Electron and proton transport in chloroplasts of pea plants after night exposures to chilling temperatures

Plant growth and development slow down when the temperature drops below a critical level (10—12 °C). One of the main mechanisms of plant adaptation to lower temperatures is an increase in the fluidity of the lipid phase of the membranes, which makes it possible to maintain the necessary membrane processes for survival, in particular, the activity of photosynthetic electron transport. In the temperate climate zone, plants often tolerate nighttime temperature drops to frost. Pea ( Pisum sativum L.) refers to cold-resistant species that can withstand a prolonged decrease in temperature. In this work, pea plants were grown at a constant temperature of 20—22 °C for 11 days, then for 6 days at night, the plants were placed in chambers at a temperature of 6 °C. The temperature dependence of the photochemical activity in chloroplasts isolated from control and chilled leaves was studied. It was shown that the maximum value of the rate of uncoupled electron transport from water to potassium ferricyanide in control chloroplasts was observed at a temperature of 22 °C, i.e. at the temperature of plant growth. In the chloroplasts of plants subjected to night cooling, the temperature dependence of the uncoupled electron transport was shifted to lower temperatures and the maximum reaction rate was recorded at a temperature of about 12 °C. When measuring the value of light-induced proton uptake ( Δ Í + ) and the rate of O 2 uptake in the reaction of electron transfer from H 2 O to methyl viologen in coupled chloroplasts, a sharp decrease in these parameters was observed with a change in the temperature of the reaction medium from 14 to 10 °C in the control, and from 10 to 6 °C in the experimental plants. The sharp decrease in the rate of photochemical reactions with decreasing temperature may be due to a phase transition of membrane lipids and a slowing down of diffuse processes. These results allow us to consider the Δ Í + as an indicator of the fluidity of the lipid phase of chloroplast membranes in comparative studies. The transmembrane proton gradient ( Δ ðÍ), which was estimated by light-dependent quenching of the fluorescent label of 9-aminoacridine, exceeded the control level in chloroplasts of plants subjected to night cooling, which may be due to an improvement in the insulating function of the membranes. The data obtained indicate that when pea plants adapt to lower night temperatures, the state of the photosynthetic chloroplast membranes changes, which ensures the preservation of their functional activity. dependence of uncoupled electron transport on temperature of the reaction media: 1 — control, chloroplasts were isolated from control plants grown at 20—22 °C; 2 — experiment, chloroplasts were isolated from experimental plants grown at day/night air temperature of 20—22/6 °C. The rate of the light-induced H 2 O → K 3 Fe(CN) 6 reaction was determined after 5 min dark incubation of chloroplasts in the reaction medium under excitation at all indicated temperature values. The reaction media contained 200 mM sorbitol, 2.5 mM MgCl 2 , 10 mM NaCl, 10 mM KCl, 0.5 mM of HEPES (pH 7.0), 1 mM tricine-NaOH, 5 mM NH 4 Cl, 0.5 µM gramicidine, 1 mM K 3 Fe(CN) 6 , and chloroplasts (0.01 mg Chl/ml) (initial pH was 7.7). The photochemical activity of chloroplasts was induced by a white light of a KGM-250 halogen lamp (PPFD 500 μ mol/(m 2 · s)