Effect of water stress on growth, water use efficiency and gas exchange as related to osmotic adjustment of two halophytes Atriplex spp

Atriplex halimus L. is known in the Mediterranean basin and along the coastal areas of Sardinia for its
adaptability to salinity, although less information is available on the resistance of this species to water stress in absence of
salinity. The effect of water stress on growth and water utilisation was investigated in two Atriplex species: A. halimus
originating of south Sardinian island and the exotic species Atriplex nummularia Lindl., originating in Australia and widely
used in land restoration of arid areas. Water stress was applied to young plants growing in 20 L pots with a sufficient water
reserve to store a potentially sufficient water reserve to maintain substrate near to field capacity (30%) between irrigations.
Watering was at 70% (control) or 40% (stress) of field capacity. In order to simulate the grazing by livestock, four plant
biomass cuttings were conducted at times T0, T1, T2 and T3, corresponding to one cutting at the end of well watered phase
(T0) before water stress induction, two cuttings after cycles of 5 weeks each during full summer (T1) and late summer (T2)
and one cutting during autumn (T3). All plants remained alive until the end of treatment although growth was strongly
reduced. Leaf dry weight (DW) and water use efficiency (WUE) were determined for all cuttings; relative water content
(RWC), turgid weight : dry weight ratio(TW : DW),water potential (Yw), osmotic potential (Ys),CO2 assimilation, osmotic
adjustment (OA), abscisic acid (ABA) and sugar accumulation were determined for the late summer cutting at T2. Water
stress induced a decrease in DW, RWC, Yw, Ys, TW:DW and CO2 assimilation for both species, but an increase in WUE
expressed in terms of dry matter production and a high accumulation of ABA and total sugars mainly for A. halimus. This
suggests a more developed adaptive mechanism in this selection. Indeed, the clone was selected from the southern part of the
island, where natural populations of saltbush are more exposed to abiotic stresses, mainly the water stress generated not by
salinity. A. nummularia showed a greaterOAand a positive net solute accumulation as than A. halimus, suggesting that water
stress resistance in A. halimus is linked to a higher WUE rather than a greater osmotic adjustment.