Offsetting effects of reduced root hydraulic conductivity and osmotic adjustment following drought.

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Root hydraulic conductivity (L(p)) and leaf osmotic potential at full turgor (Psi(pi,o)) were measured in young, drought-stressed and nonstressed peach (Prunus persica (L.) Batsch), olive (Olea europaea L.), citrumelo (Poncirus trifoliata Raf. x Citrus paradisi Macf.) and pistachio (Pistachia integerrima L.). Drought stress caused a 2.5- to 4.2-fold reduction in L(p), depending on species, but Psi(pi,o) was reduced only in citrumelo and olive leaves by 0.34 and 1.4 MPa, respectively. No differences existed in L(p) among species for nonstressed plants. A simple model linking L(p) to osmotic adjustment through leaf water potential (Psi) quantified the offsetting effects of reduced L(p) and osmotic adjustment on the hypothetical turgor pressure difference between drought-stressed and nonstressed plants (DeltaPsi(p)). For olive, the 2.5-fold reduction in L(p) caused a linear decrease in DeltaPsi(p) such that the effect of osmotic adjustment was totally negated at Psi = -3.2 MPa. Thus, no stomatal closure would be required to maintain higher turgor in drought-stressed olive plants than in nonstressed plants over their typical diurnal range of Psi (-0.6 to -2.0 MPa). For citrumelo, osmotic adjustment was offset by reduced L(p) at Psi approximately -0.9 MPa. Unlike olive, stomatal closure would be necessary to maintain higher turgor in drought-stressed citrumelo plants than in nonstressed plants over their typical diurnal range of Psi (0 to -1.5 MPa). Regardless of species or the magnitude of osmotic adjustment, my analysis suggests that a drought-induced reduction in L(p) reduces or eliminates turgor maintenance through osmotic adjustment.