Effect of short-term salinity on photosynthesis and ion relations in two sugar beet cultivars.

A factorial pot experiment was conducted based on randomized complete blocks design in green house condition to investigate the effect of short-term salt stress on leaf photosynthesis and ion relations of two sugar beet cultivars (Madison and 7233-P29). Plants were exposed to 0, 50, 150, 250, and 350 mM salinity (NaCl and CaCl2 in 5:1 molar ratio) for 48 hours. Measurements were done on youngest fully expanded leaves. Salinity caused a significant reduction in stomatal conductance and consequently leaf net photosynthesis in both cultivars. Net photosynthesis of cv 7233-P29 at low level of salinity (50 mM) at 12 h, 24 h and 48 hours after salinity application decreased by 7.4%, 15.1% and 20%, respectively, compared to control. However, reduction in photosynthesis at highest level of salinity (350 mM) at the same times were 62.4%, 73%, and 72.3%, respectively, compared to control.  For Madison the percent of reduction was significantly higher than P29. Cultivar P29 at 12 and 24 hours after salinization had significantly higher Na+ content than Madison up to 250 mM. Potassium concentration tended to decrease with increase in salinity. Salinity led to increased Cl- concentration in both cultivars. However, Madison had significantly higher Cl- content than P29 at the highest level of salinity at all three times of sampling. A strong correlation between photosynthesis and stomatal conductance (R2=0.98) and weak correlation between photosynthesis and sodium concentration (R2= 0.22), and photosynthesis and chloride concentration (R2=0.55) indicated that in short-term exposure to salinity, stomatal conductance was one of the main causes of photosynthesis.