Effects of water and nitrogen coupling on watermelon growth, photosynthesis and yield under CO2 enrichment

The accurate and efficient management of water and nitrogen is essential for the effective intensive production of greenhouse watermelon. In order to determine the optimal application of water and nitrogen to promote the growth and yield of watermelon under the increasing CO2 concentration in the future, we conducted an experiment with 12 treatments over two growing seasons in 2019 and 2020 to evaluate the effects of water and nitrogen coupling on the growth, photosynthesis, and yield of watermelon under carbon dioxide (CO2) enrichment. The treatments included three irrigation levels, I1 (80% evaporation (Ep)), I2 (100% Ep), and I3 (120% Ep); two nitrogen levels, N1 (644.04 kg/ha) and N2 (1288.09 kg/ha); and two CO2 concentrations, C1 (400 ppm) and C2 (800 ppm). The experimental results show that except for intercellular CO2 concentration (Ci) and chlorophyll, irrigation exhibited a promoting effect on other indicators in two years, and yield was most strongly improved with increased irrigation. The interaction of water and nitrogen dramatically affected the net photosynthetic rate (Pn), transpiration rate (Tr) and Ci in two years. Elevated CO2 concentration alleviated the negative effect of low nitrogen on the number of leaves, dry matter accumulation, Pn, and Ci, with the biggest improvement on dry matter accumulation, changes of 35.7% and 17.9% for two years. Meanwhile, elevated CO2 concentration also promoted Pn under low-irrigation, with increases of 36.9% and 31.1% in two years. Eight indicators of growth and photosynthesis were used to evaluate comprehensive growth based on Technique for order performance by similarity to ideal solution (TOPSIS), and dry matter accumulation achieved the largest combined weight of 0.241 and 0.232, followed by Pn. A positive correlation was found between comprehensive growth and yield. Different treatments were effective for overall promotion of growth and yield, with 115–120% Ep irrigation, 976.07–1288.09 kg/ha nitrogen fertilizer optimal for watermelon at 400 ppm CO2 concentration and 114–120% Ep irrigation, 664.04–913.06 kg/ha nitrogen fertilizer best for 800 ppm CO2. These results suggest for the expected future higher CO2 concentrations, greenhouse watermelon production should reduce application of nitrogen fertilizers to save resources, but maintain full irrigation.