LIGHT, TEMPERATURE AND SALINITY EFFECTS ON GROWTH, LEAF ANATOMY AND PHOTOSYNTHESIS OF DISTICHLIS SPICATA (L.) GREENE

The effects of light, temperature, and salinity on growth, net CO2 exchange and leaf anatomy of Distichlis spicata were investigated in controlled environment chambers. When plants were grown at low light, growth rates were significantly reduced by high substrate salinity or low temperature. However, when plants were grown at high light, growth rates were not significantly affected by temperature or salinity. The capacity for high light to overcome depressed growth at high salinity cannot be explained completely by rates of net photosynthesis, since high salinity caused decreases in net photosynthesis at all environmental conditions. This salinity-induced decrease in net photosynthesis was caused largely by stomatal closure, although plants grown at low temperature and low light showed significant increases in internal leaf resistance to CO2 exchange. Increased salinity resulted in generally thicker leaves with lower stomatal density but no significant differences in the ratio of mesophyll cell surface area to leaf area. Salinity and light during growth did not significantly affect rates of dark respiration. The mechanisms by which Distichlis spicata tolerates salt appear to be closely coulpled to the utilization of light energy. Salt-induced leaf succulence is of questionable importance to gas exchange at high salinity in this C4 species. NUMEROUS STUDIES have dealt with the effects of substrate salinity on physiology, morphology, and growth of plants (see Bernstein and Hayward, 1958; Poljakoff-Mayber and Gale, 1975; Flowers, Troke, and Yeo, 1977; Longstreth and Nobel, 1979; Waisel, 1972). The conclusions drawn from these studies have been quite variable, due both to interspecific differences in responses to salinity and to differential responses resulting from interaction of salinity with other environmental variables not standardized among the studies. For example, humidity (Gale, Naaman, and Poljakoff-Mayber, 1970), irradiance (Longstreth and Strain, 1977; Mallot et al., 1975), temperature (Mallott et al., 1975) and nitrogen levels (Haines and Dunn, 1976) can all affect plant responses to salinity. One widely reported response to salinity is an increase in leaf thickness (Jennings, 1976; Longstreth and Nobel, 1979; PoljakoffMayber, 1975; Waisel, 1972). This salt-induced succulence has been ascribed to osmoregulation in plants subjected to salinity (Jennings, 1976). Increased succulence appears to have beneficial effects on CO2 exchange by increasI Received for publication 24 March 1980; revision accepted 14 August 1980. The work reported here was partially supported by the State of New Mexico and the U.S. Department of Interior, Office of Water Research and Technology under project B-058-N MEX. ing the internal surface area per unit leaf surface over which CO2 diffusion can occur. This apparently lowers the internal leaf resistance to CO2 uptake (Jennings, 1976; Longstreth and Nobel, 1979). Most of the work demonstrating salt-induced succulence and a resulting enhancement of gas exchange has been with C3 plants (Black, 1958; Poljakoff-Mayber, 1975; Longstreth and Nobel, 1979). Many C4 plants however are also tolerant of high salinity (Duncan, 1974) and the same phenomenon has been implied in at least one C4 species (Longstreth and Strain, 1977). Because the photosynthetic physiology and leaf anatomy of C4 plants are much more functionally interrelated than in C3 plants, it might be expected that physiological and anatomical responses to salinity of C4 plants would differ from responses of C3 plants. However, the physiological and anatomical responses of salt-tolerant C4 plants to salinity are not well known. A better knowledge of these responses will improve our understanding of the mechanisms and ecology of salt tolerance in this widely distributed group of plants. Such information also has implications with regard to selecting and breeding salt-tolerant C4 crop plants. The widespread salt-tolerant C4 grass, Distichlis spicata (L.) Greene was chosen for study because its occurrence in a diversity of inland (Ungar, 1974) and coastal (Duncan, 1974) saline habitats implies both a high degree of salinity tolerance and a broad