Effects of salinity and short-term elevated atmospheric CO2 on the chemical equilibrium between CO2 fixation and photosynthetic electron transport of Stevia rebaudiana Bertoni.

The effect of water salinity on plant growth and photosynthetic traits of Stevia rebaudiana was investigated to determine its level and mechanisms of salinity tolerance. It was also attempted to assess how short-term elevated CO2 concentration would influence the boundaries and mechanisms of its photosynthetic capacity. The plants were grown in gravel/hydroponic system under controlled greenhouse conditions and irrigated with four different salinity levels (0, 25, 50 and 100 mol m-3NaCl). Low salinity did not significantly alter the plant fresh weight, which was substantially decreased by 67% at high salinity treatment. Salinity tolerance threshold was reached at 50 mol m-3 NaCl while C50 was between 50 and 100 mol m-3 NaCl, indicating that S. rebaudiana is a moderate salt tolerant species. Salt-induced growth reduction was apparently linked to a significant decline of about 47% in the photosynthetic rates (Anet) at high salinity treatment, leading consequently to a disequilibrium between CO2-assimilation and electron transport rates (indicated by enhanced ETRmax/Agross ratio). Elevated atmospheric CO2 enhanced CO2 assimilation rates by 65% and 80% for control and high-salt-stressed plants respectively, likely due to significant increases in intercellular CO2 concentration (indicated by enhanced Ci/Ca). The priority for Stevia under elevated atmospheric CO2 was not to save water but to maximize photosynthesis so that the PWUE was progressively improved and the threat of oxidative stress was diminished (decline in ETRmax/Agross). The results imply that elevated CO2 level could ameliorate some of the detrimental effects of salinity, conferring higher tolerance and survival of S. rebaudiana, a highlydesired feature with the forthcoming era of global changes.

Elevated atmospheric CO2 concentration enhances salinity tolerance in Aster tripolium L.

Our study aimed at investigating the influence of elevated atmospheric CO(2) concentration on the salinity tolerance of the cash crop halophyte Aster tripolium L., thereby focussing on protein expression and enzyme activities. The plants were grown in hydroponics using a nutrient solution with or without addition of NaCl (75% seawater salinity), under ambient (380 ppm) and elevated (520 ppm) CO(2). Under ambient CO(2) concentration enhanced expressions and activities of the antioxidant enzymes superoxide dismutase, ascorbate peroxidase, and glutathione-S-transferase in the salt-treatments were recorded as a reaction to oxidative stress. Elevated CO(2) led to significantly higher enzyme expressions and activities in the salt-treatments, so that reactive oxygen species could be detoxified more effectively. Furthermore, the expression of a protective heat shock protein (class 20) increased under salinity and was even further enhanced under elevated CO(2) concentration. Additional energy had to be provided for the mechanisms mentioned above, which was indicated by the increased expression of a beta ATPase subunit and higher v-, p- and f-ATPase activities under salinity. The higher ATPase expression and activities also enable a more efficient ion transport and compartmentation for the maintenance of ion homeostasis. We conclude that elevated CO(2) concentration is able to improve the survival of A. tripolium under salinity because more energy is provided for the synthesis and enhanced activity of enzymes and proteins which enable a more efficient ROS detoxification and ion compartmentation/transport.

Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L.

This study investigated the interaction of NaCl-salinity and elevated atmospheric CO(2) concentration on gas exchange, leaf pigment composition, and leaf ultrastructure of the potential cash crop halophyte Aster tripolium. The plants were irrigated with five different salinity levels (0, 25, 50, 75, 100% seawater salinity) under ambient and elevated (520 ppm) CO(2). Under saline conditions (ambient CO(2)) stomatal and mesophyll resistance increased, leading to a significant decrease in photosynthesis and water use efficiency (WUE) and to an increase in oxidative stress. The latter was indicated by dilations of the thylakoid membranes and an increase in superoxide dismutase (SOD) activity. Oxidative stress could be counteracted by thicker epidermal cell walls of the leaves, a thicker cuticle, a reduced chlorophyll content, an increase in the chlorophyll a/b ratio and a transient decline of the photosynthetic efficiency. Elevated CO(2) led to a significant increase in photosynthesis and WUE. The improved water and energy supply was used to increase the investment in mechanisms reducing water loss and oxidative stress (thicker cell walls and cuticles, a higher chlorophyll and carotenoid content, higher SOD activity), resulting in more intact thylakoids. As these mechanisms can improve survival under salinity, A. tripolium seems to be a promising cash crop halophyte which can help in desalinizing and reclaiming degraded land.