DIFFERENCES IN STOMATAL RESPONSE WITHIN A COTTON CANOPY

Potted cotton plants were grown in the open to investigate the causes for differences in the dynamic behavior of stomata of top and bottom leaves of cotton plants. Half of the plants were subjected to a six day long drying cycle. The leaf water potential, abaxial (lower leaf surface) and adaxial (upper leaf surface) stomatal resistance, leaf temperature, initial osmotic potential and bulk modulus of elasticity of top and bottom leaves were studied in detail. Our results show that abaxial stomata of bottom leaves have a threshold leaf water potential of -1.3MPa compared to -2.0MPa for abaxial stomata of top leaves. The threshold leaf water potential value for adaxial stomata was in the range of -1.0MPa for both top and bottom leaves. Leaf-air temperature difference was influenced to a large extent by leaf position in the canopy. Initial osmotic potential was similar for top and bottom leaves. Bulk modulus of elasticity was lower in top leaves and decreased with an increase in the level of leaf water deficit. It is postulated that the lower bulk modulus of elasticity of top leaves is responsible for their lower threshold leaf water potential actuating the stomatal feedback system. The observed differences resulted in a delay of two days in the closure of abaxial stomata of top leaves.