Insights into melatonin-induced photosynthetic electron transport under low-temperature stress in cucumber

In this study, the differences in chlorophyll fluorescence transient (OJIP) and modulated 820 nm reflection (MR₈₂₀) of cucumber leaves were probed to demonstrate an insight into the precise influence of melatonin (MT) on cucumber photosystems under low temperature stress. We pre-treated cucumber seedlings with different levels of MT (0, 25, 50, 100, 200, and 400 μmol · L^-1) before imposing low temperature stress (10 °C/6 °C). The results indicated that moderate concentrations of MT had a positive effect on the growth of low temperature-stressed cucumber seedlings. Under low temperature stress conditions, 100 μmol · L^-1 (MT 100) improved the performance of the active photosystem II (PSII) reaction centers (PIabs), the oxygen evolving complex activity (OEC centers) and electron transport between PSII and PSI, mainly by decreasing the L-band, K-band, and G-band, but showed differences with different duration of low temperature stress. In addition, these indicators related to quantum yield and energy flux of PSII regulated by MT indicated that MT (MT 100) effectively protected the electron transport and energy distribution in the photosystem. According to the results of W ≥ 1 and MR₈₂₀ signals, MT also affected PSI activity. MT 100 decreased the minimal value of MR/MR_O and the oxidation rate of plastocyanin (PC) and PSI reaction center (P700) (V ), while increased △MR_slow/MR_O and deoxidation rates of PC⁺ and P₇₀₀ ⁺ (V ). The loss of the slow phase of MT 200 and MT 400-treated plants in the MR₈₂₀ kinetics was due to the complete prevention of electron movement from PSII to re-reduce the PC⁺ and P700 ⁺. These results suggest that appropriate MT concentration (100 μmol · L^-1) can improve the photosynthetic performance of PS II and electron transport from primary quinone electron acceptor (Q_A) to secondary quinone electron acceptor (Q_B), promote the balance of energy distribution, strengthen the connectivity of PSI and PSII, improve the electron flow of PSII via Q_A to PC⁺ and P₇₀₀ ⁺ from reaching PSI by regulating multiple sites of electron transport chain in photosynthesis, and increase the pool size and reduction rates of PSI in low temperature-stressed cucumber plants, All these modifications by MT 100 treatment promoted the photosynthetic electron transfer smoothly, and further restored the cucumber plant growth under low temperature stress. Therefore, we conclude that spraying MT at an appropriate concentration is beneficial for protecting the photosynthetic electron transport chain, while spraying high concentrations of MT has a negative effect on regulating the low temperature tolerance in cucumber.