Effects of Ultraviolet Radiation on Fluorescence Induction Kinetics in Isolated Thylakoids and Intact Leaves

Abstract

Action spectra were determined for the UV-induced delay of fluorescence induction (expressed as increase of half-rise time) in leaves of Elodea densa Casp. and Oxalis deppei Lodd. Elodea leaves showed increasing sensitivity with wavelength decreasing from 310 to 280 nm, while the Oxalis leaves, when irradiated from the abaxial side, gave a rather flat spectrum with lower sensitivity than Elodea throughout the range. When Oxalis leaves were irradiated from the adaxial side, UV sensitivity was even lower. The difference is partly explainable by a higher content of water-soluble, UV-absorbing substances in the adaxial epidermis. The action spectra are compared to that determined earlier for isolated spinach thylakoids. We also analyzed in more detail the effect of ultraviolet radiation on fluorescence kinetics at selected wavelengths. The fluorescence induction in isolated spinach thylakoids is the sum of three first order processes with different time constants. The main effect of ultraviolet radiation (280 nm) is to decrease the amplitude of the intermediate component. It also slightly retards the slow component. Results with 320-nm radiation were similar.

Fluorescence induction in Elodea leaves can be described as the sum of two first order components. The main effect of 320-nm radiation was to decrease the rate constant of the slow component.

The fluorescence rise in Oxalis leaves (and also in spinach leaves) has a biphasic pattern with a plateau in the middle. It is interpreted using a model with two kinds of photosystem II units: one set of unconnected units and one set with an energy transfer probability of about 0.65. The main effect of ultraviolet radiation is to diminish the fluorescence from the unconnected units, which have the largest rate constant.