Growth response and plant water status in forage sorghum [Sorghum bicolor (L.) Moench] cultivars subjected to decreasing levels of soil moisture

A pot experiment was conducted to determine the response of two recent forage sorghum cultivars (JS-2002 and Chakwal Sorghum) and an old one (JS-263) to three levels of soil moisture (30%, 50% and 70% field capacity). Several traits were assessed addressing plant morphology, functional growth, leaf water status, biomass yield and water use efficiency (WUE). Soil moisture variation greatly affected all traits, while cultivars significantly differed in the response to drought. At low moisture the three genotypes showed similar net assimilation rate, specific leaf area, root and shoot dry weight. Conversely, at high moisture JS-2002 exhibited a higher potential than Chakwal Sorghum, in turn passing JS-263. As it concerns plant height, leaf area, leaf water potential (LWP) and relative water content (RWC), the three cultivars consistently behaved across moisture levels maintaining the same ranking between best (JS-2002) and worst performer (JS-263). Especially in LWP and RWC the gap between JS-263 and JS-2002 (LWP, -1.84 vs. -1.55 MPa; RWC, 71 vs. 78% in the two respective cultivars) points out the old genotype inadequacy to face drought. WUE outlined an increasing difference between most (S-2002) and least efficient cultivar (JS-263) at rising moisture. JS-263 also showed a higher yield response factor to water supply, meaning a stronger yield decrease under water deficit. The resilience to drought shown by recent varieties (JS-2002 and Chakwal Sorghum) is a good premise for their use in areas subjected to dry spells. Further research at field plot scale is nevertheless needed to assess actual gains in varying moisture conditions.