Responses to short-term reductions in light in soybean leaves: Effects of leaf position and drought stress'

This study examined how leaf position in the canopy affected photosynthetic and stomatal responses to short-periods during a drought cycle in soybean (Glvciize max [L.] Merr.). All soybean leaves had similar basic responses to short-termterm. minutes-long shade periods during a drought cycle in soybean (Glycine max [L.] Merr.). All soybean leaves had similar basic responses to short-term shade, including rapidly decreased photosynthetic rates (A(co2)) slower decreases in stomatal conductance (g(s)), and delayed stomatal reopening and photosynthetic recovery after leaves were reilluminated. Drought stress lowered overall A(co2) and restricted photosynthetic and stomatal responses to short-term shade with the negative effects of drought being stronger in lower than in upper leaves. Some of the negative effects of drought persisted after drought was relieved, causing reduced overall water use efficiency, especially in lower leaves. These results indicate that leaf position effects on stomatal responses to short-term shade events become important during and after stress.