A.G. Li et al. Agricultural and Forest Meteorology 106 2001 289–301 299
Table 7 Individual kernel grain filling processes of the main stem lower spikelets; maximum grain weights mg per kernel M, and grain filling
rates R
a
contrast over ambient A and elevated E CO
2
concentrations, and drought stress D and well-watered W treatments Contrasts
Kernel 1 Kernel 2
Kernel 3 Kernel 4
d.f. F
d.f. F
d.f. F
d.f. F
AD–AW 3
10.93
∗∗∗
3 11.23
∗∗∗
3 10.07
∗∗∗
3 2.78
∗
AD–ED 3
16.13
∗∗∗
3 7.94
∗∗∗
3 5.69
∗∗∗
3 5.67
∗∗∗
AW–EW 3
27.44
∗∗∗
3 13.77
∗∗∗
3 13.61
∗∗∗
3 9.46
∗∗∗
ED–EW 3
7.76
∗∗∗
3 5.96
∗∗∗
3 6.49
∗∗∗
3 2.89
∗
M
AD
–M
ED
1 4.41
∗
1 4.58
∗
1 0.84
1 8.97
∗∗
R
AD
–R
ED
2 9.55
∗∗∗
2 4.69
∗∗
2 2.75
2 7.46
∗∗∗
M
AW
–M
EW
1 9.77
∗∗
1 0.27
1 1.71
1 7.69
∗∗
R
AW
–R
EW
2 15.95
∗∗∗
2 4.05
2 6.69
∗∗∗
2 8.61
∗∗∗
M
AD
–M
AW
1 1.20
1 8.22
∗∗
1 2.33
1 1.50
R
AD
–R
AW
2 3.64
∗
2 8.02
∗∗∗
2 6.25
∗∗
2 1.84
M
ED
–M
EW
1 8.39
∗∗
1 3.78
1 5.53
∗
1 4.25
∗
R
ED
–R
EW
2 6.07
∗∗
2 4.41
∗
2 5.21
2 3.55
∗ a
R =
1 4
MB , the test of R is for difference in parameters M and B.
∗
Significant at 0.05 level.
∗∗
Significant at 0.01 level.
∗∗∗
Significant at 0.001 level.
3.3.2. Kernels on the middle spikelet Drought stress influenced the grain filling pro-
cesses of all the kernels on the middle spikelet under both elevated and ambient CO
2
levels with the excep- tion of the fourth kernel at elevated CO
2
condition Table 5. The first kernel weight increased 4.8 and
2.6 mg under ambient CO
2
and elevated CO
2
levels, respectively, due to a prolonged grain filling dura-
tion Fig. 4B. The final weight of the second kernel increased 4.1 mg in the well-watered treatment at
elevated CO
2
again due to a prolonged grain fill- ing duration Fig. 4B. The third and fourth kernels
were less responsive to drought stress conditions than the first and second kernels in this experiment
Table 5.
3.3.3. Kernels on the lower spikelet All the grain filling processes of the individual ker-
nels at the lower spikelet were influenced by drought stress conditions over both CO
2
levels Table 7. Well-watered conditions increased the final kernel
weights of the first, third, and fourth kernels under elevated CO
2
due to a longer grain filling duration Fig. 4C, but not under ambient CO
2
Tables 6 and 7. The final weight of the second kernel increased
by well-watered condition due to a longer grain fill- ing duration under the ambient CO
2
condition only Tables 6 and 7. Water treatment had the most effect
on the fourth kernel weight.
4. Discussion
Our results indicate that both the rate and the du- ration of grain growth vary substantially depending
on floret positions and environmental conditions. The combination of these changes determines the final in-
dividual kernel weight. In some cases, the change in rate or duration had no influence on the final individ-
ual kernel weight because the change in rate compen- sated for the change in duration. As a general trend,
kernels in proximal spikelets had a higher growth rate and a longer grain filling duration, however, there
were exceptions. These results agree with those of Simmons and Crookston 1979. They suggested that
the higher grain filling rate of the proximal kernels is related to their earlier anthesis and greater growth
immediately after anthesis. Furthermore, grain filling rates and durations of individual kernels responded
differently to drought stress or elevated CO
2
treat- ments in this study. Generally, well-watered condition
tends to extend grain filling duration and elevated CO
2
apparently stimulates the grain filling rate of in- dividual kernels. The higher rate of grain filling could
300 A.G. Li et al. Agricultural and Forest Meteorology 106 2001 289–301
be associated with the increased individual leaf or canopy assimilation Pinter et al., 1996; Garcia et al.,
1998. Also kernels that are more proximal or distal to the rachis were affected proportionately more than
those towards the center of a spikelet.
The ranking of individual kernel weights within a spikelet was the second the first the third
the fourth kernels with the exception in the upper spikelet. It was not affected by elevated CO
2
or wa- ter stress treatments in this experiment even though
the differences among the kernels might change over various treatment and floret positions. The study of
the vascular anatomy of a spikelet Zee and O’Brien, 1971; Hanif and Langer, 1972 has indicated that the
first, second, and third kernels are connected to the transfer cells in the rachis by independent vascular
strands and the fourth kernel is connected to the main assimilate source by strands originating near the base
of the third kernel, and so on. By this anatomical construction, the first kernel has priority to the assim-
ilate supply and also it has an advantage because it is fertilized earlier than the rest of kernels on the same
spikelet. However, the morphogenesis was modified in the later stage of grain growth and the second
kernel exceeded the first one in weight at maturity Bremner and Rawson, 1978. This may be because
the second kernel has a larger growth potential than the other kernels. Within a spikelet the order of the
individual kernel weight is determined structurally and genetically. In this experiment, water stress and
elevated CO
2
treatments did not change the ranking of kernel weights, even though the absolute weights
of kernels were changed. The grain filling process is determined by assim-
ilate supply, intrinsic resistance to assimilate trans- portation, and kernel growth potential Bremner and
Rawson, 1978. For a specific kernel within a spikelet, the combination of these three factors, as well as en-
vironmental factors, determines the grain filling rate and duration. The combination of rate and duration
changes determines the final kernel weight. Elevated CO
2
and well-watered conditions may directly affect the grain filling process by providing more assimilate
Pinter et al., 1996, and may also potentially influ- ence the development of the plant at cellular or molec-
ular levels, further impacting the growth potential and intrinsic resistance. We reported in a previous paper
that the final grain weight of the main stem spike is increased 4 by elevated CO
2
of 550 mmol mol
− 1
and 7 by well-watered conditions Li et al., 2000. It is
probable that the increase from distal and proximal kernels contributes more to the main stem total grain
weight.
5. Conclusion
