Effects of Light Chilling on Photophosphorylation in Cucumber

Summary

The response of in situ photophosphorylation in attached cucumber (Cucumis sativus L.) leaves to chilling (5°C) under strong illumination (1000 µE·m^‒2·1) was investigated. A single-beam kinetic spectrophotometer fitted with a clamp-on, whole leaf cuvette was used to measure the flash-induced electrochromic absorbance change at 518–540 nm (ΔΔ_518–540) in attached leaves. The relaxation kinetics of the electric field-indicating ΔΔ_518–540 measure the rate of depolarization of the thylakoid membrane. Since this depolarization process is normally dominated by proton efflux through the coupling factor during ATP synthesis, this technique can be used, in conjunction with careful controls, as a monitor of in situ ATP formation competence.

Whole, attached leaves were chilled at 5°C and 1000 µEm^‒2·s^‒1 for up to 6 h, then rewarmed in the dark and 100% R.H. at room temperature for 30 min. Leaf water potential, chlorophyll content, and the effective optical path length for the absorption measurements were not affected by the treatment. Light-and CO₂-saturated leaf disc oxygen evolution and the quantum efficiency of photosynthesis were inhibited by approx. 50% after 3 h of chilling and by approx. 75% after 6 h. Despite the large inhibition to net photosynthesis, the measurements of ΔΔ_518–540 relaxation kinetics showed photophosphorylation to be largely unaffected by the chilling and light exposure. Our measurements showed that the chilling of whole leaves in the light caused neither an uncoupling of photophosphorylation from photosynthetic electron transport nor any irreversible inhibition of the chloroplast coupling factor in situ. The sizeable inhibition observed after light chilling in net photosynthesis cannot, therefore, be attributed to a reduced capacity for photophosphorylation. The amplitude of the ΔΔ_518–540, however, was reduced by about half after 3 h of treatment. The cause of this decreased amplitude and its significance to the overall inhibition is under investigation.