Responses of wheat plants to nutrient deprivation may involve the regulation of water-channel function

The sap flow (Jv) and the osmotic hydraulic conductance (L(0)) of detached, exuding root systems from wheat (Triticum aestivum L. cv. Chinese Spring) plants deprived of nitrogen for 5 d (- N) or of phosphorus for 7 d (- P), were measured and compared with controls receiving a complete nutrient supply. In the roots of - N and - P plants, Jv and L(0) decreased markedly, but between 4 and 24 h after resupplying N to - N plants (NRS plants) and P to - P plants (PRS plants), Jv and Lo recovered to values similar to those of control plants. Values of Jv and L(o) were always greater during the light period than during the dark, due to the diurnal variation of these parameters. Reducing transpiration in the light had no effect on Jv and L(0) of - N and - P plants. Sap flow and Lo were also determined using individual axes from plants which had been grown with their roots divided between nutrient-deficient( - N or - P) solution and a complete nutrient solution. Differences were observed in Jv and L(0) between axes of the same plant, but stomatal conductance (Gs), which was also measured, was not affected in these split-root experiments. In control plants, Jv and L(0) declined sharply to values similar to - N and - P plants after HgCl2 treatment (50 mu M), but were restored by treating 5 mM dithiothreitol. In plasma membranes from - N and - P roots, the amount of stigmasterol increased relative to sitosterol compared with control roots. The degree of unsaturation of bound fatty acids also increased, compared with controls, as a result of a decline in the relative amounts of 16:0 and 18:0 and an increase in 18:2. Plasma-membrane fluidity, estimated by steady-state fluorescence polarisation using 1,6-diphenyl hexatriene, showed that the plasma membranes from nutrient-deprived plants were less fluid than those from control plants, measured during both the light and dark periods and in split-root experiments. In NRS plants, the relative abundance of sitosterol increased, so that the stigmasterol/sitosterol ratio returned to a value similar to that of controls. However, in PRS plants, the difference in stigmasterol/sitosterol ratio was maintained, compared with controls. The degree of unsaturation of bound fatty acids, membrane fluidity and the hydraulic conductivity of root systems also recovered in NRS and PRS plants to values similar to those of control plant plasma membranes. The results obtained suggested that - N and - P treatment decreased L(0), by reducing either the activity or the abundance of Hg-sensitive water channels. Also, there may be an interaction between the increase in membrane lipid ordering and the decrease in L(0).