Growth, Yield, and Water Relations of Sorghum Genotypes Differing in Root Architecture Under Deficit Irrigation.

Concern over the impact of agriculture on water resources is increasing in many regions. Crop root system architecture (RSA) offers the potential to use water inputs more effectively and/or efficiently, thereby reducing the need for irrigation. Therefore, the objective of this study was to evaluate the effects of deficit irrigation on growth, yield, water use and water-use efficiency (WUE) in two sorghum [Sorghum bicolor (L.) Moench] genotypes differing in RSA. Deep-rooted Early Hegari (EH) and shallow-rooted BK7 sorghum  genotypes were grown to maturity under six irrigation levels (25%, 50%, 75%,100%, 50/100%, and 100/50% of full transpiration, where irrigation in the latter two was switched at boot stage) in large pots (with a 30-cm diameter and 50-cm height ). The amounts of water use until maturity were 175, 338, 494, 615, 421, and 518 mm under 25, 50, 75,100, 50/100, and 100/50 levels, respectively. Root biomass increased as irrigation decreased, especially in the deep rooted genotype (EH), and root biomass was greater in EH compared to BK7 at low irrigation (25%). Despite differences in RSA, no genotype benefits in grain yield were observed at any irrigation level. In fact, grain yields were negatively related (P < 0.05; R² = 0.66) to root biomass across all data. Taken together, optimal grain yield and water use efficiency for both genotypes was between the 50 and 75 % irrigation levels. The results of this study support the potential for deep rooting to increase access to soil water, which may increase yield under water deficit, but could also result in a tradeoff between partitioning to root and grain that may reduce its benefit for grain yield.