Importance of leaf versus whole plant CO2 environment for photosynthetic acclimation

The reduction of photosynthetic capacity in many plants grown at elevated C O2 is thought to result from a feedback effect of leaf carbohydrates on gen e expression, Carbohydrate feedback at elevated CO2 could result from limit ations on carbohydrate utilization at many different points, for example ex port of triose phosphates from the chloroplast, sucrose synthesis and phloe m loading, transport in the phloem, unloading of the phloem at the sinks, o r utilization for growth of sinks. To determine the relative importance of leaf versus whole plant level limitations on carbohydrate utilization at el evated CO2, and the possible effects on the regulation of photosynthetic ca pacity, we constructed a treatment system in which we could expose single, attached, soybean leaflets to CO2 concentrations different from those exper ienced by the rest of the plant. The single leaflet treatments had dramatic effects on the carbohydrate contents of the treated leaflets. However, pho tosynthetic capacity and rubisco content were unaffected by the individual leaflet treatment and instead were related to the whole plant CO2 environme nt, despite the fact that the CO2 environment around the rest of the plant had no significant affect on the total non-structural carbohydrate (TNC) co ntents of the treated leaflets. These results necessitate a re-evaluation o f the response mechanisms to CO2 as well as some of the methods used to tes t these responses. We propose mechanisms by which sink strength could influ ence leaf physiology independently of changes in carbohydrate accumulation.