vated concentrations of CO 2 were used for specific time periods. Also, this appears to be in conflict with
shading experiment reported previously for wheat.
If projected warming occurs, temperatures may move out of the desired range for sink establish-
ment, and therefore lower productivity would result. Such an environmental change may require chang-
ing crops or time of planting. Impacts of increasing CO 2 concentration are extensively discussed by
Bowes [121].
For factors that can be controlled but have a cost, such as water, pesticides, and fertilizer, cost/ben-
efit ratios, both economic and environmental, can be enhanced by an awareness of the most critical time
for development of economic sinks. Therefore, if some input is limited, emphasis should be placed on
the critical phase of plant development. One must also be aware that the critical phase might be at very
different calendar dates for different crops. For example, floral initiation in annual crops occurs in late
spring or early summer, whereas initiation occurs in late summer for the next season’s crop on woody
perennials. No matter how favorable the conditions are during seed and fruit growth, partitioning of as-
similates into those economically important parts can occur only if those parts form earlier in develop-
ment.
D. Plant Disease and Partitioning
Recently, the emphasis of crop protection has changed. Using either traditional genetic techniques or re-
combinant DNA, genes for the production of materials toxic to pests are being incorporated into crop
plants. However, this approach is not always successful, for these materials can be toxic to humans or
farm animals or even to the plant itself [122,123]. Therefore, we will probably need to continue to use
chemical pest control, but the timing will be much more important so that the most economic benefit can
be obtained with the smallest amount of material, lowest possible rate of resistance development by pests,
and lowest possible environmental impact. Timing of pesticide application has emphasized the vulnera-
ble part of pest life cycles. Timing of crop vulnerability to pests must also be considered, for economi-
cally important plant parts are not equally vulnerable at all portions of a plant’s life cycle. In addition, we
will be able to lower total pesticide use by avoiding many prophylactic applications if we learn how pes-
ticides are transported through plants, how plant parts are targeted, and use appropriate timing of appli-
cation. It seems reasonable to apply this principle to transgenic plants that produce their own pesticide
such that this production is confined to vulnerable plant parts at appropriate times.
Control of assimilate partitioning and/or availability of assimilates may be a mechanism by which
some pathogens alter a plant in the manifestation of disease symptoms. Blunt ear syndrome of corn, man-
ifested by stunted development of ears with little evidence of disease in the vegetative parts of the plant
[124,125], may result from disruption of normal partitioning. Failure of plants to accumulate carbohy-
drates in stems or supply it to developing ears would lead to suppression of flower development, discussed
earlier, and result in this syndrome.
Dickinson et al. [126] created a transgenic tomato plant that had the appearance of one infected with
Clavibacter michiganensissubsp.sepedonicus, the causative agent of potato ring rot (CA Ishimura, per-
sonal communication). The transgenic tomato plants expressed an apoplastic yeast invertase. This re-
sulted in the hydrolysis of most of the apoplastic sucrose of source leaves. Because tomato uses the
apoplastic pathway of phloem loading (see later), little sucrose was available to be loaded and the resul-
tant hexoses were reabsorbed by mesophyll cells. These cells accumulated large amounts of starch and
became chlorotic, a typical symptom of this disease and of excess starch accumulation in chloroplasts.
Von Schaewen et al. [127], using the same principle, had similar results with tobacco but much less se-
vere symptoms on Arabidopsis.
More recently, Balachandran et al. [128,129] showed that partitioning was altered in transgenic to-
bacco by the expression of the movement protein of tobacco mosaic virus. The transgenic plants had
lower dry mass of roots and stems compared with leaves, again indicating interference with assimilate
export. Furthermore, unloading of the phloem is influenced by parasitism. It has been shown that higher
plant parasites such as Cuscuta[130,131] and a parasitic root nematode [132] induced phloem unload-
ing.
These results indicate that at least some disease conditions are caused by the disruption of normal
partitioning. Use of transgenic plants to mimic disease physiology may make it possible to learn the mech-
anisms of pathogenesis and to develop procedures to interfere with such mechanisms.
PRODUCTION-RELATED ASSIMILATE TRANSPORT 429