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Mongolian pine introduced to a semiarid area of northern China is very important for ensuring ecological and environmental security. However, the main natural conditions of water shortage and calcium enrichment in this area limit the growth of introduced Mongolian pine, and the effect of calcium-water coupling on its growth is still unclear. In this study, twenty treatments with combinations of four soil water conditions (e.g., 20%, 40%, 60% and 80% of the field water capacity) and five calcium concentration gradients (e.g., 0, 50, 100, 200, and 400 mg·kg−1) were conducted to investigate this effect by measuring the indices of growth, biomass, photosynthetic pigment, gas exchange rate and photosynthates, chlorophyll fluorescence parameters, water use efficiency (iWUE) and antioxidant enzyme activity based on a pot experiment. The results showed that calcium-water coupling significantly affected the growth, photosynthetic characteristics and antioxidant enzyme activities of Mongolian pine seedlings. These indices of the Mongolian pine seedlings reached their maximum values at a calcium concentration of 100 mg·kg−1 under no drought stress. Moreover, the indices of growth status, biomass, net photosynthetic rate, transpiration rate, stomatal conductance, chlorophyll content, chlorophyll fluorescence parameters, soluble sugar, starch and antioxidant enzyme activities in the Mongolian pine seedlings increased with increasing water gradients under the same calcium concentration, while the iWUE values exhibited an opposite response. Under the same drought stress, the growth, photosynthetic characteristic and antioxidant enzyme activity indices of the Mongolian pine seedlings first increased and then decreased with increasing calcium concentrations and exhibited positive effects at calcium concentrations of 50–200 mg·kg−1. When the calcium concentration was 400 mg·kg−1, the physiological indices of the Mongolian pine seedlings under both drought stress gradients were significantly reduced, which indicated that the growth of the Mongolian pine seedlings was seriously affected by excessive exogenous calcium levels. |
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