Changes in the temperature dependence of the maximum carboxylation capacity (VCmax) of Rubisco during thermal acclimation of PN remain controversial. I tested for acclimation of the temperature dependence of VCmax in quinoa, wheat, and alfalfa. Plants were grown with day/night temperatures of 12/6, 20/14, and 28/22°C. Responses of PN to substomatal CO2 (Ci) and CO2 at Rubisco (Cc) were measured at leaf temperatures of 10-30°C. VCmax was determined from the initial slope of the PNvs. Ci or Cc curve. Slopes of linear regressions of 1/VCmaxvs. 1/T [K] provided estimates the activation energy. In wheat and alfalfa the increases in activation energy with growth temperature calculated using Ci did not always occur when using Cc, indicating the importance of mesophyll conductance when estimating the activation energy. However, in quinoa, the mean activation energy approximately doubled between the lowest and highest growth temperatures, whether based on Ci or Cc., J. A. Bunce., and Obsahuje bibliografii
The thermal photoacoustic signal (279 Hz) and the chlorophyll (Chl) fluorescence of radish cotyledons (Raphanus sativus L.) were measured simultaneously. The signals were recorded during a photosynthetic induction with actinic radiation of different quantum fluence rates [20, 200, and 1200 µmol(PAR-quantum) m-2 s-1]. The rise of these signals upon irradiation saturating photosynthesis was followed in the steady state of the induction and during the subsequent dark-recovery (i.e., in dark periods of 1, 5, 15, and 45 min after the induction). From these values various parameters (e.g., quantum yield, photochemical loss, different types of quenching coefficients) were calculated. The results show that heat dissipation detected by photoacoustic measurements is neither low, constant, nor always parallel to Chl fluorescence. Therefore, the thermal signal should always be measured in order to fully understand the way leaves convert energy taken up by PAR absorption. This helps in the interpretation of photosynthesis under different natural and anthropogenic conditions (stress and damage effects).
The relative size of the pool of electrons accumulated in stroma reductants during actinic irradiation, which can be donated to P700+ via the intersystem chain, was estimated after short-term exposure of intact Zea mays leaves to elevated temperatures. When the temperature increased from 25 to 50 °C by 5 °C steps, the relative size of the stroma electron pool went through a maximum at around 30 °C, and decreased gradually thereafter. and Ming-Xian Jin, Hualing Mi.
Thermoluminescence (TL) in green plants arises from charge recombination of charged molecules in the reaction centre (RC) of photosystem 2 (PS2) in chloroplasts. The TL technique is used for detection of alterations in the architecture of PS2 RCs. The donor side 'S-states' and the acceptor side quinone molecules (QA and QB) are involved the charge recombination processes of PS2. High temperature (70-75 °C) glow peaks are also used to detect non-photosynthetic peroxidation processes in thylakoid membranes. The TL peaks with their characteristic charge recombination can be utilised for the study of chloroplast development, ageing, chemical, biotic, and abiotic stress induced alterations in the PS2 RC and for the study of the primary photochemical events of photosynthesis. The technique has been used successfully in the characterisation of transgenic plants in the study of genetically engineered organisms. and A. N. Misra ... [et al.].
The thermoluminescence (TL) emission of photosynthesising materials originates from the recombination of charge pairs created by a previous excitation. Using a recently described TL set-up the effect of chilling stress on TL bands occurring at positive temperatures (AG, B, and HTL) was investigated in intact leaves. The far-red irradiation of leaves at low, but non-freezing temperatures induced a TL band peaking at around 40-45 °C (AG band), together with a B band peaking between 20 and 35 °C. Low temperature stress first caused a downshift and a temporary increase in the AG band after 4 h at 0 °C in the light, then a decrease in the AG and B TL bands after 1 d at 0 °C in the light. This decrease was less pronounced in cold-tolerant genotypes and in those grown at acclimating temperatures. Furthermore, an additional band appeared above 80 °C after severe cold stress. This band indicates the presence of lipid peroxides. Thus TL is a useful technique for studying the effects of low temperature stress. and T. Janda, G. Szalai, E. Páldi.
The species specific response of photosystem 2 (PS2) efficiency and its thermotolerance to diurnal and seasonal alterations in leaf temperature, irradiance, and water relations were investigated under alpine field conditions (1 950 m) and in response to an in situ long-term heat treatment (+3 K). Three plant species were compared using the naturally occurring microstratification of alpine environments, i.e. under contrasting leaf temperatures but under similar macroclimatic conditions. Thermotolerance of PS2 showed a high variability in all three species of up to 9.6 K. Diumal changes (increases or even decreases) in PS2 thermotolerance occurred frequently with a maximum increase of +4.8 K in Loiseleuria procumbens. Increasing leaf temperatures and photosynthetic photon flux density influenced thermotolerance adjustments. Under long-term heating (+3 K) of L. procumbens canopies with infra-red lamps, the maxima of the critical (Tc) and the lethal (Tp) temperature of PS2 increased by at least 1 K. Thermotolerance of the leaf tissue (LT50) increased significantly by +0.6 K. The effects of slight water stress on thermotolerance of PS2 were species specific. High temperature thresholds for photoinhibition were significantly different between species and increased by 9 K from the species in the coldest microhabitat to the species in the warmest. Experimental heating of L. procumbens canopies by +3 K caused a significant (p>0.01) upward shift of the high temperature threshold for photoinhibition by +3 K. Each species appeared to be very well adapted to the thermal conditions of its microhabitat as under the most frequently experienced daytime leaf temperatures no photoinhibition occurred. The observed fine scale thermal adjustment of PS2 in response to increased leaf temperatures shows the potential to optimise photosynthesis under varying environmental conditions as long as the upper thermal limits are not exceeded. and V. Braun, O. Buchner, G. Neuner.
The common bean (Phaseolus vulgaris L.) is sensitive to high temperature, while an ecologically contrasting species (Phaseolus acutifolius A. Gray) is cultivated successfully in hot environments. In this study, the two bean species were respectively acclimated to a control temperature of 25 °C and a moderately elevated temperature of 35 °C in order to compare the thermotolerance capabilities of their photosynthetic light reactions. Growth at 35 °C appeared to have no obvious adverse effect on the photosynthetic activities of the two beans, but changed their thermotolerance. After a short period of heat shock (40 °C for up to 4 h), the photosynthetic activities of 25 °C-grown P. vulgaris declined more severely than those of P. acutifolius grown at 25 °C, implying that the basal thermotolerance of P. vulgaris is inferior to that of P. acutifolius. But after acclimating to 35 °C, the thermotolerances of the two species were both greatly enhanced to about the same level, clearly demonstrating the induction of acquired thermotolerance in their chloroplasts, and P. vulgaris could be as good as P. acutifolius. Temperature acclimation also changed plants' resistance to photoinhibition in a manner similar to those toward heat stress. In addition, acquisition of tolerance to heat and strong irradiance would reduce the dependency of the two beans on xanthophyll pigments to dissipate heat, and also seemed irrelevant to the agents with antioxidant activities such as SOD. and C. M. Tsai, B. D. Hsu.
The effects of phenylmercuric acetate (PMA) and apoferredoxin (apoFd) on the diaphorase activity of spinach ferredoxin:NADP+ oxidoreductase (FNR) in the presence of dibromothymoquinone (DBMIB) or cytochrome c (Cyt c) were studied. PMA inhibited effectively (I50 = < 5 μM) ferredoxin-dependent Cyt c reduction but did not affect evidently the enzyme activity in the presence of DBMIB as an electron acceptor. ApoFd caused also inhibition of Cyt c reduction but slightly stimulated, like ferredoxin, DBMIB reduction. We confirm a hypothesis according to which three binding sites for substrates [NADP(H), Fd-Cyt c, quinone/dichlorophenol indophenol] occur within the molecule of isolated FNR. and M. Bojko, S. Więckowski.