Susan Dunford, University of Cincinnati, Cincinnati, OH, USA
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:
- 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.
- 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.
- 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.
Du Y-C, Nose A, Kondo A, Wasano K (2000) Diurnal Changes in Photosynthesis in Sugarcane Leaves: I. Carbon dioxide exchange rate, photosynthetic enzyme activities and metabolite levels relating to the C4 pathway and the Calvin cycle. Plant Production Science 3: 3–8.
Koch KE (1996) Carbohydrate-modulated gene expression in plants. Annual Review of Plant Physiology and Plant Molecular Biology 47: 509–540.
Krapp A, Stitt M (1995) An Evaluation of Direct and Indirect Mechanisms for the Sink-Regulation of Photosynthesis in Spinach - Changes in Gas-Exchange, Carbohydrates, Metabolites, Enzyme-Activities and Steady-State Transcript Levels after Cold-Girdling Source Leaves. Planta 195: 313–323.
Riesmeier JW, Flügge UI, Schulz B, Heineke D, Heldt HW, Willmitzer L, Frommer WB (1993) Antisense Repression of the Chloroplast Triose Phosphate Translocator Affects Carbon Partitioning in Transgenic Potato Plants. Proceedings of the National Academy of Sciences of the United States of America 90: 6160–6164.