Calcium Can Prevent Toxic Effects of Na+ on Tomato Leaf Photosynthesis but Does Not Restore Growth

The availability of good-quality irrigation water is decreasing worldwide, and salinity is an increasingly important agricultural problem. To determine whether detrimental effects of NaCl on plant growth and leaf physiology can be minimized by additional Ca 2+ supply, tomato ( Solanum lycopersicum L.) ‘Supersweet 100’ was grown hydroponically. The basic nutrient solution contained 11.1 m m NO 3 − and 2.8 m m Ca 2+ . Three levels of NaCl (14.1, 44.4, and 70.4 m m ) were added to the basic solution to determine Na + effects on leaf physiology and growth. To determine if Ca 2+ could alleviate the toxic effects of Na + , treatments with 10 or 20 m m Ca 2+ combined with 44.4 or 70 m m NaCl were included as well. To distinguish between osmotic and ion-specific effects, there were three treatments in which all nutrient concentrations were increased (without NaCl) to obtain electrical conductivity (EC) levels similar to those of the NaCl treatments. Nutrient solutions with 70.4 m m NaCl reduced leaf photosynthesis, chlorophyll content, gas-phase conductance, carboxylation efficiency, and dark-adapted quantum yield. Inclusion of 20 m m Ca 2+ prevented these effects of NaCl. NaCl also decreased leaf length and elongation rate. This could not be prevented by adding extra Ca 2+ to the solution; reductions in leaf elongation were due to osmotic effects rather than to Na + specifically. Likewise, plant dry weight was negatively correlated with solution EC, suggesting an osmotic effect. Leaf area development apparently was more important for dry matter accumulation than leaf photosynthesis. Adding 20 m m Ca 2+ to the 70 m m NaCl solution reduced the Na + concentration in the leaf from 79 to 24 mg·g −1 .