Abstract: The objective of the study was to investigate the effects of calcium concentration on photosynthesis，chlorophyll fluorescence，pollen germination and fruit quality of watermelon under salt stress. Mini-watermelon（Citrullus lanatus Mansfeld） cultivar‘Xiuli’was used to carry out the experiment by supplementing exogenous calcium to nutrient solution containing 100 mmol · L-1 NaCl. The results showed that the stomatal conductance（Gs），intercellular carbon dioxide concentration（Ci），net photosynthesis rate（Pn），PSⅡphotochemical quantum yield（Fv/Fm），PSⅡactual photochemical efficiency（ΦPSII），relative electron transport rate（rETR），photochemical quenching（qP），non-photochemical quenching（qN）were significantly increased when calcium concentration in salty nutrient solution increased from 4 to 6 mmol · L-1 and stomatal limitation（Ls），photoinhibition（1–qP/qN）were decreased in salt-stressed seedlings. Fruit weight and the contents of ascorbic acid，soluble solids，soluble sugar，soluble protein and free amino acid in salt-stressed fruit flesh were remarkably increased by supplemental calcium from 4 to 6 mmol · L-1. However，organic acid content was decreased. The contents of Ca2+，Mg2+，Fe，Cu，and Mn were increased but Na+ was reduced in salt-stressed fruit flesh. Besides，supplemental calcium obviously promoted pollen germination and pollen tube elongation. These results indicated that supplemental calcium could effectively reduce the salt-induced stomatal limitation of photosynthesis and alleviate salt stress damage on photosynthetic apparatus. This was conducive to maintain photosynthetic performance and promote growth of salt-stressed watermelon. Meanwhile，the reproductive growth and mineral nutrition balance in fruit were improved. It could be concluded that supplemental calcium enhanced the tolerance of watermelon plants to salt stress by regulating photosynthetic and reproductive metabolisms of plants under salt stress.

Key words: Citrullus lanatus Mansfeld, salt stress, calcium, photosynthetic performance, fruit, quality