Ontogeny modifies the effects of water stress on stomatal control, leaf area duration and biomass partitioning of Pennisetum glaucum

Experiments are presented that test the relative importance, during ontogen y, of stomatal control and leaf area expansion to optimum seasonal water us e in pearl millet (Pennisetum glaucum). These parameters play a key role in the compromise between plant growth and water saving under unpredictable c onditions of semiarid environments. The response of growth and water use of crops to successive 15 d drought pe riods was measured under field conditions in Niger (West Africa). From emergence to anthesis, biomass partitioning to stems and panicles depe nded strongly on leaf area development. Water use was linearly related to g reen leaf area duration in well watered plots, but was reduced proportional ly more than green leaf area in drought-affected plots. The relations of cr op growth rate and transpiration efficiency to leaf area depended on ontoge netic changes in biomass partitioning. In P. glaucum, stomata play a dominant role in reducing crop water use unde r preanthesis drought, although this control becomes negligible after anthe sis because of ontogenetic decline in the range of stomatal conductance. Th e rate of leaf senescence after anthesis is not drought-dependent.