Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO2 Levels

Abstract

The effects of water stress and CO2 enrichment on photosynthesis, assimilate export and sucrose-P synthase [SPS] activity were examined in field grown soybean plants. In general, leaves of plants grown in CO2-enriched atmospheres (300 .mu.l/l above unenriched control, which was 349 .+-. 12 .mu.l/l between 0500 and 1900 h over the entire season) had higher C exchange rates (CER) compared to plants grown at ambient CO2, but similar rates of export and similar activities of sucrose-P synthase. On most sample dates, essentially all of the extra C fixed as a result of CO2 enrichment was partitioned into starch. CO2-enriched plants had lower transpiration rates and therefore had a higher water use efficiency (milligrams CO2 fixed per gram H2O transpired) per unit leaf area compared to nonenriched plants. Water stress reduced CER in nonenriched plants to a greater extent than in CO2-enriched plants. As CER declined, stomatal resistance increased, but this was not the primary cause of the decrease in assimilation because internal CO2 concentration remained relatively constant. Export of assimilates was less affected by water stress than was CER. When CER were low as a result of the imposed stress, export was supported by mobilization of reserves (mainly starch). Export rate and leaf sucrose concentration were related in a curvilinear manner. When sucrose concentration was above about 12 mg/dm2, obtained with nonstressed plants at high CO2, there was no significant increase in export rate. Assimilate export rate was also correlated positively with SPS activity and the quantitative relationship varied with CER. Thus, export rate was a function of both CER and C partitioning.