Effects of High Temperature on Chlorophyll Fluorescence Induction and the Kinetics of Far Red Radiation-Induced relaxation of apparent F0 in maize leaves

Temperature dependence (25–50 °C) of chlorophyll (Chl) fluorescence induction, far-red radiation (FR)-induced relaxation of the post-irradiation transient increase in apparent F₀, and the trans-thylakoid proton gradients (pH) was examined in maize leaves. Temperatures above 30 °C caused an elevation of F₀ level and an enhancement of F₀ quenching during actinic irradiation. Millisecond delayed light emission (ms-DLE), which reflects the magnitude of pH, decreased strikingly above 35 °C, and almost disappeared at 50 °C. It indicates that the heat-enhanced quenching of F₀ under actinic irradiation could not be attributed mainly to the mechanism of pH-dependent quenching. The relaxation of the post-irradiation transient increase in apparent F₀ upon FR irradiation could be decomposed into two exponential components (₁ = 0.7–1.8 s, ₂ = 2.0–9.9 s). Decay times of both components increased with temperature increasing from 25 to 40–45 °C. The bi-phasic kinetics of FR-induced relaxation of the post-irradiation transient increase in apparent F₀ and its temperature dependence may be related to plastoquinone (PQ) compartmentation in the thylakoid membranes and its re-organisation at elevated temperature.