support the hypothesis that the period of flower development is the most critical portion of the life of crop
plants if sexual parts are economically important.
A positive relationship between seed number and yield, as indicated earlier for grain, has been
demonstrated for soybeans [87]. Guffy et al. [88] also demonstrated a stronger correlation with nitrogen
supply (nodulated vs. nonnodulated) than total assimilate. It should be pointed out that nitrogen fixation
is an energy- (assimilate-) expensive process [89]; therefore, competition by weeds, cited before, could
result in lowered N 2 fixation rates. Salado-Navarro et al. [90] observed that 80 to 90% of the plant’s pro-
tein is in seeds at harvest, further indicating the importance of nitrogen partitioning to soybean produc-
tion. It should also be noted that much of this nitrogen that accumulates in soybean seeds is stored tem-
porarily in specialized cells within leaves [91].
TREE CROPS It is common for some fruit trees, especially certain apple varieties, to have a pattern of
alternate years of heavy and light crops [92]. Floral initiation occurs in late summer, a time when a large
crop would compete for carbohydrates. This would result in low flower bud formation and few flowers
the following spring. During the subsequent summer of a small crop, there would be little competition for
carbohydrates by the crop, so large numbers of flower buds would form. A common practice has been to
break this alternate year cycle by thinning the heavy crop. Unfortunately, a frost during flowering one
year can restart this cycle. This scenario is supported by Ryugo [93]; however, Westwood [92] seems to
favor a concept of hormonal control. It is likely that there is an interaction between carbohydrate supply
and hormones that provides control of floral development.
Treating apple or peach trees by shading (10% natural light) or with a photosynthetic inhibitor
[94,95] induced abscission of fruit comparable to a “June drop” but markedly increased the proportion of
fruit lost. Minchin et al. [96] demonstrated that lowering the supply of photosynthate available to two sim-
ilar young apple fruit on a single spur resulted in one being favored in the receipt of the limited supply. If
this limitation of assimilate had continued, it seems likely that the less favored fruit would abscise. These
observations indicate that a similar mechanism based on assimilate supply controls sexual reproduction
in trees as in annual plants.
Miller and Walsh [97] compared partitioning of assimilate in peach trees where fruit had been either
thinned or not. This is a matter of economic importance, for thinning is a common cultural practice used
to obtain larger fruit, which have greater economic value. The HI of unthinned trees was 0.50 but that of
thinned trees only 0.37. It is interesting that the economically valuable part of fruit, the fleshy mesocarp,
is markedly increased by thinning, but the size of the energy-expensive seed is not increased nearly as
much. In a closely related tree crop, almond, from which the seeds are marketed, thinning is not practiced
for it would result in economic loss. Girdling of table grapes has been a common practice for many years
for it is effective in trapping carbohydrate above the girdle to increase fruit size.
Little work has been done on evergreen trees except for citrus. In their review, Goldschmidt and
Koch [98] reported that citrus trees differentiate flower buds in the winter, for, unlike deciduous trees,
they have a continuing supply of photosynthate. Even so, a supply of stored assimilate seems to be im-
portant to floral development, for some varieties bear crops in alternate years. In “off” years ‘Wilking’
mandarin trees accumulated more starch and produced more flowers than in “on” years. The impact of
thinning on fruit size and total production is as reported above for other fruit.
VEGETATIVE CROPS Unlike growth of seeds and fruit, growth of vegetative sinks appears to be
limited by source capacity rather than sink capacity. Working in a cool climate, Engels and Marschner
[99–101] reported on a series of experiments with potato. They demonstrated that current photosynthate
was rapidly used in tuber growth. In addition, tubers that initiated only 2–4 days after the first initiated tu-
bers were at a large disadvantage in accumulating assimilate. Fourteen days after first tuber initiation, tu-
bers that initiated only 4 days later were about one tenth the size of earlier tubers and were growing more
slowly.
When Engels and Marschner altered source/sink ratios by removing over half of the tuber mass, to-
tal tuber growth rate (cm^3 plant^1 ) returned to the previous rate within 4 days. When 50% of the leaves
were removed with no reduction in tuber mass, tuber growth rate was halved almost at once. Both of these
experiments support their conclusions about immediate use of photosynthate in tuber growth and that
growth of potato tubers was source limited. However, Midmore et al. [102,103] reported that shading
potato plants, especially early in development, enhanced tuber production. This would seem to contradict
the preceding study, but Midmore’s work was done in Peru, where soil temperatures were reportedly as
PRODUCTION-RELATED ASSIMILATE TRANSPORT 427