Photosystem I response to high irradiance in plant leaves of different sensitivity to light stress

Abstract

Preferential photoinhibition of photosystem I (PSI) in comparison with photosystem II (PSII) has been observed under certain conditions. In order to clarify the physiological significance of PSI photoinhibition, the response of PSI to high-light stress was studied in leaves of several chilling-sensitive and -tolerant crop plants and in shade and sun leaves of tropical forest species. As compared to PSII photoinhibition, a preferential photoinhibition of the capacity of PSI-driven electron transport (measured as the maximum absorbance change of P700 at 810 nm) was observed only in the chilling-sensitive Cucumis sativus. In the tropical plant species tested, PSI capacity remained fully stable under high light that strongly affected potential PSII efficiency (FV/FM ratio of chlorophyll fluorescence). However, in all species, light stress caused a remarkable (up to 40-fold) increase in the far-red light intensity required to reach 50% P700 oxidation in the steady state (`saturation constant¿, Ks) (see ref.). Natural UV-B radiation enhanced this effect in shade leaves of tropical plants. Studies with transformants of Nicotiana tabacum deficient in functional NAD(P)H dehydrogenase (NDH) indicated that the increase in Ks was not related to enhanced cyclic electron transport around PSI via NDH. A possible explanation for this novel effect of high light on PSI photochemistry might be an increased rate of charge recombination in the PSI reaction center enhancing thermal dissipation of excess energy and thus contributing to the stability of PSI.