Abstract
To gain insights into mechanisms of salt tolerance we studied the effect of shading during salnity stress using well-fertilized 2-year-old potted 'valencia' orange trees on either Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive) rootstocks grown in Candler fine sand. We wanted to determine if shading could reduce the negative effects of salinity stress. Trees were grown either under 50% shadecloth or left unshaded in full sun light. Half of the trees in sun and shade were salinized with 50mm Cl- during two 9-week salinity periods in the spring and fall, interrupted by our normal 11-week summer rainy period, while the other half recieved no salinity treatment. The shade treatment generally reduced midday leaf termpature and evaportive demand while the salinity treatment reduced growth. In non-salinized trees, the shade effect increase midday photosynthesis and stomatal conductance but not leaf transpiration.Shade also increased growth of trees on Cleo. Shade decreased Cl- concentrations in leaves of salinized CArr trees shaded than for sun-exposed trees. Shaded trees on both rootstocks had higher leaf Na+ than sun-exposed trees after the first salinity period and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Although shading reduced Cl- accumulation in 'Valencia' on Carr, shadinng did not alleviate the negative effects of salinity on growth and Na+ accumulation in trees on either rootstock.
