Stomatal Responses of Field-Grown Cotton to Radiation and Soil Moisture
Field research conducted on stomatal responses to water stress has generally focused on sunlit leaves and ignored the role of shaded leaf conductance. This study was carried out on a cotton crop (Gossypium hirsutum, cv. Acala SJ2) to quantify and compare sunlit and shaded stomatal conductance under variable water stress conditions, and to identify the primary environmental parameters regulating the stomatal conductance of sunlit and shaded leaves. Photosynthetically active radiation (PAR), leaf temperature, humidity, and stomatal conductances for both abaxial and adaxial leaf surfaces were measured on cloudless days during two field seasons in 1988 and 1989. Average daytime shaded leaf conductance was approximately 41 and 36% that of sunlit leaves in non-stress and severe water stress conditions, respectively. The primary environmental parameter affecting sunlit leaf conductance was PAR. Shaded leaf conductance was weakly correlated to PAR and leaf temperature, respectively, under non-stress conditions. Under water stress, shaded and sunlit leaf conductances were not correlated to leaf temperature. Leaf conductance was not correlated to humidity under any test condition. The derivative of stomatal conductance with respect to PAR decreased as PAR increased, with most of the change occurring in the PAR range typically obtained in the shaded fraction of the canopy. Increasing moisture stress progressively decreased the stomatal response to PAR. When sunlit and shaded leaves were pooled together, a curvilinear regression improved the correlation with PAR. Abaxial conductance comprised a greater percentage of leaf conductance for shaded than for sunlit leaves.
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