Decrease In Electron Transport Rate Research Articles

Temperature response of photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation in Beta vulgaris L.

Photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation at different temperatures (5–35 °C range) were studied in plants of Beta vulgaris L. grown outdoors at springtime. Gas exchange and chlorophyll fluorescence measurements were performed in plants exposed to different temperatures for two hours at saturating irradiance (1000 μmol photons m −2 s −1). The photosynthetic rate decreased significantly either by lowering the temperature below 25 °C or by increasing it above 25 °C. At low temperatures (5, 10 and 15 °C) a significant decrease of electron transport rate (ETR) was found and processes alternative to CO 2 fixation, as sinks of electrons, occurred but they were not affected by the temperature. However, at 5 °C a low and statistically significant photorespiratory rate was observed and the non-photochemical quenching (NPQ) was the highest. At higher temperature (30–35 °C) ETR saturated and hence non-assimilative processes, alternative to CO 2 fixation, sustained the photochemical activity. Among processes consuming O 2 photorespiration increased at 30–35 °C, while processes non-consuming O 2 were non-affected. By increasing the temperature up to 30–35 °C NPQ values did not show appreciable differences compared to 25 °C. The obtained results indicate that in B. vulgaris plants the thermal dissipation represents the main dissipative process of the excess excitation energy at low temperatures. On the contrary, at high temperatures (30–35 °C) the main dissipation of the excess excitation energy is due to alternative pathways to CO 2 assimilation.

Limitations in the use of PAM fluorometry for measuring photosynthetic rates of macroalgae at high irradiances

Pulse amplitude modulated (PAM) fluorometry can be used for measuring photosynthetic electron transport rates (ETR) of marine angiosperms and macroalgae both in the laboratory and in situ. Regarding macroalgae, quantitative values and linear correlations between ETR and rates of photosynthetic O2 evolution have so far been shown only for a few species under low irradiances. As a logical continuation of such work, the aim of the present study was to (a) assess to what degree high irradiances would limit such measurements and (b) evaluate whether PAM fluorometry could be used quantitatively also for other marine macroalgae from different phyla. This was done by comparing ETR with rates of gross O2 evolution (net O2 exhcange corrected for dark respiration) at various irradiances for the green alga Ulva lactuca grown at two irradiances, the brown algae Fucus serratus and Laminaria saccharina and the red algae Palmaria palmata and Porphyra umbilicalis. At low irradiances, there was a clear positive correlation between O2 evolution and fluorescence-based ETR. At high irradiances, however, all algae featured an apparent decrease in ETR while O2 evolution remained relatively constant, and this resulted in markedly increasing O2/ETR ratios. This anomaly could be nicely illustrated in plots of O2/ETR as a function of the effective quantum yield of photosystem II (Y). Such plots showed that the O2/ETR ratio generally started to increase when Y reached a critical low value of c. 0.1. It was further found that the irradiance at which this value was reached varied with species and previous light histories. Thus, it is the Y value, rather than the irradiance per se

Different kinetics of photoinactivation of photosystem I-mediated electron transport and P700 in isolated thylakoid membranes

Photoinactivation kinetics of photosystem I (PSI)-mediated electron transport rate was compared to that of P700 content at room (22 °C) and low (4 °C) temperatures in isolated spinach thylakoid membranes. The high light treatment was carried out under aerobic and anaerobic conditions. At 22 °C the decrease of electron transport rate showed first order exponential kinetics. The amount of P700 decreased linearly, being less affected in the first hours of illumination. During photoinhibition at 4 °C in the presence of oxygen, the kinetics of inactivation of PSI photochemical activity and the content of P700 were different. It was found that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) had different protective effect on the electron transport rate and on P700 content at both temperatures. Treatment with high light intensity under N 2 atmosphere had no effect on the electron transport rate or P700 content. The possible degradation of PSI reaction centre proteins was determined using immunoblot methods. In the presence of linear electron transport at 22 °C correlation between formation of toxic hydroxyl radicals and inhibition of oxygen uptake was observed.

Atividade fotossintética e peroxidação de lipídios de membrana em plantas de aroeira-do-sertão sob estresse hídrico e após reidratação

Avaliaram-se os efeitos do estresse hídrico e da reidratação sobre parâmetros de fluorescência, condutância estomática e peroxidação de lipídios em plantas de Myracrodruon urundeuva Fr. All. (Anacardiaceae). As plantas foram cultivadas em casa de vegetação e a seca foi induzida por suspensão da irrigação durante 14 dias. Sob estresse hídrico, observou-se queda acentuada no conteúdo relativo de água foliar, o qual alcançou valores próximos a 60 %. Essa queda foi acompanhada por grande diminuição na condutância estomática e na taxa aparente de transporte de elétrons (ETR) e extensivos danos às membranas evidenciados por aumentos na peroxidação de lipídios. Apesar do decréscimo nos valores de ETR, não se observaram alterações nos valores do rendimento quântico potencial do fotossistema II (Fv/Fm) em conseqüência do estresse hídrico. Durante a fase de recuperação, as plantas apresentaram valores de condutância estomática, parâmetros de fluorescência e peroxidação lipídica similares àqueles verificados antes do período de seca. Todos os parâmetros fotossintéticos investigados mostraram valores normais 6h após o início de reidratação. O ajuste linear entre os valores de condutância e ETR mostrou alta correlação entre essas duas variáveis, sugerindo que os estômatos representam o principal fator limitante da fotossíntese em plantas de Myracrodruon urundeuva sob estresse hídrico.

Heterogeneous inhibition of photosynthesis over the leaf surface of rosa rubiginosa L. during water stress and abscisic acid treatment: induction of a metabolic component by limitation of CO(2) diffusion

The contribution of changes in stomatal conductance and metabolism in determining heterogeneous photosynthesis inhibition during dehydration and abscisic acid (ABA) feeding was investigated using detached leaves of Rosa rubiginosa L. The steady-state and maximal rates of electron transport under a transient high CO(2) concentration were monitored using chlorophyll fluorescence imaging. The decrease in electron transport rate induced by dehydration and ABA treatment almost reverted to the control rate under transient high CO(2) availability. Therefore, inhibition of photosynthesis was mainly mediated through stomatal closure. However, since reversion was not complete, a metabolic inhibition was also identified as a decrease in the maximal electron transport rate driven by carboxylation. Under dehydration or ABA feeding, as under low ambient CO(2) treatment, in 21% or 0.4% O(2), the lower the steady-state electron transport was, the lower was the maximal electron transport rate during transient high CO(2) availability. We conclude that low CO(2) availability reduced the capacity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) to drive electron transport. The potential contribution of Rubisco deactivation mediated by stomatal closure is discussed.