Factors affecting CO2 assimilation, leaf injury and growth in salt-stressed durum wheat

To examine the factors that affect tolerance to high internal salt concentrations, two tetraploid wheat genotypes that differ in the degree of salt-induced leaf injury (Wollaroi and Line 455) were grown in 150 mM NaCl for 4 weeks. Shoot biomass of both genotypes was substantially reduced by salinity, but genotypic differences appeared only after 3 weeks, when durum cultivar Wollaroi showed greater leaf injury and a greater reduction in biomass than Line 455. Ion accumulation, water relations, chlorophyll fluorescence and gas exchange were followed on one leaf (leaf 3) throughout its life. Salinity caused a large decrease in stomatal conductance (gs) of both genotypes. This was not due to poor water relations, as leaf turgor of both genotypes was higher in the salt treatment than in controls, so chemical signals were likely to have caused the decrease in gs. Reductions in assimilation rate were initially due to gs and, with time, were due to a combination of stomatal and non-stomatal limitations. The non-stomatal limitations were associated with a build up of Na⁺ above 250 mM. The efficiency of PSII photochemistry in Line 455 was unaffected throughout. However, in Wollaroi, the potential and actual quantum yield of PSII photochemistry began to decline as the leaf aged and the thermal energy dissipation of excess light energy (NPQ) increased. This coincided with high Na⁺ and Cl^- concentrations in the leaf and with chlorophyll degradation, indicating that these later reductions in CO2 assimilation in Wollaroi were a consequence of a direct toxic ion effect. The earlier reduction in CO2 assimilation and greater leaf injury explain why growth of Wollaroi was less than Line 455. The most sensitive indicator of salinity stress was gs, followed by CO2 assimilation, with fluorescence parameters other than NPQ being no more sensitive than chlorophyll itself.