In the past decades, it has become clear that superoxide radical (O2 .-) can be generated from photosystem II (PSII) during photosynthesis. Depending on the extent of its accumulation, O2 .- plays an important role in plant physiology and pathology. The photoinhibition/repair cycle is a typical process in PSII which is mainly responsible for the survival of plants under the photoinihibition condition. It is therefore of significant importance to determine O2 .- production in this cycle, and then explore how O2 .- is controlled by PSII within a normal physiological level. With this in mind, we herein investigate the variation of the O2 .- levels in PSII under Mn-depleted and photoactivated conditions mimicking the photoinhibition/repair cycle in vitro. The effect of intrinsic SOD-like component on the O2 .- levels was also studied. Results show that PSII has the ability to regulate the O2 .- levels in these two processes by simultaneously modulating the O2 .- generation activity and intrinsic SOD-like activity. This finding could shed new lights on the photoprotective property of PSII against O2 .- and other reactive oxygen species. and Y. G. Song ... [et al.].
Tomato and pepper leaves were clipped with black leaf clips for dark adaptation under solar radiation in the late spring or early summer 2010 in southern Italy. The leaves showed highly variable maximum PSII quantum yield (Fv/Fm = 0.026-0.802) using a continuous-excitation fluorometer Pocket PEA. These results were confirmed using the modulated fluorometer FMS1 on tomato leaves in mid summer, with Fv/Fm as low as 0.222 ± 0.277 due to nearly equal minimum (F0) and maximum (Fm) fluorescence emission. A significant clip effect on Fv/Fm occurred after only 12 (tomato) or 25 (pepper) min. Increasing the leaf temperature from 25 to 50°C reportedly induced an F0 increase and Fm decrease so that Fv/Fm approached zero. The hypothesis that black leaf clips overheated under intense solar irradiance was verified by shrouding the clipped leaves with aluminum foil. In clipped leaves of pepper, Fv/Fm with the black clip/Pocket-PEA was 0.769 ± 0.025 (shrouded) and as low as 0.271 ± 0.163 (nonshrouded), the latter showing a double F0 and 32% lower Fm. An 8% clip effect on Fv/Fm was observed with the white clip/FMS1. To avoid the clip effect in high irradiance environments, Fv/Fm measurements with black clip/Pocket PEA system required leaf dark adaptation with
radiation-reflecting shrouds. It would be useful if manufacturing companies could develop better radiation-reflecting leaf clips for the Pocket PEA fluorometer. and P. Giorio.
The use of black leaf-clips for dark adaptation under high solar radiation conditions is reported to underestimate the maximum quantum yield of PSII photochemistry (Fv/Fm) measured by the continuous-excitation fluorometer Pocket PEA. The decrease in Fv/Fm was due to a rise in minimum fluorescence emission (F0), probably resulting from increased leaf temperature (Tl). In
field-grown tomato and pepper, fluorescence parameters and Tl in the region covered by the black leaf clip were measured in clipped leaves exposed to solar radiation during dark adaptation (clipped-only leaves) and in clipped leaves protected from solar radiation by aluminium foil (shrouded clipped leaves). Results confirmed significant Fv/Fm underestimates in clipped-only leaves primarily due to increased F0. In one tomato experiment, Tl increased from 30 to 44.5°C in clipped-only leaves, with a negligible rise in shrouded clipped leaves. In two respective pepper experiments, Tl in clipped-only leaves increased from 27 to 36.2°C and 33 to 40.9°C. Based on the results of this study, a clip-effect parameter (PCE) on fluorescence emission is proposed as the difference for Fv/Fm (or -F0/Fm) between shrouded clipped leaves and clipped-only leaves, which resulted to be 0.706 for tomato, and 0.241 and 0.358 for the two pepper experiments., P. Giorio ... [et al.]., and Obsahuje bibliografii