Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light

Abstract

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m(-2)). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P 700 (+) (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.

Interactions between water stress, sun‐shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

“…As found in pea leaves (Havaux et al 1991), light protected photochemical activity against inactivation by heat. This photoprotection was saturated at low PFD (65 jjivaoi vcT s~').…”

Stress Tolerance of Photosystem II in Vivo

“…Within the photosynthetic apparatus, PSII seems to be the most heat-sensitive function, whereas PSI activity, stromal enzymes, or chloroplast envelope are comparatively much more thermostable (3,5,12,18,29). It is believed that increasing temperature leads first to a blockage of PSII reaction centers and then to a dissociation of antennae pigmentprotein complexes from the central core of the PSII lightharvesting apparatus (2,3,10).…”

“…The photoacoustic signals generated by small leaf discs (diameter, 1 cm) at 250C were measured with a custom-made photoacoustic spectrometer that has been described (12) …”

“…For instance, light has been shown to markedly reduce damage to PSII during heat stress depending on its intensity and spectral characteristics (12, 31, 36). Increased thermostability of PSII has been observed in leaves exposed to physicochemical stresses such as high salinity (21) or hypertonic stress (17).…”

The chloroplast Ndh complex mediates the dark reduction of the plastoquinone pool in response to heat stress in tobacco leaves

“…E¡ect of heat stress on the rate of re-reduction of P700 + after far-red illumination in the ndh mutant v1 Previous work has shown that mild heat stress of pea [7] and potato [8] leaves induces a dramatic increase in the rate of re-reduction of P700 in the dark after a period of far-red illumination. This type of assay gives a measure of the rate of PSII-independent reduction of PSI and is comprised of both cyclic electron transfer around PSI and electron donation to the intersystem electron transfer chain by stromal reductant [8,12].…”