The Involvement of Dual Mechanisms of Photoinactivation of Photosystem II in Capsicum annuum L. Plants

Abstract

For plants, light is an indispensable resource. However, it also causes a loss of photosynthetic activity associated with photoinactivation of photosystem II (PSII). In studies of the mechanism of this photoinactivation, there are two conflicting hypotheses at present. One is that excess energy received by leaves, being neither utilized by photosynthesis nor dissipated safely in non-photochemical quenching, causes the photoinactivation. The other involves a two-step mechanism in which excitation of Mn by photons is the primary cause. In the former hypothesis, photoinactivation of PSII should not occur in low light that provides little excess energy, but in the latter hypothesis it should. Therefore, we tested these two hypotheses in different irradiances. We used a system that can measure the fraction of functional PSII complexes under natural conditions and over a long period in intact leaves, which were attached to a plant treated with lincomycin taken up via the roots. The leaves were photoinactivated in low, medium or high light (30, 60 or 950 micromol m(-2) s(-1)) with white, blue, green or red light-emitting diode arrays. Our results showed that the extent of photoinactivation per photon exposure was higher in high light than in low light, consistent with the abundance of excess energy. However, photoinactivation did occur in low light with little excess energy, and blue light caused the greatest extent of photoinactivation followed by white, green and red light in this order, an order that can be predicted from the Mn absorbance spectrum. These results suggest that both mechanisms occur in the photoinactivation process.

The action spectrum of Photosystem II photoinactivation in visible light

“…However, it cannot account for the features observed in the 600-700 nm region of all reported action spectra. Studies by Oguchi et al [60][61][62] suggest both hypotheses to be valid within the cross section of a leaf. It was observed that at the uppermost chloroplasts in the leaf tissue, photoinactivation by blue and red light is more effective compared to green light, consistent with chlorophyll absorption.…”

“…More recently, a hybrid paradigm which suggests that both photosynthetic pigment and Mn 4 CaO 5 cluster paradigms are valid that both operate simultaneously [60][61][62][63]76]. The outcome indicating the predominant paradigm is highly dependent upon the chosen experimental conditions.…”

Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light

“…At low irradiance, the excess absorbed irradiance was approximately 0.1 µmol photons m −2 s −1 , compared with 97 µmol photons m −2 s −1 at high irradiance. Using whole capsicum plants that took up lincomycin through their roots, Oguchi et al (2009) were able to demonstrate that the extent of PS II photoinactivation was higher in high irradiance of white light compared with low or moderately low irradiance, at the same photon exposure.…”

Systemic Regulation of Leaf Anatomical Structure, Photosynthetic Performance, and High-Light Tolerance in Sorghum

“…7, C and F), owing to their depressed photosynthetic capacity. There are two mechanisms that are primarily responsible for initiating the photoinactivation of PSII, one of which operates when excess light energy is not utilized by photosynthesis (Oguchi et al, 2009). The lower the photosynthetic capacity, as was the case in the MS and S treatments, the greater was the excess energy, consistent with the exacerbation of photoinactivation of ) was calculated using the method of Thain (1983) with the assumption that the bundle sheath cells were spheroid.…”