Mid-Season water stress on yield and water use of millet (Panicum miliaceum) and sorghum (Sorghum bicolour L. Moench)

In water-limited environments, plant productivity is determined jointly by the amount of water available and the water use or evapotranspiration efficiency (WUE or ETE). Mid-season water deficit can lead to a halving of yield. Some genotypes of Millet (Panicum miliaceum) and of Sorghum [Sorghum bicolor (L.) Moench] with different maturity status (Early; E, Middle; M, and Late; L) were cultivated in sand/nutrient media in a greenhouse. Control plants were maintained at field capacity. The mid-season water stress was imposed at 50% flowering for ten consecutive days, followed by re-irrigation to field capacity until harvest. Yield reductions by water stress averaged 77% in millet and 37% in sorghum and these differences were substantiated when genotypes of the same maturity were compared. This is pointing to a higher stress susceptibility of millet, particularly in M and L genotypes. Evapotranspiration efficiency, ETE, showed genotypic variation in well-watered plants and tended to be higher in sorghum. Evapotranspiration efficiency was generally raised by stress, however, difference between genotypes and crops were almost quenched at maturity. Nevertheless, water use efficiency, WUE differed significantly among genotypes in each crop and each water regime, but changes in rankings of genotypes gave evidence of some interactions with water regimes. In both crops WUE was correlated to its components ETE and HI (harvest index) under well-watered conditions, but only to HI under stress condition. Negative correlations of WUE with CID (carbon-13-isotope discrimination) were weak to poor and some conflicting results with regard to genotypic ranking were observed. These outcomes show the worth of continuous attention to harvest index in breeding for improved water use efficiency in C-4 crops.