Interaction of UV-B Radiation and the Photosynthetic Process

Abstract

Alterations in morphology, ultrastructure, and pigments may be caused by exposure to UV radiation. This in turn may result in many indirect effects on cell processes and subsequent growth. One example of this is the altered distribution of photosynthetically-active radiation (PAR, 400–700 nm) with depth in a leaf after exposure of plants to UV-B radiation (Bornman and Vogelmann, 1991), with resultant change in the microenvironment for processes such as photosynthesis. Different protective mechanisms such as UV-B screening pigments modify the UV response, however. The effectiveness of these pigments is reflected in light gradients within a leaf as measured by fiberoptic microprobes.

The composition of photosystems (PS) I and II suggests that UV radiation would be effective in inducing changes in both systems, although PS II is generally more affected. The different components of the PS II reaction center seem to be variably influenced by UV-B radiation.

Simultaneous exposure to both UV-B radiation and high or low PAR was investigated with regard to the quantum yield (variable fluorescence/maximum fluorescence, F_v/F_m), photochemical (q_p) and non-photochemical quenching processes (q_NP), and D1 turnover. Low and high PAR (400 and 1600 mol m^-2 s^-1) were used with respective levels of UV-B radiation (13 and 3.25 kJ m^-2 day^-1), to simulate a period of cloudy weather followed by a period of high light. Photoinhibition and increased turnover of D1 were observed under high PAR and high UV-B radiation; no photoinhibition was observed with simulation of a cloudy day (400 mol m^-2 day^-1), and lower level of UV-B (3.25 kJ m^-2 day^-1), although increased turnover of the Dl protein still occurred under low PAR and UV-B radiation compared to low light conditions without UV-B radiation. Thus, even though small amounts of UV-B radiation did not enhance photoinhibition, the increased turnover of Dl may partly explain the often-reduced productivity in many studies (C. Sundby-Emanuelssonand J.F. Bornman, unpubl. data). However, photo-inhibition by visilble light and UV inhibition may not always reflect damage via similar mechanisms.

Thus, although rates of CO₂ assimilation as a measure of net photosynthesis do not always follow the sometimes-larger changes due to UV radiation found within PS H, disturbances of these partial reactions may be reflected in more subtle changes of the carbon cycle.

Please see Dr. Bornman’s full manuscript, p. 345, this publication.