Differential Physiological and Growth Responses of Lettuce Seedlings to Extreme Thermal Stress Analyzed through Chlorophyll Fluorescence

This study sought to monitor changes in growth parameters, chlorophyll fluorescence (CF) levels, and proline and chlorophyll contents in lettuce seedlings grown under different temperature conditions. Ten-day-old lettuce seedlings grown under a fluorescent lamp in a closed plant production system having a constant temperature (24/18°C), humidity of 60 ± 3%, and a light period of 14/10 h (day/night) were kept at 24/18°C for three days in a closed chamber under white warm LED lighting at 220 ± 10 µmol∙m^-2∙s^-1. The seedlings were then grown under different temperature conditions [low: 8/4°C (day/night), control: 24/20°C, and high: 40/36°C] in a closed and controlled plant production system for eight days. The CF parameters, proline amount, and chlorophyll content were measured at 0, 2, 4, 6, and 8 d after the initiation of the treatment, while the growth parameters (fresh weight, dry weight, leaf length, and hypocotyl length) were measured at the end of the experiment. The quantum yield of nonregulated energy dissipation in PSII [Y(NO)] increased on the second day and then decreased at low temperatures. Non-photochemical quenching (NPQ) and the fluorescence decrease ratio (Rfd) significantly decreased under both high-and low-temperature stress compared to those in the control. Growth parameters were retarded at low temperatures, whereas leaves became dry at high temperatures. The chlorophyll content decreased significantly, whereas the proline content increased only at low temperatures. These results indicate that Y(NO), NPQ, and Rfd can be used to detect temperature stress in lettuce seedlings.