Severity and duration of osmotic stress on partial root system: effects on root hydraulic conductance and root growth

Effects of severity and duration of osmotic stress on partial root system were investigated on root hydraulic conductance (L (p) ) and growth in each sub-root system. The maize seedlings were raised in nutrient solution with the roots divided equally into two containers. At 12 days after transplanting, half of root system was subjected to osmotic stress of -0.2, -0.4 and -0.6 MPa using PEG 6000 added in the solution, and no PEG 6000 solution was taken as control (CK). L (p) and root growth in each sub-root system were measured at 0, 0.25, 0.5, 1, 3, 5, 7 and 9 days after treatment (DAT). Results show that compared to CK, -0.2 MPa treatment enhanced the hydraulic conductance per root area (L (pr) ) in non-stressed sub-root system by 12 % at 0.5 DAT. In -0.4 MPa treatment, L (pr) in non-stressed sub-root system was significantly increased at 9 DAT. But in -0.6 MPa treatment, significant decrease of 12-40 % was observed in non-stressed sub-root system during 0.5-3 DAT if compared to CK, indicating that the threshold of osmotic stress for the compensatory effect of water uptake in non-stressed sub-root system was between -0.2 and -0.4 MPa. In addition, the root area growth rate in non-stressed sub-root system was significantly higher than that in CK during 0.25-0.5, 1-3 and 5-7 DAT for -0.2, -0.4 and -0.6 MPa treatments, respectively. Similar trend also was observed for root length growth rate, indicating that osmotic stress could still stimulate the compensatory effect of root growth other than water uptake under partial root system. Moreover, the occurrence of such compensatory effect delayed with the increasing osmotic stress. Meanwhile, -0.2 MPa treatment had no significant effect on maize shoot growth but -0.4 and -0.6 MPa treatments inhibited shoot biomass accumulation, which resulted in higher root/shoot ratio in -0.4 and -0.6 MPa treatments. Thus root plasticity in hydraulic conductance and root growth in different root zones varied largely depending on the severity and duration of osmotic stress under partial root system.