Salinity, Stomatal Responses and Whole-Tree Hydraulic Conductivity of Orchard 'Washington Navel' Orange, Citrus sinensis (L.) Osbeck

Abstract

Diurnal patterns in stomatal conductance and bulk water potentials were measured over a 6-month period for spring flush leaves on 24-year-old Washington Navel orange trees that had been irrigated with water containing either 5 mol m^-3 NaCl or 20 mol m^-3 NaCl for 5 years prior to measurements.

During summer and autumn, at early morning measurement times, stomatal conductances of leaves on trees irrigated with 20 mol m^-3 were significantly below those on trees irrigated with 5 mol m^-3 NaCl. Lower values on high salinity trees were not attributable to more negative water potentials or lower turgor pressures but were apparently due to an inability of stomata on leaves from salinised trees to open in response to low vapour pressure deficits (VPDs). There was little effect of salinity on stomatal conductances during afternoon measurements when high vapour pressure deficits prevailed. Laboratory studies confirmed that stomata on salinised trees are less responsive to VPD than those from unsalinised trees.

When measurements were made during winter months there was no effect of salinity on diurnal patterns of stomatal conductances but leaf water potentials were less negative for leaves of salinised trees during daylight hours.

Hydraulic conductance (G) of trees to liquid water flow was greater for trees irrigated with 5 mol m^-3 NaCl in summer, but seasonal reductions in G for trees irrigated with 5 mol m^-3 NaCl occurred to a far greater extent than for trees irrigated with 20 mol m^-3 NaCl. This may have been a consequence of a reduction in leaf areas of salinised trees during summer and autumn without concomitant decreases in root length.