Yield, transpiration efficiency, and water-use variations and their interrelationships in the sorghum reference collection

Sorghum is well adapted to water-limited conditions, but the traits responsible for this enhanced adaptation under drought conditions remain unclear. In this study, yield, transpiration efficiency (TE) and water extraction were assessed in 149 germplasm entries from the sorghum reference set (plus three control cultivars) using a lysimetric system under terminal water stress and fully irrigated conditions outdoors. A 10-fold range for grain yield and harvest index (HI), 2-fold range for TE and a 1.25-fold variation for water extraction were observed under terminal water stress conditions. Transpiration efficiency and water extraction under water stress related poorly to that under fully irrigated conditions, reflecting a large genotype-by-water treatment interaction. Under drought stress, total water extraction varied by similar to 3 L plant(-1) among germplasm. Entries from the Durra race had highest water extraction capacity, whereas Caudatum-Bicolor and Caudatum-Durra intermediate races had poor water extraction. Durra, Caudatum and Caudatum-Guinea races had highest TE, whereas the Guinea race had the lowest. Although yield was closely related to HI, at any level of HI there were substantial yield differences that remained unexplained, and these residual yield variations were closely related to TE (R-2 = 0.60). Similarly, substantial yield variations that were still not explained by HI or TE were closely related to the total water extracted under water stress (R-2 = 0.35). Amultilinear regression analysis confirmed these results and showed the importance of water extraction during grain filling. Therefore, next to HI, the yield differences under terminal drought in sorghum were driven by TE, and then next by water extraction. The large genetic variation for TE and water extraction offer great breeding opportunities and in particular, highlight the Durra race as a critical source of variation.