Stomatal responses of pearl millet (Pennisetum americanum [L.] Leeke) to leaf water status and environmental factors in the field

. Factors affecting stomatal conductance (g1) of pearl millet (Pennisetum americanum [L.] Leeke), cultivar BJ 104, were examined in the field in India during the dry season. Diurnal changes in g1 were evaluated for upper expanded leaves at flowering on two occasions using plants subjected to varying degrees of water stress. Except for the most severely stressed treatment, diurnal changes in g1 closely matched changes in irradiance (I), the promotive effect of which largely overcame opposing influences on g1 of increasing atmospheric vapour pressure deficit, and decreasing leaf water and turgor potentials (Ψ, Ψp). Two main effects of water stress on g1 were evident: (i) a decrease in the amplitude of the mid‐day peak in g1, and (ii) a decrease in the time over which high g1 was maintained, resulting in early (mid‐day) closure and hysteresis in the relationship between g1 and I. Leaf conductance was greatest for upper leaves and decreased down the canopy. At equivalent depths in the canopy g1 was higher in flowering than in photoperiodically‐retarded plants of the same age. The magnitude of water stress‐induced stomatal closure increased down the plant, and was more marked in retarded than in flowering plants. Within individual stress treatments Ψ of upper leaves decreased linearly as transpiration flux increased. It is concluded that stomatal behaviour of upper leaves of pearl millet at flowering largely operates to maximize assimilation rather than to minimize water loss.