Effects of phosphorus and water supply on yield, transpirational water-useefficiency, and carbon isotope discrimination of pearl millet

Several studies have identified low soil P and water availability as major constraints to pearl millet Pennisetum glaucum (L.) R. Br.] production in s emi-arid West Africa. To evaluate the effects of phosphate and water supply on yield, transpirational water-use efficiency (WUEI), and carbon-isotope discrimination (Delta), two varieties of pearl millet were cultivated in po ts in a glasshouse at the ICRISAT Sahelian Centre, near Niamey, Niger. Phos phate and water supply had significant effects on yield, WUEI, and Delta. C ompared with the control plants, which had adequate water and P availabilit y, yield was reduced 34% by low crater supply and 48% by low P supply. Unde r high P-supply, water stress increased WUEI by approximately 37%. Under lo w P-supply, no effect of water supply on WUEI was observed, Water stress in creased Delta by approximately 0.6 parts per thousand for low P plants, and 0.9 parts per thousand for high P plants. Added P increased Delta by 0.3 t o 0.4 parts per thousand. WUEI and Delta did not differ significantly betwe en varieties. Differences in Delta between green and necrotic leaves were f ound within both P treatments under low water supply. We attribute changes in Delta to changes in the ratio of external to internal concentration of C O2, (p(i)/p(a)), leakage rates of CO2 out of bundle-sheath cells, respirati on rates. or chemical composition of the plant material.