Interactive effects of salinity and boron on the performance and water use of eucalypts

Agroforestry has been promoted as a viable "reuse" method for reducing the volumes of saline drainage effluents in California's San Joaquin valley. The effectiveness of eucalyptus at reducing saline drainage volumes is dependent upon its tolerance to the conditions where they are established over the long term, in particular, high concentrations of salts (up to 28 dS/m) and boron (up to 30 mg/L). Consequently we are currently conducting a three-year study in a large outdoor sand-tank facility at the new US Salinity Lab in Riverside California to determine the potential and limits by which eucalyptus (Eucalyptus camaldulensis, Dehn. Clone 4544) can utilize saline drainage effluent containing variable amounts of boron. Data from trees harvested between 28 April and 1 May indicate that salinity had profound effects on water use, biomass, branch number, leaf area, and canopy size. At low salinity, boron reduced biomass, but this was not evident at higher levels of salinity. Salinity and boron reduced transpiration as measured by sap flow and total water consumption. The older leaves on trees treated with high levels of B (25 and 30 mg/L) but relatively low levels of salinity (ECw 2 and 6 dS/m) showed foliar injury characteristic of B toxicity only two months after exposure to these treatments. Leaf B concentrations support this supposition. Injured tissue was found to contain B in excess of 1,500 mg/kg d.w. Much less injury was found on trees exposed to the same levels of B but in the presence of high salinity (ECw > 10 dS/m). Leaf B concentrations decreased as salinity increased indicating that salinity reduces leaf-B accumulation.