Physiological And Biochemical Responses Of Spring Wheat To Drought Stress Under Non-hydraulic Root-sourced And Hydraulic Root Signals

In this study, five different spring wheat cultivars were planted in pots to measure the physiological and biochemistry reaction which include antioxidant enzymes activities,Malonaldehyde (MDA), solutble sugar (WSS) and abscisic acid (ABA) content under non-hydraulic root-sourced and hydraulic root signals to drought stress, and in order to study the relationship with their feature of root-sourced signals. Non-hydraulic root-sourced signal (nHRS) is generally referred to as the significant decrease in stomatal conductance without obvious change in leaf water status. It is the early-warning response made by plants at the beginning of drought stress in soil. Afterwards, further decrease in soil water content significantly would lead to a decrease in leaf water status, and further lower stomatal conductance, which is traditionally defined as hydraulic root signal (HRS).(Experiment in 2005) Five wheats, which were MO1, DM22, HST, GY602 and LC8275, were applied to undergo gradual drought stress involving in non-hydraulic root-sourced signals (nHRS) and hydraulic root signals (HRS). At the same time, the change of antioxidant enzymes activity, MDA and WSS content were measured to find the relationship with their non-hydraulic root-sourced signal threshold ranges.(Experiment in 2006) Three wheat cultivars which included HST, GY602 and LC8275 were planted in pots to measure their non-hydraulic root-sourced signal threshold ranges basing on the change of their stomatal conductance and leaf water potential under soil gradual drought condition. The endogenesis abscisic acid (ABA) content in leaf and root, the MDA content in root, the WSS content in leaf, the yield and water use efficiency (WUE) were masured under non-hydraulic and hydraulic signals to drought stress.The results show that:1. The non-hydraulic root-sourced signal threshold ranges: Period 1, LC8275 (0.49Mpa) > GY602 (0.47Mpa) > HST (0.41Mpa) > DM22 (0.28Mpa) > MO1 (0.16Mpa); Period 2, LC8275 (0.47Mpa) > GY602 (0.43Mpa) > HST (0.37Mpa) > DM22 (0.23Mpa) > MO1 (0.12Mpa). The non-hydraulic root-sourced signal threshold ranges of all the cultivar are narrower in Period2 than in Period 1.2. The MDA content in all cultivars expect HST in Period2 measured under non-hydraulic root-sourced signals (nHRS) did not have remarkably change compared to the contrast (CK); but remarkably higher than CK under hydraulic root signals (HRS). It shows that: non-hydraulic root-sourced signal (nHRS) is the early-warning response made by plants at the beginning of drought stress in soil and at this stage plants can keep themself from suffering drought stress.3. Compared with the other three cultivars DM22 and MO1 had a later initiation of the non-hydraulic root-sourced signal and a earlier initiation of hydraulic root signal, and they begin their early-warning response system later and lost more water and suffered more damage made by drought stress so they had higher MDA content than the other three cultivars.4. The grain yield under well water condition and drought stress were: LC8275 > GY602 > HST > DM22 > MO1; the percentage of the reduction of the grain yield under drought stress was: DM22 (54%) > MO1 (50%) > HST (45%) > GY602 (35%) > LC8275 (27%). The reduction of the grain yield were remarkably negatively correlated with the non-hydraulic signal threshold range, R=-0.859*.5. The cultivars of DM22 and MO1 had higher WSS content under nHRS, LC8275, GY602 and HST had a lower WSS content than MO1 and DM22 under nHRS and HRS.6. The cultivars of MO1 and DM22 had a higher SOD activities under non-hydraulic root-sourced signal (nHRS) and hydraulic root signal (HRS) while SOD activities did not have a obviously change in LC8275. LC8275, GY602 and HST had a higher CAT activities and HST had a higher POD activities than other cultivars. It shows that: using one or two indexes to estimate the plant's ability of resist a drought is unilateral, so we combines the physiological-biochemistry reaction and the root-sourced signal threshold ranges to study the adaptive mechanism in response to drought stress. The results of experiment 2006 show that:1. The contents of ABA in all the three spring wheat cultivars under non-hydraulic signals (nHRS) have increased significantly compared to the contrast. The contents of ABA in leaf were remarkably negatively correlated with the stomatal conductance, R=-0.85; The contents of ABA in roots were remarkably negatively correlated the soil water potential, R=-0.86. The contents of ABA in roots were remarkably positively with the contents of ABA in leaf, LC8275: R=-0.816*, GY602: R=-0.711, HST: R=-0.828*. It shows that ABA responses to soil drought stress rapidely and causes the stomatal conductance reduction to decrease water transpiration.2. The ABA content in HST is remarkably higher than the other two cultivars under nHRS. LC8275 had a lower ABA content and earlier initiation of non-hydraulic root-sourced signal at the beginning of drought stress. It shows that LC8275 is more sensitivity to ABA accumulation, and low ABA concentration can bring the initiation of non-hydraulic root-sourced signal to LC8275. HST is dull to the soil drought stress until ABA concentration arising higher; it initiated the non-hydraulic root-sourced signal, when it is under HRS it also had a higher ABA concentration than LC8275 and GY602. At this time ABA played as an effete signal.3. The contents of MDA in LC8275 and GY602 under non-hydraulic signals (nHRS) have no obvious change compared to the contrast, while in HST the MDA contents increased evidently. HST has a higher MDA content all the time.4. The contents of WSS in all three spring wheat cultivars under non-hydraulic signals (nHRS) have no obvious change compared to the contrast. HST has a highest WSS level under nHRS and HRS while LC8275 has the lowest WSS concentration.5. The grain yield of the all three spring wheat cultivars under drought stress were remarkably decreased compared to the contrast. Under well water condition, GY602 has the highest grain yield, while under MS (mild stress) and SS (severe stress) LC8275 has the highest grain yield and HST has the lowest grain yield all the time. The WUE under MS (mild stress) of all three cultivars are higher than it under CK and SS.LC8275 has the obvious higher WUE than HST under MS, HST has the lowest WUE.