Effect of Salinity on Leaf Gas Exchange in Two Populations of a C 4 Nonhalophyte

Abstract

Gas exchange measurements were made on plants from two natural populations differing in salt tolerance of Axdropogon glomeratus, a C4 nonhalophyte, to examine the effect of salinity on components responsible for differences in photosynthetic capacity. Net CO2 uptake and stomatal conductance decreased with increasing salinity in both populations, but to a greater extent in the inland (nontolerant) population. The intercellular CO2 concentrations increased with increasing salinity in the inland pop-ulation, but decreased in the marsh (tolerant) population. Water use efficiency decreased as salinity increased in the inland population, and remained unchanged in the marsh population. Carboxylation efficiency decreased and CO2 compensation points increased with increasing salinity in both populations, but to a lesser extent in the marsh population. Carboxylation efficiencies were higher with 2% relative to 21% atmos-pheric 02 in salt stressed plants, suggesting that a decrease in the carboxylation:oxygenation ratio of ribulose 1,5-bisphosphate carboxyl-ase/oxygenase was partly responsible for the decrease in photosynthetic capacity. Populational differences in photosynthetic capacity were the result of greater salinity-induced changes in carboxylation efficiency in the inland population, and not due to differences in the stomatal limitation to C02 diffusion. Photosynthetic inhibition due to salinity stress has been de-scribed in both salt-tolerant and salt-sensitive species (2, 3, 5, 15, 19, 22, 23, 28, 29). Decreases in photosynthetic CO2 uptake with increasing salinity have been ascribed to lower stomatal con-ductance (15, 22, 28), a depression of biochemical capacity for carbon uptake and reduction (2, 3, 5, 19, 23), a decrease in photochemical capacity (1), or some combination of these. En-zymatic sensitivity to salinity under uniform conditions is thought to be the same for halophytes, species native to saline habitats, and nonhalophytes, species potentially inhabiting mar-ginally saline areas, but which are more widespread geographi-cally in nonsaline habitats (9, 10, 12, 17, 26). Genetic differen-tiation in the response to salinity between natural populations has been described for a number of species, yet the sensitivity of CO2 uptake to salinity in populations differing in salt tolerance