Hydraulic and Non-hydraulic Root-sourced Signals in Old and Modern Spring Wheat Cultivars in a Semiarid Area

Non-hydraulic root-sourced signal (NRS) is so far affirmed to be a unique “early-warning” response to soil drying in plants, but little is known about the quantitative effect of this early-warning mechanism on crop production. To evaluate the link of NRS to a drought tolerance profile, a pot-culture study was carried out in a plant growth chamber with eight spring-wheat (Triticum aestivum L.) cultivars bred in semiarid China. The NRS was judged to begin when there was a significant lowering of stomatal conductance without any change in leaf relative water content (RWC), and the hydraulic root signal (HRS) was judged to begin when leaf RWC changed significantly. Soil water contents (SWC), at which the NRS and HRS were switched on, differed among the eight cultivars. For “Monkhead” and “Jinby,” representing “old” cultivars, the NRS and HRS were initiated successively at about 60% FWC (field water capacity) and 45% FWC, respectively. Conversely, “Longchun8139-2” and “Plateau 602” (recent cultivars) showed the NRS and HRS occurring between 70% FWC and 35% FWC, a much wider range. The events of the other four non-old cultivars were generally intermediate. This threshold range (TR) of soil FWCs between the onset of NRS and HRS also narrowed over the successive developmental stages from seedling to seed filling. Fewer survival days (SD), lower maintenance rate of grain yield (MRGY), and higher lethal leaf water potentials (LLWP) had been found in old cultivars. Widening TR was significantly correlated with increasing SD and MRGY (r = 0.8713** and 0.7318*, respectively), and with decreased LLWP (r = 0.8591**). This survey of different-decade cultivars suggests that advances in grain yield and drought tolerance would be made by targeted selection for a wider TR of root-sourced signals.