Differentiate effects of non-hydraulic and hydraulic root signaling on yield and water use efficiency in diploid and tetraploid wheat under drought stress

Non-hydraulic root signals (nHRS) and hydraulic root signals (HRS) are important mechanisms that play a major role in determining plant productivity under drought stress. In the present study, six wheat varieties (three diploid and three tetraploid) were used to investigate the variation of yield, its components, water use efficiency (WUE) and biomass allocation under two signals. At the jointing stage, three water treatments were set up: i) well-watered control (WW, 80 % field water capacity (FC)), ii) moderate water stress (MS, 50 % FC) and iii) severe water stress (SS, 30 % FC). The results showed that around 50 % FC, wheat activated the nHRS to respond to drought. While around 30 % FC, wheat activated the HRS as a response to deeper drought. Under the operation of nHRS, the reduction of yield, aboveground biomass and WUE of diploid species was greater than that of tetraploid ones, but there were no differences under the operation of HRS. It could be argued that two different wheat species followed differentiate positive defense strategy in response to nHRS, but fell into a similar negative adaptive strategy under the HRS. The ploidy level of wheat also had a significant effect on wheat yield composition, WUE and biomass allocation model. As such, grain yield, grain number and spike weight per plant, aboveground biomass, harvest index (HI) and WUE were significantly greater in the tetraploid species than the diploids ones. In addition, the retention rate of aboveground biomass and grain decreased under the operation of nHRS and HRS. Path analysis showed that spikelet number, thousand grain weight and grain number were the main factors affecting yield formation. Grain number had the greatest direct effect on yield, while spikelet number and its grain number exerted the main effect on yield. These results were consistent under both signals.