–DN 526
46
a
All shoot axes were numbered: dead, live non-ear-bearing and ear-bearing axes. A shoot axis is a tiller or a main stem. A few main stems can be dead. Phyllochron: elapsed thermal time between the emergence of two successive leaves. Ability to tiller is estimated and additional tillering duration is simulated according to the model
by Masle-Meynard and Se´billotte 1981b — see Section 2.3 and Fig. 1 in Section 2. ME, SGE, LE and ML, situation treatments as in Table 1 or Fig. 2; SO, SN, sparse stands without or with nitrogen fertilization; DO, DN, dense stands without or with nitrogen fertilization as in Fig. 5.
b
The plants which were able to tiller within the model after the sampling date are: a the tillering plants that had not stopped to tiller at the sampling date; b the plants without tiller which bore less than six unfolded leaves.
c
Simulation was carried out for each plant which was able to tiller within the model. For each further phyllochron of simulated tillering on all plants able to tiller, the new tillers were added to the previous shoot number of the stand in each plot, and this sum was compared to the ‘pasty-ripe’ strength by anova.
sheath are subsequently needed before the first leaf of the tiller emerges Fig. 1. Kirby and Faris
1972 observed that the elongation of the pro- phyll in the subtending sheath always ended in the
emergence of the tiller. Young barley seedlings in the field are unlikely to experience the effects of
nutritional shortage. This is because seed reserves satisfy their nutritional needs until 120 – 200 de-
gree-days after germination Metivier and Dale, 1977; Kullmann and Greef, 1992. At this time,
the unfolding of the second leaf occurs with the usual phyllochrons at spring time sowing Kirby
et al., 1982. Therefore, nitrogen cannot be so short that cell division in the buds of the first
tillers was prevented. The nitrogen effect which was reported above in the last paragraph of
Section 3.2 is not necessarily related to competi- tion. It could be a hormonal effect: Samuelson et
al. 1992 showed barley seminal roots to produce cytokinins when nitrates rise in the growing
medium, and cytokinins favour tiller bud growth Johnston and Jeffcoat, 1977.
Neighbourhood effects may be related to light quality effects Ballare´ et al., 1987; Casal et al.,
1990. In the literature on the effect of light quality on tillering e.g. Kasperbauer and Karlen,
1986; Barnes and Bugbee, 1991; Skinner and Sim- mons, 1993; Davis and Simmons, 1994 investiga-
tions on the beginning of tillering on seedlings were not reported. However, Davis and Simmons
1994 recorded the presence of specified tillers on their sampled plants. These results on May-sown
barley show that the far-red-enriched light from 2 cm-neighbouring plants on a row in a stand, is
enough to delay the start of tillering up to T2 on some plants. In shorter days, Kirby and Faris
show that very high plant densities prevented TC bud development in barley Kirby and Faris,
1972 and advanced floral initiation Kirby and Faris, 1970. The earlier far-red enrichment from
neighbourhood rather than from canopy closure which usually signals the onset of competition
can condition phenotypes by delaying the onset of tillering in an interaction with floral initiation.
One possible reason for this effect, is the produc- tion of gibberellins which inhibit tiller growth
Johnston and Jeffcoat, 1977. A flow of gibbere- llins is produced at floral initiation and this is
triggered by increased daylength Pharis et al., 1987 or the addition of far-red light Deitzer et
al., 1979. In the dense stands of SGE, LE and ML, 1 cm
or less separated the plants in a row because of the 25-cm-spaced rows of the experimental design.
Long days and close neighbourhood strongly in- teracted to advance shooting Fig. 3 and to lock
the first tiller buds Fig. 5 in dense stands. In sparse stands, the small number of WTT3 +
plants at ML compared to LE Fig. 5, in spite of the same daylength and density, can be explained
by the low photothermal quotient at ML Fig. 2: the developmental effect of inductive daylengths is
weakened by low irradiance at a given tempera- ture in spring barley Aspinall and Paleg, 1963;
Faris et al., 1969.
With similar photothermal quotients, high ni- trogen supply and low plant density, a markedly
increased daylength produced a higher level of phenotypes with delayed start of tillering com-
pare SN conditions at ME and LE.
4
.
3
. End of tillering, competition and growth capacities
The dynamics of the cessation of tillering reflected the ways of the onset of competition
which occurred in the stands. At LE, some plants stopped tillering early at
the beginning of stem elongation while others continued, or some even began to tiller see 3.3.
When the canopy was not closed during stem elongation Fig. 4, the competition remained very
weak for a long time, except locally, among direct neighbours. This is consistent with a situation of
reduced growth potential: when the growth with- out competition is low, the restriction of growth
by the same arrangement of neighbours is reduced results by Lindquist et al., 1994. In the dense
stands, the tillering continued late, as in the sparse stands Table 2. The general onset of competition
was not hastened by four times higher plant den- sity, because this density had brought too high
proportion of weak phenotypes WTT3 + Fig. 5.
At SGE, the general cessation of tillering at the beginning of stem elongation was likely due to
competition for restricted soil resources, because of a sudden drought at that time Fig. 2. After-
wards, in sparse stands the late closure of the canopy may have allowed for a resumption of
tillering — such as in pasture grasses — as a consequence of rains after the drought Luebs and
Laag, 1969. In dense stands, the earlier closure of the canopy prevented the resumption of tillering.
In these stands, three times higher plant density caused more weak phenotypes Fig. 5, but
started an earlier onset of competition Table 2.
At ME, the early and definitive cessation of tillering reflected an early establishment of compe-
tition between all plants, including the sparse stands, as a result of the high growth potential of
the individuals.
At ML, the tillering phenotypes seem to be close to those of SGE in sparse stands and to
those of LE in dense stands Fig. 5. In spite of these likely low growth potentials, no significant
further tillering was observed Table 2. Competi- tion must have been established by soil resources
restricted by long-lasting drought Fig. 2.
5. Conclusions
