Photosynthetic acclimation to elevated CO2 is dependent on N partitioning and transpiration in soybean

Physiological processes that modulate photosynthetic acclimation to rising atmospheric CO2 concentration are subjects of intense discussion recently. Apparently, the down-regulation of photosynthesis under elevated CO2 is not understood clearly. In the present study, the response of soybean (Glycine max L.) to CO2 enrichment was examined in terms of nitrogen partitioning and water relation. The plants grown under potted conditions without combined N application were exposed to either ambient air (38 Pa CO2) or CO2, enrichment (100 Pa CO2) for short (6 days) and long (27 days). Plant biomass, apparent photosynthetic rate, transpiration rate and N-15 uptake and partitioning were measured consecutively after elevated CO2 treatment. Long-term exposure reduced photosynthetic rate, stomatal conductance and transpiration rate. In contrast, short-term exposure increased biomass production of soybean due to increase in dry weight of leaves. Leaf N concentration tended to decrease with CO2 enrichment, however such difference was not true for stem and roots. A close correlation was observed between transpiration rate and 15N partitioned into leaves, suggesting that transpiration plays an important role on nitrogen partitioning to leaves. In conclusion existence of a feed back mechanism for photosynthetic acclimation has been proposed. Down-regulation of photosynthetic activity under CO2 enrichment is caused by decreasing leaf N concentration, and reduced rate of transpiration owing to decreased stomatal conductance is partially responsible for poor N translocation. (C) 2009 Published by Elsevier Ireland Ltd.