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 deta ched, exuding root systems from wheat (Triticum aestivum L. cv. Chines e Spring) plants deprived of nitrogen for 5 d (- N) or of phosphorus f or 7 d (- P), were measured and compared with controls receiving a com plete 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 recov ered 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, d ue to the diurnal variation of these parameters. Reducing transpiratio n 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 wh ich had been grown with their roots divided between nutrient-deficient ( - N or - P) solution and a complete nutrient solution. Differences w ere observed in Jv and L(0) between axes of the same plant, but stomat al conductance (Gs), which was also measured, was not affected in thes e split-root experiments. In control plants, Jv and L(0) declined shar ply to values similar to - N and - P plants after HgCl2 treatment (50 mu M), but were restored by treating 5 mM dithiothreitol. In plasma me mbranes from - N and - P roots, the amount of stigmasterol increased r elative to sitosterol compared with control roots. The degree of unsat uration of bound fatty acids also increased, compared with controls, a s a result of a decline in the relative amounts of 16:0 and 18:0 and a n increase in 18:2. Plasma-membrane fluidity, estimated by steady-stat e fluorescence polarisation using 1,6-diphenyl hexatriene, showed that the plasma membranes from nutrient-deprived plants were less fluid th an those from control plants, measured during both the light and dark periods and in split-root experiments. In NRS plants, the relative abu ndance of sitosterol increased, so that the stigmasterol/sitosterol ra tio returned to a value similar to that of controls. However, in PRS p lants, the difference in stigmasterol/sitosterol ratio was maintained, compared with controls. The degree of unsaturation of bound fatty aci ds, membrane fluidity and the hydraulic conductivity of root systems a lso recovered in NRS and PRS plants to values similar to those of cont rol plant plasma membranes. The results obtained suggested that - N an d - P treatment decreased L(0), by reducing either the activity or the abundance of Hg-sensitive water channels. Also, there may be an inter action between the increase in membrane lipid ordering and the decreas e in L(0).