high as 32°C for the unshaded plots. It seems likely that lowering soil temperature was more important
than loss of light in a region of high temperatures and high irradiance. They used this study to demon-
strate the desirability of intercropping potatoes and corn. Although differences between vegetative and
sexual crops have been emphasized, the demonstration that high sucrose concentrations enhanced potato
tuber formation by cultured potato stems [104] is reminiscent of the impact of sugar supply on flower bud
differentiation [42,43].
In a study of sweet potatoes, Nakatani et al. [105] made several reciprocal grafts between various
clones to obtain differences in source and sink capacities. They reported that tuber dry mass was corre-
lated with leaf area. Because there was no difference in rate of photosynthesis (mg CO 2 dm^2 hr^1 ), they
concluded that rate of tuber growth was limited by the source. This principle of source limitation proba-
bly applies to forage crops as well.
B. Metabolic Consequences of Source/Sink Manipulation
During the day, as much as half of the carbon fixed in current photosynthesis is incorporated into starch
within chloroplasts [106]. This is critical for the well-being of a plant, for starch degradation at night sup-
plies carbohydrate for respiration of that leaf and for export to the rest of the plant to maintain respiration
and growth. The importance of this reserve starch was illustrated by transgenic Arabidopsiswith limited
ability to synthesize starch [107]. As a result of that metabolic deficiency, soluble carbohydrates accu-
mulated in leaves, inhibited photosynthesis, and resulted in little carbohydrate being available for night
export, thereby limiting growth to daytime.
When sinks are limiting, accumulation of assimilate can feed back and inhibit photosynthesis [106].
Claussen and Lenz [108], working with eggplant, demonstrated that removal of fruit resulted in accumu-
lation of sucrose and starch in source leaves. They also reported that this treatment lowered the activity
of sucrose phosphate synthase and the rate of photosynthesis. These changes were observed over periods
of days to weeks. Azcon-Bieto [109] demonstrated inhibition of photosynthesis within hours in wheat
leaves when export of assimilate was inhibited. The feedback mechanisms of assimilates on the enzymes
of photosynthesis are quite complex and not completely understood. For reviews on this subject, see Refs.
106 and 110–114. In addition to direct feedback on photosynthesis, Mandahar and Garg [115] reported
that removal of sinks from barley plants resulted in loss of chlorophyll from flag leaves.
C. Environmental Control
This section is intended not to cover in full detail the many factors of the environment that influence plant
productivity but rather to relate environmental control to factors discussed earlier. Adjustments in envi-
ronment, genetics, or cropping cycle to enhance floral development and seed or fruit set are based on the
realization that sink development will increase HI and very likely yield.
Plants have an ability to adjust partitioning when exposed to different environments. Logendra et al.
[116] exposed tomato plants to photoperiods of 8, 16, or 20 hr each day. Those exposed to the shortest
light periods retained a higher proportion of their photosynthate for later export during the long dark pe-
riod, thereby maintaining some supply for growth and maintenance during that dark period. This occurred
even though the total amount of carbohydrate accumulated in the light period by leaves of plants with the
shortest photoperiod was less than that of those exposed to longer photoperiods. Grange [117] obtained
similar results for pepper.
Increasing CO 2 concentration in the atmosphere can increase crop yields only if plants are able to in-
crease sinks in response to increased photosynthetic opportunity. If they are not able to increase their
sinks, assimilate accumulation will inhibit photosynthesis and no production increase will occur. Nie et
al. [118] found that wheat exposed to an elevated CO 2 concentration (550 vs. 360 mol mol^1 ) had higher
leaf concentrations of carbohydrate and lower expression of messenger RNA (mRNA) of photosynthetic
enzymes. However, Wang and Nobel [119] demonstrated that an elevated CO 2 concentration induces Op-
untiato form larger sinks, increases tissue concentrations of carbohydrate, and enhances activity of non-
photosynthetic enzymes of carbohydrate metabolism. Jitla et al. [120] demonstrated that rice grain yield
was increased if an elevated CO 2 concentration was supplied from the time of planting but not if that sup-
ply was delayed until only 15 days later. The importance of the early growth period is that it aids the es-
tablishment of sinks. These data should be used in the interpretation of other experiments in which ele-
428 HENDRIX