assimilate partitioning and may even feed back on supply by affecting photosynthesis. From an evolu-
tionary standpoint, these patterns seem reasonable, for a plant will be more successful in producing
progeny if it produces only the number of seeds that assuredly will be viable. So, at some stage, a plant
must sense the number of seeds that it can successfully supply, even if a disaster, such as drought or loss
of leaves to insects, should occur. Once the critical developmental stages have passed, a plant is commit-
ted to seeds that are set, and seed number is determined. One possible mechanism of the abortion process
is suggested by the demonstration [49,50] that more basal sites on inflorescences are more adequately vas-
cularized than more distal sites. Thus, a more distal location would receive adequate assimilate supply
only if that supply was larger or if the more proximal sites were unoccupied. From the economic stand-
point, the floral development period is critical, for it is during this period that maximum potential sink ca-
pacity is established. Subsequent events may diminish that capacity, but capacity cannot be increased
when that critical time has passed.
- Productivity of Specific Crops
SMALL GRAIN Shanahan et al. [51] demonstrated that grain number was a stronger indicator of win-
ter wheat yields than grain size, even in the plains of eastern Colorado, where low moisture and high tem-
peratures during grain filling often result in large variability in grain size.
Bremner and Rawson [52] reported that position within the spike and within each spikelet had a large
influence on grain mass. In control plants, the largest grains within the spike were located about one third
the distance from the base. Within each spikelet, the most basal grain was largest. Removal of some grains
9 days after anthesis resulted in some increase in size, primarily at the ends of the spikes. In no case did
the most distal grain on the thinned spikelet attain the mass of those at the base of the unthinned spikelet.
These investigators interpreted the results as indicating that the basal position of each spikelet had a more
adequate vascular system supplying assimilates rather than suggesting a limiting supply of assimilates
within the plants. That conclusion is supported by others [49,50].
Further evidence that sink capacity is limiting to production was presented by several others. Blade
and Baker [53] demonstrated that changing the source/sink ratio by lowering plant density, removing de-
veloping grain, or removing the flag leaf had little influence on the mass of individual grains. Other work
supports the conclusion that wheat [54] and oats [55] are sink limited during grain filling.
To illustrate the impact of lowered supply of photoassimilate on wheat grain production, Fischer [56]
conducted several shading experiments. In his more severe treatments, shaded plants received only 35%
of the natural light during a single 21-day treatment period. When the shading period was centered dur-
ing vegetative development or early in floral development, there was little impact on grain production
when compared with unshaded controls. The shading period centered near the midpoint of floral devel-
opment lowered grain production to just over 40% of controls, shading centered at anthesis lowered yield
to 80% of controls, whereas subsequent shading periods had progressively less impact, with the final pe-
riod, which extended to maturity, having little impact. Work by Wardlaw [57] and Caldiz and Sarandon
[58] supports these results. Supporting data were also provided by Kiniry [59] in the demonstration that
shading sorghum plants during inflorescence development resulted in production of far fewer grains than
in controls. Removal of the shade at anthesis resulted in larger grains than in controls, but not nearly
enough larger to compensate for loss in number.
High temperature alone [60] or in combination with low light prior to anthesis resulted in lower ker-
nel number [57,61] than high temperature at any other developmental stage. Either of these environmen-
tal impacts would lower assimilate accumulation during inflorescence development.
Willenbrink et al. [62] demonstrated that shading (50%) starting at 22 days after anthesis had little
impact on grain mass or number; loss of assimilate from photosynthesis was made up by added mobi-
lization of stored fructan from vegetative parts. They further demonstrated that removing about two thirds
of the grains increased individual grain mass only slightly. These data added support to the idea that grain
production is sink limited during grain filling.
High temperature during grain filling inhibited starch synthesis [63] and shortened the grain-fill-
ing period [64,65], thereby lowering production of wheat by lowering grain mass [66]. Tashiro and
Wardlaw [67] obtained similar results for rice and in addition demonstrated that starch content was
more negatively influenced at elevated temperatures than protein content. Jenner and coworkers
[68–70], studying the impact of high temperatures on starch-synthesizing enzymes of wheat and bar-
ley, found lower activity whether high temperature was applied in vivo or in vitro. They also reported
PRODUCTION-RELATED ASSIMILATE TRANSPORT 425