Drought performance of millet (Panicum miliaceum) and sorghum (Sorghum bicolor (L.) Moench)

In water-limited environments, plant productivity is determined jointly by the amount of water available and the phenological, morphological, and physiological strategies employed to cope and/or acclimatise. Mid-season water deficit, which is frequent in semi arid regions, can lead to a halving of yield. Three genotypes representing three different maturity (early, middle, and late) groups of each crop were cultivated in sand/nutrient media in a greenhouse. Control plants were maintained at field capacity and mid-season water stress was imposed at 50% flowering at soil water potentials of –200hPa and –400hPa for ten consecutive days. This was followed by re-irrigation to field capacity until harvest. The objectives of this experiments were: (1) To determine if there were any differences in the physiological and morphological response of both crops to mid-season water stress; (2) To verify any differences between the maturity groups of each crop in response to varying mid-season water stress. And (3) to identify selection parameter(s) and the role(s) they may play in sustaining yield under drought conditions. Drought stress effect on assimilation and transpiration rates increased with stress intensity, and was stronger in the late maturity of both crops. However, maturity differences in photosynthetic rates did not reflect differences in yields. In the late millet, basal tillers became a liability to yield with increased stress intensity; producing fewer fertile panicles and acting as additional sinks. The early and late maturity of millet and sorghum respectively ceased reproductive (panicle) development under stress, a strategy to avoid kernel abortion. High stem pre-anthesis assimilate storage and delayed leaf senescence (particularly in the late sorghum) contributed positively to yield by respectively remobilising and providing assimilates and photosynthetate for grain filling during and after mid-season water deficit. Water use efficiency was reduced as stress level increased; with stronger reductions observed in millet than sorghum. Carbon Isotope Discrimination (CID) was higher in millet than sorghum and increased with maturity groups, but it was not affected by water stress. Yield under stress was positively associated to CID in both crops. There was a strong significant negative correlation between CID and WUE within each maturity group of each crop. However, the use of CID to select for WUE was not quite feasible within crop and between crops. Harvest index was seemingly higher in the early maturity, and showed significant positive correlations to WUE irrespective of treatments or stress intensity in both crops. General analyses of the relative reductions in assessed parameters by water stress showed sorghum to be more drought resistant than proso millet.