Fotoproteção do PSI induzida por estresses abióticos que restringem a assimilação de CO2 é contrastante em J. curcas e R. communis

Abstract

Several abiotic stresses induced photodamage to photosystem II (PSII). This photodamage is generated by an imbalance between the reducing power generated by the reductive stage of photosynthesis and limitations in consumption of the Calvin cycle. Stresses that promote stomatal closure with salinity and water deficiency are well known for this purpose. When the stress is prolonged, damage can induce photoinhibition of PSII. However, PSI can also be inhibited. In vivo, studies have revealed that the PSI is more resistant than PSII. Moreover, in vitro studies show that PSI in isolated thylakoid membranes suffers inhibition even at low light levels (50 μmol.m-2s-1). This suggests that PSI in vivo requires photoprotective mechanism to prevent a collapse of the plant. Also, due to the low turnover rate of PSI protein complex, photoinhibition is more severe in PSI than in PSII, because this phenomenon is irreversible. On the other hand, there have been few studies of the photoinhibition of PSI in plants due to three factors: the difficulty of measuring the activity of PSI, the limited combination of plant species and environmental conditions in which the phenomenon could occur, and the non-regulatory aspect of photoinhibition of PSI. However, new methodologies capable of measuring in vivo the PSI and capable of inducing the PSI photoinhibition techniques have been developed and used to better understand how plants avoid this phenomenon. Ricinus communis and the Jatropha curcas are two said species adapted to arid and semi-arid regions. Both species have developed mechanisms to deal with the energy imbalance generated by drought and salinity. For this reason, this two species were selected. In this study, both species were subjected to salt stress and the answers were analyzed using biophysical methods such as gas exchange, chlorophyll fluorescence, absorbance of P700. Surprisingly, the plant Ricinus communis had a larger pool of oxidation potential of P700 under stress even having reduced the cyclic electron flow (CEF). Jatropha curcas showed a reduction in the pool P700 and an increase in the EFC. The oxidation of P700 is displayed in the literature as a way of avoiding photoinhibition PSI, however this mechanism is generally attributed to the increase in proton motive force (PMF) generated by an increase in the EFC. The pmf actually increase because there was an increase in power dissipation as heat measured by the non-photochemical quenching (NPQ). The plant Ricinus communis had the PSI inhibited artificially. The plants under salt stress, better able to tolerate the photoinhibition of PSI despite having increased the hydrogen peroxide synthesis (H2O2). Subsequently, the experiment was repeated in Ricinus communis changing treated water deficit. The results presented by plants in salt stress and inhibited PSI were similar to stressed plants by water deficit stress and inhibited PSI. In conclusion, the study suggests that plant Ricinus communis under stresses which limit the Calvin cycle activate photoprotective mechanisms possibly increase the pmf oxidize P700, but this mechanism is not dependent on CEF.