OFFSETTING EFFECTS OF REDUCED ROOT HYDRAULIC CONDUCTIVITY AND OSMOTICADJUSTMENT FOLLOWING DROUGHT

Root hydraulic conductivity (L(p)) and leaf osmotic potential at full turgor (Psi(pi,o)) were measured in young, drought-stressed and nonstr essed peach (Prunus persica (L.) Batsch), olive (Olea europaea L.), ci trumelo (Poncirus trifoliata Raf. x Citrus paradisi Macf.) and pistach io (Pistachia integerrima L.). Drought stress caused a 2.5- to 4.2-fol d reduction in L(p), depending on species, but Psi(pi,o) was reduced o nly in citrumelo and olive leaves by 0.34 and 1.4 MPa, respectively. N o differences existed in L(p) among species for nonstressed plants. A simple model linking L(p) to osmotic adjustment through leaf water pot ential (Psi) quantified the offsetting effects of reduced L(p) and osm otic adjustment on the hypothetical turgor pressure difference between drought-stressed and nonstressed plants (Delta Psi(p)). For olive, th e 2.5-fold reduction in L(p) caused a linear decrease in Delta Psi(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 h igher turgor in drought-stressed olive plants than in nonstressed plan ts over their typical diurnal range of Psi (-0.6 to -2.0 MPa). For cit rumelo, osmotic adjustment was offset by reduced L(p) at Psi = -0.9 MP a. Unlike olive, stomatal closure would be necessary to maintain highe r turgor in drought-stressed citrumelo plants than in nonstressed plan ts over their typical diurnal range of Psi (0 to -1.5 MPa). Regardless of species or the magnitude of osmotic adjustment, my analysis sugges ts that a drought-induced reduction in L(p) reduces or eliminates turg or maintenance through osmotic adjustment.