Growth, Photosynthesis, and Nutrient Uptake at Different Light Intensities and Temperatures in Lettuce

Light and temperature are two crucial factors affecting plant growth. Light intensities vary considerably with season and weather conditions. Reasonable light regulation at different temperatures is a key issue in environmental regulation. In this study, we determined the effects of light intensity and temperature on crop growth and development. Furthermore, we determined an optimal light value and a suitable light range at different temperatures for producing the lettuce Lactuca sativa L. Artificial climate chamber experiments were conducted at five light intensities (100, 200, 350, 500, and 600 μmol·m−2·s−1), as well as at low (15 °C/10 °C), medium (23 °C/18 °C), and high (30 °C/25 °C) temperatures. In these experiments, we investigated the photosynthetic rate; chlorophyll fluorescence parameters; total N, P, and K uptake; and growth of lettuce plants. The results indicated that at a low temperature, the values of effective quantum yield of photosystem II photochemistry (ΦPSII), net photosynthetic rate (Pn), stomatal conductance (gS), and transpiration rate (Tr) —as well as those of N, K, and P uptake—were the highest at 350 μmol·m−2·s−1, followed by 500 μmol·m−2·s−1, which resulted in higher values for leaf number (LN), leaf area (LA), dry weight (DW), and fresh weight (FW). At the medium temperature, the values of ΦPSII, Pn, gS, and Tr, as well as those of N, K, and P uptake were higher at 350, 500, and 600 μmol·m−2·s−1 than at other light intensities, resulting in high values for LN, LA, DW, and FW of lettuce plants. The LN, LA, and FW of lettuce plants were the highest at 500 μmol·m−2·s−1, whereas DW was the highest at 600 μmol·m−2·s−1. At a high temperature, lettuce plants exhibited the highest values of Fv/Fm, ΦPSII, Pn, gS, and Tr, as well as those of N, K, and P uptake for the 500 μmol·m−2·s−1 treatment; whereas LN, LA, FW, and DW were the highest at 600 μmol·m−2·s−1. In addition, the values of Fv/Fm indicated that lettuce plants were under stress under the following combinations: 600 μmol·m−2·s−1 at the low temperature, 100 μmol·m−2·s−1 at the medium temperature, and 100–350 μmol·m−2·s−1 at the high temperature. Based on these results, an optimal regulation strategy for light intensity at different temperature environments was proposed for lettuce cultivars similar to L. sativa L. in some regions, such as the subtropical regions of China. Specifically, for low temperatures, light intensities of 350 to 500 μmol·m−2·s−1are recommended for production, and an intensity of 350 μmol·m−2·s−1 provides optimal supplementary light during early spring and winter in greenhouses. For medium temperatures, light intensities of 350 to 600 μmol·m−2·s−1 are recommended, and 500 μmol·m−2·s−1 is the optimal value during the middle of spring and autumn. For high temperatures, light intensities of 500 to 600 μmol·m−2·s−1are recommended, and 600 μmol·m−2·s−1 is the optimal value of light intensity during late spring and early autumn.