Soil phosphorus availability and pearl millet water-use efficiency

Pearl millet (Pennisetum glaucum (L.) R. Br.) production in the West African Sahel is constrained by low, erratic rainfall and low soil nutrient (particularly P) availability. Outdoor pot and growth chamber experiments tested the hypothesis that increasing soil P supply increases transpirational water-use efficiency (WUE(sub T)), under water-stressed and non-water-stressed conditions. Pearl millet was grown outdoors under semiarid conditions in covered pots containing 85 kg of acid, P-deficient Betis sand (sandy, siliceous, thermic Psammentic Paleustalf). Plants were treated with four P levels and two water treatments, and harvested at 14-d intervals. Significant main and interactive effects on WUE(sub T) due to P level, water treatment, and time of harvest were found. The slope of the curve relating DM to cumulative transpiration (T(sub cum)) increased with P level and water stress when data from all harvests were pooled. In the growth chamber, WUE(sub T) of non-water-stressed plants ranged with increasing P level from 3.22 to 9.12 g kg to the -1 at 29 days after sowing (DAS) in pots containing 6 kg soil, and from 0.84 to 9.24 g kg to the -1 at 49 DAS in pots containing 18 kg soil. The ratio of leaf net photosynthetic rate to transpiration (WUE(sub gas)) at 500 micron mol m to the -2 s to the -1 photosynthetic photon flux density (PPFD) ranged from 1.88 micron g mg to the -1 for plants receiving no P to 10.25 micron g mg to the -1 for those receiving 0.310 g P 6 kg to the -1 soil. Between PPFD levels of 500 and 2000 micron mol m to the -2 s to the -1, plants receiving no P increased WUE(sub gas) to only 3.60 micron g mg to the -1, whereas those receiving higher levels of P increased WUE(sub gas) to as much as 18.2 micron g mg to the -1. Our finding that increasing soil P availability increases WUE(sub T) under water-stressed and non-water-stressed conditions reinforces previous conclusions that water supply in the Sahel and similar semiarid environments cannot be effectively managed for improved crop production without addressing soil fertility constraints.