that plants treated with high temperatures were able to recover some lost enzyme activity after being
returned to a cool environment.
It appears that environmental impacts during grain filling do not lower yield as much as comparable
environments during inflorescence development. A further indication that assimilate supply during veg-
etative growth does not limit grain production was the observation that grazing of wheat prior to floral
initiation did not lower grain yield [71].
MAIZE Maize appears to be more closely balanced than small grain on the basis of source-sink capac-
ity during grain filling. Jones and Simmons [72] demonstrated that removing a portion of the developing
ear had little if any impact on grain size, whereas defoliation of the plants 12 days after midsilking re-
sulted in lower grain number and mass per grain and defoliation 24 days after midsilking resulted in only
lowered mass per grain. Defollation at either date resulted in rapid depletion of carbohydrates from stems,
while control plants had repartitioned about 40% of their maximum stem mobilizable carbohydrates at
maturity. Maize is more sensitive than wheat to loss of photosynthetic capacity during grain filling, for
stem-stored carbohydrates are mostly sugars [72] rather than larger polymers, resulting in a lower storage
capacity. Shading corn plants during reproductive development lowered the quantity of harvested grain
more that shading at any other time [73]. Unlike the results for wheat, shading during the vegetative stage
caused significantly lower grain production. None of the shading treatments caused lower stover yield.
Setter and Flannigan [74] demonstrated that decreasing source by shading early in grain development re-
sulted in a correlation between lowered number of endosperm nuclei and lowered grain dry mass. Work
by Hueros et al. [75] indicated that development of transfer cells at the base of developing grains is crit-
ical to grain filling and prevention of abortion, probably because these cells are involved in supplying as-
similate to developing grain. Another interesting phenomenon in maize is the relationship of carbohydrate
metabolism in “sweet corn.” A lowered ability to accumulate starch [76,77] resulted in lower grain mass,
which was then reflected in lower Hl.
LEGUMES It is more difficult to associate the pattern of carbohydrate partitioning and accumulation
with reproductive activity in indeterminate plants. Aufhammer and coworkers [78–80] demonstrated
that unlike the effect in wheat, lowering the number of seed-bearing sites by removal of flower buds
fromVicia fabadid not lower production, for these plants were able to compensate by increasing seed
number and/or size at other sites. In addition, they demonstrated that removal of basal flower buds in-
creased the fruit set at more distal positions. It is likely that some minimum carbohydrate concentration
must be met if buds are to develop into flowers and then into fruit. Sage and Webster [81] demonstrated
that more distal buds, flowers, or fruit of a raceme of Phaseolusare more likely to abort than those at
the more proximal positions. These studies imply that development at more distal positions was limited
by insufficient assimilate supply. Mauk and Breen [82] were able to support that hypothesis with data
of^14 C assimilation studies. This appears to be parallel to the conclusion of Fischer and HilleRisLam-
bers [50] that more distal reproductive sites are limited by vascularization. White et al. [83] reported
that genetically determined seed size and yield (including HI) are negatively correlated in Phaseolus
vulgaris.This suggests that the growth of small seeds in a basal fruit on a raceme or on a basal raceme
of a stem results in less depletion of the carbohydrate supply, thus allowing more fruit and/or seeds to
develop. However, Stockman and Shibles [84] showed that neither increasing light intensity nor re-
moving leaves altered flower and pod abscission. They concluded that carbohydrate supply was not
immediately involved in abscission.
Kelly and Spanswick [85] demonstrated that competition for assimilates between vegetative and re-
productive growth can affect seed size. Using nearly isogenic lines of pea, they found that the lines that
stopped vegetative growth early and even lost their leaves produced larger seeds, even though their total
seasonal photosynthesis was less than that of lines that maintained active vegetative growth and photo-
synthesis longer.
Wiles and Wilkerson [86] studied soybean production in competition with cocklebur. They reported
that little production loss occurred if the cocklebur plants were removed before the fifth week of plant
growth, about the time the soybean plants started to flower. Losses increased as time of competition con-
tinued through the 16th week, when flowering was complete. No additional loss of seed production oc-
curred when cocklebur was allowed to compete to harvest. These results are supported by result for shad-
ing of indeterminate Vicia faba[79,80] beginning at the start of flowering. Those treatments resulted in
lowering of seed number but not seed size. As with the shading studies cited earlier for wheat, these data
426 HENDRIX