Effects of different concentrations of dissolved oxygen or temperatures on the growth, photosynthesis, yield and quality of lettuce

With the continuous improvement of living standards, people pursue vegetables of higher quality, and increasing the yield and quality of vegetables has become the goal of many researchers. To accurately study the effects of different dissolved oxygen concentrations (6.5, 7.5, and 8.5 mg L−1) in the irrigation water or climatic chamber temperatures (16, 18, and 20 °C) on the growth, photosynthesis, yield and quality of lettuce (Lactuca sativa L. var. Grand Rapids Tbr), a comparative design based on a three-color light climate chamber was used. An increase in the dissolved oxygen concentration and the temperature had significant effects on the plant height, transpiration rate (E), soluble protein and nitrate contents (P < 0.05), net photosynthetic rate (A), stomatal conductance (gsw), intercellular carbon dioxide concentration (Ci), leaf temperature (Tleaf), vitamin C content, yield and irrigation water use efficiency (IWUE) of lettuce (P < 0.01). Compared with a dissolved oxygen concentration of 6.5 mg L−1 (O1), the plant height, leaf area index (LAI), chlorophyll content, A, dry matter accumulation (Dm), vitamin C content, soluble protein content, yield, IWUE and water use efficiency (WUE) of O3 (i.e., higher dissolved oxygen content) increased by 22.86%, 35.69%, 12.09%, 163.95%, 14.70%, 39.90%, 77.21%, 32.79%, 32.79%, and 5.84% (P < 0.05), respectively, while the nitrate content decreased by 14.37% (P < 0.05). Compared with a climate chamber temperature of 16 °C (T1), the plant height, LAI, chlorophyll, A, Dm, vitamin C mass fraction, yield, IWUE and WUE of T3 (i.e., higher temperature) increased by 18.33% (P < 0.05), 24.09% (P> 0.05), 9.79% (P < 0.05), 114.89% (P < 0.05), 14.70% (P < 0.05), 55.05%, 38.05%, 17%, 38.17% and 23.5% (P < 0.05), respectively, and decreased the nitrate and soluble protein contents by 72.73% and 11.66%, respectively (P < 0.05). Increasing the dissolved oxygen concentration in the irrigation water or the climate chamber temperature enhanced the growth, photosynthetic ability, and WUE of lettuce, thus increasing the Dm, yield, and nutritional value and improving the quality and flavor of lettuce. Regarding dissolved oxygen, at an irrigation quota of 48 mm, a fertilization rate of 1.30 g pot−1, and a climate chamber temperature of 18 °C, the lettuce yield and quality were optimal at a dissolved oxygen concentration of 8.5 mg L−1. Regarding temperature, at an irrigation quota of 48 mm, a fertilization rate of 1.30 g pot−1, and a dissolved oxygen concentration of 7.5 mg L−1, the yield and quality of lettuce in the climate chamber were optimal when the climate chamber temperature was 20 °C. These results provide conditions viable for increased yield and quality of lettuce.