Effects of elevated atmospheric carbon dioxide concentration and drought on soybean leaf photosynthesis

Soybean (Glycine max (L.) Merr. cv. Jack) was grown in the field in rain-protected plots to study effects of drought and atmospheric CO$\sb2$ enrichment on leaf gas exchange. Midday depressions in net leaf photosynthetic CO$\sb2$ exchange rates (Pn) were found in drought-stressed plants and the diurnal changes were mostly stomatal regulated, although accumulated drought stress eventually resulted in some non-stomatal limitations. However, seasonal changes in Pn were mostly limited by non-stomatal factors. Water use efficiency was always higher for drought stressed plants and depended on the severity of stress and associated stomatal or nonstomatal limitations. At enriched atmospheric CO$\sb2$ levels, stomatal limitations to Pn under drought stress were less important than at ambient atmospheric CO$\sb2$ levels. Morning and afternoon leaf starch levels were enhanced in both irrigated and nonirrigated plants in enriched CO$\sb2$. Afternoon starch levels were higher in stressed leaves than in non-stressed leaves at normal CO$\sb2$ levels.Soybean (Glycine max (L.) Merr. cv. Jack) grown under both controlled environment and field conditions was used to study acclimation of leaf photosynthesis to long-term atmospheric CO$\sb2$ enrichment. Plants grown at 350-400 $\mu$L CO$\sb2$/L air and at 700-800 $\mu$L/L were switched back and forth between treatments to follow dynamic changes in Pn and Gs. Pn of enriched plants was less than that of ambient-grown plants when measured at the ambient CO$\sb2$ level, but it was higher than that of ambient-grown plants when measured at elevated CO$\sb2$ levels, in both controlled environment and field studies. Stomatal conductance (Gs) of leaves on field-grown plants and pot-grown plants responded similarly to changes in air CO$\sb2$ level; although the timing and extent of response differed, the general response pattern was identical. Gs values in plants grown in an enriched CO$\sb2$ environment responded differently to rapid changes in CO$\sb2$ than Gs values in ambient-grown plants; such behavior suggested stomatal acclimation to long-term exposure to an CO$\sb2$-enriched atmosphere.Leaf non-structural carbohydrate levels were greater in plants exposed to long-term atmospheric CO$\sb2$ enrichment, but we could not prove that this was the main cause for lower photosynthetic performance when such plants were placed in ambient air. Pn of CO$\sb2$-enriched plants in ambient air did recover to that of ambient-grown plants after 15 days of exposure to ambient air.Both field-grown and pot-grown plants in controlled environments acclimated similarly to CO$\sb2$-enrichment.U of I OnlyRestricted to the U of I community indefinitely during batch ingest of legacy ETD