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

Abstract

The sap flow (Jv) and the osmotic hydraulic conductance (L₀) 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 L₀ 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 L₀ 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 L₀ of — N and — P plants. Sap flow and L₀ 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 L₀ 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 L₀ declined sharply to values similar to those of roots from — N and — P plants after HgCl₂ treatment (50 μM), but were restored by treating with 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 L₀, 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 L₀.