Susan Dunford, University of Cincinnati

Photosynthetic rate and sink demand are linked processes, particularly in species which store starch. Increased sink demand is correlated with increased photosynthesis, and vice versa. An accumulation of photosynthate (starch, sucrose, or hexoses), in the source leaf could be one of the signals coupling photosynthetic rate with sink demand. Possible mechanisms include:

1. Inhibition by starch. When sink demand is low, high starch levels in the source could physically disrupt the chloroplasts, interfere with CO₂ diffusion, or block light absorption. Little evidence supports this hypothesis.
2. Phosphate availability. When sink demand is low, photosynthesis could be restricted by a lack of free orthophosphate in the chloroplast (Du et al. 2000). Under conditions of low demand by the sink, sucrose synthesis is usually reduced, and less phosphate is thus available for exchange with triose phosphate from the chloroplast (via the phosphate translocator). If starch synthesis, which releases orthophosphate in the chloroplast, could not release phosphate fast enough, a deficiency in phosphate would ensue. ATP synthesis and thus CO₂ fixation would decline. A study with potato plants transformed with antisense DNA to the phosphate translocator provides support for this hypothesis (Riesmeier et al. 1993). The transformed plants, which displayed reduced phosphate translocator activity, allocated proportionately more carbon into starch and less into sucrose. These effects were accompanied by a reduction in the light- and CO₂-saturated rates of photosynthesis in young plants.
3. Regulation by sugars. High sugar levels decrease the transcription rate and expression of genes for many photosynthetic enzymes (Koch 1996). The changes in gene expression occur over the same time frame as the source adjustments already described. For example, in source leaves of spinach (Spinacia oleracea), mRNA for several photosynthetic enzymes decreased when soluble carbohydrates accumulated as a result of inhibition of export from the leaf (Krapp and Stitt 1995). Although transcript levels began to decline almost immediately, changes in photosynthetic enzyme activity were apparent only after several days. In this species at least, photosynthesis appeared to be inhibited because of changes in gene expression, not because of phosphate limitation, as discussed earlier.