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Growth and transpirational water use efficiency of pearl millet in response to water and phosphorus supply
In the Sahel, pearl millet (Pennisetum americanum (L.) Schum) is cultivated on sandy soils low in water holding capacity and available nutrients, particularly phosphorus. Because rainfall tends to be low and erratic, soil management must be designed to use available water as efficiently as possible to improve food supply. Towards this end, the influence of soil water and phosphorus supply upon growth and transpirational water use efficiency (WUE(T)) of pearl millet was investigated. Studies reported include 1) a lysimeter/growth analysis experiment using four phosphorus and two water treatments, 2) determination of transpiration and WUE(T) under the conditions stated above, and 3) a growth chamber experiment in which gas exchange characteristics, WUE(T), and starch and chlorophyll contents were determined as a function of phosphorus application in non-water stressed millet plants. Results indicate 1) phosphorus supply strongly influences amount and distribution of millet biomass under water stressed and non-water stressed conditions; however, relative growth rate and net assimilation rate are relatively unaffected by water and phosphorus supply; 2) increasing phosphorus increases WUE(T) under both water stressed and non-water stressed conditions. Water stress also increases WUE(T), which is not constant over time; and 4) the ratio of photosynthesis to transpiration drops considerably during phosphorus stress, thus offering a physiological explanation for lowered WUE(T). Additionally, chlorophyll and starch content are increased. Results offer agronomic and physiological evidence that water conservation in the Sahel will be a relatively ineffective means of stabilizing and raising millet yields without first addressing soil fertility constraints.