- Carbohydrate Metabolism
Of course, a plant’s development and response to the environment are mediated by gene expression.
Therefore, the demonstration by Xu et al. [33] that the expression of different isozymes of carbohydrate
metabolism is either enhanced or suppressed by the environment is of great importance but not surpris-
ing. For the establishment of economic sinks in many plants, starch synthesis is essential (see review by
Smith et al. [34]). Also, the understanding of sugar-metabolizing enzymes is illustrated for sugarcane in
that differences in expression of acid invertase and sucrose phosphate synthase determine the accumula-
tion of sucrose in different cultivars [35]. In addition, Geiger et al. [36] reported that invertase activity in
developing bean pods and corn grains increased before the rate of assimilate import increased, indicating
that utilization of sucrose enhanced its import (also see Sturm [37]). It is also interesting to note that these
events occurred a few days before the time of abortion, suggesting that assimilate supply is critical in in-
hibiting abortion. Zeng et al. [38] demonstrated that activity of invertase in maize roots was markedly
lowered by anoxia (0% O 2 ) or hypoxia (3% O 2 ). Because low O 2 would limit the root’s ability to metab-
olize sugars, this seems reasonable, for lowered use of sucrose would lower the sink capacity; however,
the activity of sucrose synthase was not lowered by these treatments. This supports Sturm’s view [37] that
invertase activity is important in controlling sink strength.
In her review, Koch [39] categorized “famine” and “feast” genes. “Famine” genes enhance supply
and suppress utilization of assimilates, whereas “feast” genes operate in the opposite way. An ability to
understand and control these genes may enhance productivity and HI. Koch et al. [40] also reported that
there are two genes for sucrose synthase in maize roots. One is maximally expressed in a sugar-depleted
environment, the other in a high-sugar environment. Furthermore, they are expressed in different cells,
potentially having an impact on assimilate partitioning.
- Floral Evocation
For plants from which sexual parts are harvested, maximum potential harvest is established at flower for-
mation. Clearly, plants are variable in the stage of development at which reproduction is controlled. Some
plants continue to flower after fruit form, but most of those later flowers abscise if the earlier fruit has not
been removed. Ornamentalists have known for centuries that removal of old flowers and young fruit (car-
bohydrate sinks) markedly increases subsequent flowering. It is reasonable to conclude that assimilate
supply is critical in determining degree of flowering, fruit, and seed set.
Hendrix et al. [41] demonstrated that the content of fructan in wheat inflorescences 7 days before an-
thesis was highly correlated with grain number. Bodson [42] and Bodson and Outlaw [43] reported that
accumulation of carbohydrates in buds of Sinapsiswas associated with floral evocation. In their review,
Bodson and Bernier [44] stated:
Available evidence suggests that an early change in carbohydrate concentration in the apical bud is criti-
cal to floral initiation, but that this modification is not sufficient. alone to trigger initiation. It is not possi-
ble to conclude whether assimilate accumulation in the reproductive structures is responsible for inflores-
cence development since the timing of events that are integral parts of reproductive development is
generally very poorly known.Several workers [45–47] have demonstrated that floral induction of the long-day plants Sinapsis alba
andArabidopsis thallanaby one photoinductive cycle resulted in an increase in sucrose concentration in
sieve tube exudate. Ishiora et al. [48] were able to induce floral development on cultured Pharbitisapices
by raising sucrose and/or lowering ammonium concentrations. Nitrate did not inhibit floral induction.
One might conclude that floral induction induces greater sucrose translocation. However, inductive pho-
toperiods altered the diurnal timing of carbohydrate supplied from photosynthesis. That alteration, alone,
may account for altered concentrations of sucrose in exudate. One could just as easily conclude that a flo-
ral inductive signal increases sink capacity of buds, thereby enhancing flowering, or that once floral de-
velopment has been initiated, the degree of reproductive development is controlled by assimilate supply.
It also appears that the importance of carbohydrate supply extends to shortly after anthesis, a period of
potential abortion. Once the abortion period has passed, studies cited subsequently indicate that the ca-
pacity of sinks rather than sources is limiting.
It appears that in all crops, assimilate supply and partitioning patterns determine the degree of flow-
ering and/or fruit set. The amount of fruit, seed, or grain that develops determines subsequent patterns of
424 HENDRIX