Abstract:
Non-hydraulic root-sourced signal (nHRS) is so far affirmed to be a unique and positive ‘early-warning’ response of plant to drying soil, but its functional role and potential evolutionary implication is little known in dryland wheat. Three spring wheat cultivars, Monkhead (1940-1960s), Dingxi 24 (1970-1980s) and Longchun 8139 (1990-present) with different drought sensitivity were chosen as materials for the research. Physiological and agronomic parameters were measured and analyzed in two relatively separated but closely related trials under environment-controlled conditions. The results showed that characteristics of nHRS and its eco-physiological effects varied from cultivars. Threshold ranges (TR) of soil moisture at which nHRS was switched on and off were 60.1-51.4% (% of FWC) in Monkhead, 63.8-47.3% in Dingxi 24 and 66.5- 44.8% in Longchun 8139 respectively, suggesting that earlier onset of nHRS took place in modern cultivars. Leaf abscisic acid (ABA) concentration was significantly greater and increased more rapidly in old cultivars, Monkhead and Dingxi 24 than that of Longchun 8139 during the operation of nHRS. As a result of nHRS regulation, maintenance rate of grain yield was 43.4%, 60.8% and 79.3%, and water use efficiency was 1.47, 1.65 and 2.25 g/L in Monkhead, Dingxi 24 and Longchun 8139 respectively. In addition, drought susceptibility indices were 0.8858, 0.6037 and 0.3182 for the three cultivars, respectively. This suggests that earlier trigger of nHRS led to lower ABA-led signal intensity and better drought adaptability. It can be argued that the advances in yield performance and drought tolerance might be made by targeted selection for an earlier onset of nHRS. Finally, we attempted developing a conceptual model regarding root-sourced signal weakening and its evolutionary cue in dryland wheat.
