Long term effects of saline irrigation on the yield and growth of mature Williams pear trees

Salt tolerance of mature Williams Bon Cretien pear trees was assessed in a field trial on a duplex, slowly permeable clay loam. The trees were irrigated with a range of salinities; electrical conductivity of irrigation water (ECw) of 0.2 to 1.4 dS/m by flood for seven years or 0.2 to 2.1 dS/m by microjet sprinklers for nine years. Water-table levels were maintained below 3 m by a groundwater pump. Yield and leaf ion content were assessed during the treatment period. Aspects of growth and physiology were monitored in the 0.2 and 2.1 dS/m microjet treatments during the seventh irrigation season. Soil profile salinities varied between 3.0 and 4.3 dS/m for the most saline flood treatment and from 1.5 to 2.6 dS/m for the most saline microjet treatment. Soil sodicity (sodium absorption ratio) increased during the experiment, reaching a maximum of 9 in the most saline treatments. The salinity treatments caused reduced yields after seven years. In the most saline treatment (ECw=2.1 dS/m, microjet-irrigated), yield decreased to about 60 and 50% of the control in the eighth and ninth years, respectively, and 40% of trees were dead in the ninth year. Leaf ion concentrations (in January) of the most saline treatment were at "excess" levels (>0.1% Cl and >0.02% Na) from 1982 to 1990. There were significant (P<0.01) negative linear relationships between yield in 1990 and leaf Na and Cl, measured both in 1990 and in 1989. During the seventh season of saline irrigation, lateral shoot growth was reduced, leaves and fruit were smaller and leaf fall was earlier in the 2.1 dS/m treatment compared with the control. Dawn and midday water potential and osmotic potential were not significantly affected by saline irrigation. Midday CO2-assimilation rates (A) and leaf conductance to water vapour diffusion (g) were similar for 2.1 dS/m irrigated and control trees, however there was a trend towards a reduction in A and g of these salt-treated trees late in the irrigation season when leaf Na and Cl had increased to 250 and 240 mM (tissue water basis) respectively.