Photosynthetic Responses and Salt Tolerance of Seedlings of Four Shrub Species Used for Revegetation in the Qaidam Basin Under Short-Term NaCl Stress

Selecting shrubs that maintain photosynthesis under salinity is essential for revegetating saline land in the Qaidam Basin, northwestern China. Potted seedlings of Nitraria tangutorum, Tamarix chinensis, Haloxylon ammodendron and Potentilla fruticosa were irrigated with 0, 100, 200, 400 or 600 mmol L−1 NaCl for 30 days, and leaf gas exchange (diurnal courses, temporal changes and light responses), growth, leaf water status and salt injury were measured. NaCl significantly reduced the net photosynthetic rate (Pn), stomatal conductance and transpiration rate of all species (repeated-measures ANOVA, p < 0.001), increasingly with salt concentration and treatment duration. After 30 days at 600 mmol L−1, Pn was 61.2%, 72.2%, 79.9% and 92.1% lower than in the control in T. chinensis, N. tangutorum, H. ammodendron and P. fruticosa, respectively (p < 0.05). Early declines were accompanied by a lower intercellular CO2 concentration and a higher stomatal limitation value, whereas after 25–30 days the stomatal limitation value of P. fruticosa and H. ammodendron under high salinity fell below that of the control, a pattern consistent with an increasing contribution of non-stomatal limitation. Salinity decreased the maximum net photosynthetic rate and apparent quantum yield and increased the light compensation point. N. tangutorum retained the highest absolute photosynthetic rates, whereas T. chinensis showed the smallest relative decline. A membership-function index ranked the short-term salt tolerance of the seedlings as T. chinensis > N. tangutorum > H. ammodendron > P. fruticosa; this ranking was reproduced by principal component analysis and in 92–100% of the alternative indicator weightings, indicator subsets and uncertainty simulations tested, with P. fruticosa ranking last in all cases. These results support T. chinensis and N. tangutorum as priority species for revegetating saline land, pending field validation.