Forty-day-old seedlings of Atriplex halimus were treated either with NaCl (50, 300 and 550 mM) for the subsequent 30 days or with 15% PEG for the subsequent 10 days. As much as 50 mM of NaCl significantly increased shoot fresh and dry weight and height; nevertheless, 300 or 550 mM NaCl seemed to have no effect. On the other hand, these growth parameters were not affected by drought after 3 or 6 days, but were reduced after 10 days. The gas exchange parameters (photosynthetic rate, stomatal conductance and transpiration rate) were increased by 50 mM NaCl, but decreased by 300 and 550 mM. These parameters were decreased in response to drought only after 10 days of withholding water. In contrast to Na⁺, K⁺ was significantly decreased by NaCl but not by drought. The time course effect revealed that phosphoenol pyruvate carboxylase (PEPC) protein was doubled in response to NaCl after 1 and 5 h and continued thereafter, higher than control, while drought had no significant effect. Rubisco seemed unchanged by NaCl or drought. It could be concluded that the decrease in fresh weight might be attributed to the decrease in water content. Moreover, the decrease in photosynthesis could result from a decrease in stomatal conductance, a protective mechanism against water loss to improve water use efficiency. These findings indicate that Atriplex halimus tolerates NaCl and drought through decreasing growth, reducing gas exchange parameters to improve water use efficiency, uptake Na⁺ and saving, if any, the photosynthetic enzyme particularly PEPC.
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