Abstract: This experiment was designed to investigate the factors affecting the daily change of photosynthesis in the salt-tolerant leguminous plant Alhagi pseudoalhagi by comparing with salt-sensitive Vigna radiata. The plants were cultivated in a greenhouse and subjected to 0 (controlled), 50, 100 and 200 mmol·L^-1 NaCl treatments for 2 weeks. The daily change of photosynthesis, chlorophyll content and expression of Rubisco large subunit gene were measured. The results showed that the curves of controlled plants of both A. pseudoalhagi and V. radiata were unimodal, while the response of photosynthetic daily change with increasing salinity was different between the salt-sensitive plant V. radiata and the salt-tolerant one A. pseudoalhagi. For A. pseudoalhagi, the rates of Pn in 50-200 mmol·L^-1 NaCl-treated plants increased faster than that in the untreated control plants, and there was no significant difference among other treatments in the max peak of Pn values. The increase of photosynthetic rate of V. radiata was faster in the controlled plants than that in salinity treatments, and salt-treated plants had a lower photosynthetic rate than the untreated plants. There was no midday depression for A. pseudoalhagi in 50-100 mmol·L^-1 and V. radiata in 50 mmol·L^-1 NaCl, which was observed in A. pseudoalhagi 200 mmol·L^-1 treatment and in V. radiata 100 mmol·L^-1 treatment. In A. pseudoalhagi, the ‘midday depression’ was mainly caused by decreased Gs under severe salt stress, while in V. radiata, the contribution of the inhibition of Rubisco to the Pn ‘midday depression’ was more significant than that of the Gs. In V. radiata, the expression of rbcL was decreased with increasing salt stress, and in A. pseudoalhagi, the expression of rbcL was increased at 50 and 100 mmol·L^-1, however, it was decreased at 200 mmol·L^-1 NaCl.

Key words: Alhagi pseudoalhagi, Vigna radiata, salt stress, photosynthetic daily change, photosynthetic midday depression, chlorophyll