RESPONSES OF SOYBEAN LEAF PHOTOSYNTHESIS TO CO2 AND DROUGHT

Soybean [Glycine max (L.) Merr. cv. Jack] was grown in the field in ra in-protected plots to study effects of drought and atmospheric CO2 enr ichment. on leaf gas exchange. Midday depressions in leaf photosynthet ic CO2 exchange rates (P(N)) were found in drought-stressed plants and the diurnal changes were mostly stomatal-regulated, although accumula ted drought stress eventually resulted in some non-stomatal limitation s. However, seasonal changes in P(N) were mostly limited by non-stomat al factors. Water use efficiency was always higher for drought stresse d plants and depended on the severity of stress and associated stomata l or nonstomatal limitations. At enriched atmospheric CO2 levels, stom atal limitations to P(N) under drought stress were less important than at ambient atmospheric CO2 levels. Morning and afternoon leaf starch levels were enhanced in both irrigated and nonirrigated plants in enri ched CO2. Afternoon starch levels were higher in stressed leaves than in non-stressed leaves at normal CO2 levels.