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There was considerable variability in grain weight /spike recorded in
69 wheat genotypes which ranged from 1.4 to 3.3 g with mean 2.1 g and
coefficient of variation (26 %) (Table 2).
However, in response to defoliation, variability reduced to 18.4 %
indicating that the differences ingrain weight/spike might be partly
due to variation in current photosynthesis. This was also evident in
genotypes subjected to delayed sowing, wherein CV for grain
weight/spike was as high as 23.9 %. These results are on par with
observations in which contribution of current photosynthesis plays a
major role both under normal and stress condition (Setter et al.
1998). The pooled analysis revealed that manual defoliation
significantly reduced the grain weight/spike but not the grain
number. In contrast to this, remarkable reduction in both of these
traits was observed in response to KI spray and delayed sowing.
Significant correlation coefficient between reductions in grain
weight/spike and grain numbe/spike in response to KI spray (r=0.73,
p=0.01) indicated that the reduction in grain weight/spike was
largely due to reduction in grain number rather than reduction in
assimilate availability. Further, there was significant correlation
(r=0.83, p=0.01) between number of grains in untreated control and
those in KI-treated plants. Presumably KI might have selectively and
uniformly inhibited initial post anthesis processes associated with
grain development. It might be due to toxic effect of KI on
development of grains at disadvantageous location within the spikes
or spikelets of all the accessions. Reduced availability of
assimilate for florets located in the distal part of the spike and
spikelets has been reported when source was a limiting factor (Slafer
et al. 1996). Reduction in grain weight/spike in response. to
defoliation could not be explained by reduction in grain number.,
Instead, it was obvious from reduction in individual grain weight
that there was reduction in assimilate supply to the developing
grains. This was further supported by significantly high correlation
(r=0.63, p=0.01) between grain weight/spike in defoliated and those
in control plants irrespective of variation in flowering time. Hence,
it was inferred that response to defoliation could explain the
potential of the accessions to mobilize stem reserves in a better way
as compared to KI treatment.
No perceptible relation was observed between response to KI spray and
defoliation. This could he attributed to difference in effect of
these two treatments on grain number/spike. Response of accessions to
defoliation could partly explain their performance under late sown
condition as there was significant correlation between grain weights
in response to these two treatments (r=0.54, R2=0.3).
However, failure of perfect simulation as reflected by low
R2 value was mainly because of the fact that the reduction
in grain weight/spike was largely due to reduced grain growth period
under late sown condition. Under such conditions, both the amount and
rate of supply of assimilate must have played a significant role.
It is concluded that KI (0.25%) used to stop leaf photosynthesis also
affected processes other than assimilate supply during grain growth.
Hence, it can not be used to determine potential of wheat genotypes
to use the carbohydrate accumulated in stem for-grain development.
However, lesser concentration or novel chemicals with no effect on
biological activities in the developing grain may be used to explain
the translocation of assimilate for grain development and. yield
potential of wheat genotypes under heat stress environments.
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