THE ROLE OF TRANSIENT STARCH IN ACCLIMATION TO ELEVATED ATMOSPHERIC CO2

Although increased concentrations of CO2 stimulate photosynthesis, thi s stimulation is often lost during prolonged exposure to elevated carb on dioxide, leading to an attenuation of the potential gain in yield. Under these conditions, a wide variety of species accumulates non-stru ctural carbohydrates in leaves, It has been proposed that starch accum ulation directly inhibits photosynthesis, that the rate of sucrose and starch synthesis limits photosynthesis, or that accumulation of sugar s triggers changes in gene expression resulting in loser activities of Rubisco and inhibition of photosynthesis. To distinguish these explan ations, transgenic plants unable to accumulate transient starch due to leaf mesophyll-specific antisense expression of AGP B were grown at a mbient and elevated carbon dioxide. There was a positive correlation b etween the capacity for starch synthesis and the rate of photosynthesi s at elevated CO2 concentrations, showing that the capability to synth esize leaf starch is essential for photosynthesis in elevated carbon d ioxide, The results show that in elevated carbon dioxide, photosynthes is is restricted by the rate of end product synthesis, Accumulation of starch is not responsible for inhibition of photosynthesis, Although transgenic plants contained increased levels of hexoses, transcripts o f photosynthetic genes were not downregulated and Rubisco activity was not decreased arguing against a role of sugar sensing in acclimation to high CO2, (C) 1998 Federation of European Biochemical Societies.