Impact of late-season drought on water relations in a sparse canopy of millet (Pennisetum glaucum (L) R Br)

Although millet is known to be adapted to very dry conditions, little is known about its canopy water relations. The control of water loss and dehydration tolerance were studied during the grain filling for two cultivars presumed resistant and sensitive to moisture deficit at this stage. Two experiments were conducted in the dry hot season at Niamey, using crops based on the traditional system of hill sowing at low density: a preliminary trial with low leaf area (index of 0.7 at flowering); and a main trial with higher leaf area (index of 1.8) induced by tillering where parameters were recorded at a short time scale. The preliminary trial with small initial leaf area revealed a less apparent leaf water deficit according to leaf water potential and stomatal regulation but the main results were in agreement with those of the main trial. The cultivars were similar in their water relations. During drought, water losses declined quickly due essentially to a large decrease in the green leaf area through senescence. The potential effect of stomatal regulation was reduced by a natural decline of 50% after earing. The water deficit was relatively small and osmotic adjustment was absent in the upper leaves of eared shoots, which stayed green. These results reveal for millet, that rapid control of leaf area by senescence is the predominant mechanism at this stage, inducing long-term avoidance of dehydration of the upper leaves on eared shoots. The quick adjustment of canopy conductance to the reduction of soil water availability is an hypothesis advanced.