Abstract:In order to compare the differences of physiological responses of cucumber seedlings under the Ca(NO3)2 and NaCl stress, the effects of two iso-osmotic salts on growth, photosynthetic characteristics, reactive oxygen level, antioxidant enzymes activity and cations absorption of cv.‘Jinchun No. 2’(salt sensitive cultivar) cucumber seedlings cultivated hydroponically were investigated. The results showed that NaCl inhibited the growth of both shoots and roots of cucumber seedlings, whereas the inhibiting effect by Ca(NO3)2 was mainly on shoots, but not on roots. The results of photosynthetic gas exchange parameters indicate that NaCl stress restricted the entry of CO2 from environment into mesophyll cells resulting in reduction of photosynthetic rate. According to analysis of chlorophyll a fluorescence parameters, the capacity of carbon assimilation of cucumber seedlings was also inhibited by NaCl stress. However, Ca(NO3)2 stress only affected the entry of CO2, but did not significantly affect the capacity of carbon assimilation. The portion of excessive energy in photosystem Ⅱ (PSⅡ) reaction center (Ex) was enhanced by both Ca(NO3)2 and NaCl, which led to an increase of the production rate of superoxide anion radical (O2-·) and content of hydrogen peroxide (H2O2) in cucumber seedling leaves. The increase of Ex, O2-· production rate and H2O2 content by NaCl stress was more than that by Ca(NO3)2. Changes of antioxidant enzymes activity of cucumber seedling leaves were different between Ca(NO3)2 and NaCl stress. For example, the activity of peroxidase (POD) and catalase (CAT) was increased, but the activity of superoxide dismutase (SOD) in leaves was reduced by NaCl stress. However, the activity of antioxidant enzymes such as SOD, POD, CAT was all enhanced by Ca(NO3)2. We speculated that reactive oxygen species (ROS) quenching was limited under NaCl stress, leading to its accumulation and the serious lipid peroxidation. However, ROS was eliminated by antioxidant enzymes under Ca(NO3)2 stress, so the lipid peroxidation damage by Ca(NO3)2 stress was less than that by NaCl. Besides, NaCl stress remarkably increased accumulation of Na+, leading to reduction of ratio of K/Na, Mg/Na and Ca/Na in each plant organ of cucumber seedlings, and disturbing of ionic equilibrium in plants. Ca(NO3)2 increased Ca2+ accumulation in cucumber seedlings, but decreased K+ and Mg2+ content, which reduced ratio of K/Ca and Mg/Ca, and also disturbed ionic equilibrium in plants. However, the injury to cucumber seedlings by excessive Na+ was more severe than by excessive Ca2+. Our results suggested that physiological responses of cucumber seedlings under the iso-osmotic Ca(NO3)2 and NaCl stress were different. NaCl stress increased the accumulation of Na+ in cucumber seedlings, which not only restricted the entry of CO2 from environment into mesophyll cells, but also reduced carbon assimilation. Because of limitation of ROS quenching, the rapid accumulation of ROS duo to the rise of excess energy in PSⅡ reaction center might activate lipid peroxidation damage, and reduce the growth and photosynthetic performances of cucumber seedlings. Carbon assimilation capacity of cucumber seedlings was not significantly affected by Ca(NO3)2, because that ROS could be removed by antioxidant system in a timely manner without causing lipid peroxidation damage and significant reduction of photosynthetic rate as well as growth of cucumber seedling.
