It appears that a sustained stimulation of photosynthesis and growth in response to elevated atmospheric CO 2 requires an increase of sink organ development to utilize and store additional photosynthate. Without sufficient sink capacity, photosynthate can feed back such that source capacity is brought back into balance with sink capacity. Thus, the relative magnitude of source and sink activities is an important determinant of crop yield in future climates. In general, plants are able to adjust their development of sink organs in response to altered photosynthetic rates. But little is known about the factors that limit the capacity for up-regulation of sink capacity and the regulatory mechanisms that are at work.
Heat stress is believed to decrease plant growth due to inhibition of photosynthetic rate.
Thus, it has been proposed that elevated CO 2 treatment, which is expected to enhance photoassimilate supply, may alleviate the effect of heat stress on tuber growth. Our findings indicate that elevated CO 2 substantially increases the rate of photosynthetic carbon assimilation, apparently avoiding feedback effects by upregulating sink development. To evaluate how sink capacity was modulated, we have measured cell numbers, cell size, and extent of endoreduplication with flow cytometry. We are also studying cambial glucose and sucrose concentrations, and activities of several enzymes of sugar metabolism. We have used shade as an additional test of the effects of altered photosynthate status.