Detection of temperature stress using chlorophyll fluorescence parameters and stress-related chlorophyll and proline content in paprika (Capsicum annuum L.) seedlings

Thirty three-day-old paprika (Capsicum annuum L.) seedlings were grown under different temperature conditions (low: 10°C, moderate: 25°C, and high: 35°C) in a closed plant production system for 32 days and their chlorophyll (Chl) fluorescence and growth parameters, and Chl and proline contents were measured at 0, 1, 2, 4, 8, 16, 24, and 32 days after the initiation of treatment. Minimal fluorescence (F₀) sharply increased from 8 days and continued until the end of the experimental period under all three temperature treatments, with the highest increase at the low temperature condition. Maximum quantum yield (F_v/F_m) and the efficiency of excitation capture of open photosystem II (PSII) center (F’_v/F’_m) significantly decreased at low temperature compared with those at moderate and high temperatures. Non-photochemical quenching (NPQ) and the ratio of the fluorescence decrease (Rfd) were significantly affected, particularly at the high temperature, followed by the low and moderate temperatures; quantum yield of non-regulated energy dissipation in PSII (ϕNO) increased under all treatments. Furthermore, Chl content showed a relatively greater decrease at the low temperature compared to the high temperature throughout the experiment; moderate temperature showed a stable chlorophyll content throughout the experiment. Proline concentration increased significantly at the high and low temperatures, but not under moderate temperature. Plant height and shoot and root weight were the lowest at the low temperature. Overall, our results suggest that paprika plants were more severely affected by low temperature than high temperature with respect to photosynthetic activity as well as growth, which was significantly slowed at low temperature.