Mecanismos fotossintéticos em cultivares de arroz sob excesso de ferro e ondas de calor

Abstract

Rice cultivars differ in their tolerance to excess iron (Fe). In sensitive cultivars, Fe excess toxicity can cause increased production of reactive oxygen species, reduced carbon assimilation rate, reduced quantum efficiency of photosystem II, and concomitant inability to suppress excess energy by non-photochemical dissipation. The water and light energy use efficiency, and photoprotection mechanisms in rice cultivars may vary according to Fe concentrations, or between the interaction of excess Fe with increasing temperature. Thus, the objective of the work is to understand how rice cultivars contrasting in tolerance to Fe excess are affected by photochemical, biochemical, or stomatal limitations, and photosynthetic efficiency at higher temperatures. In the contrasting rice cultivars, Fe concentrations and Fe exposure time were essential to determine sensitivity and tolerance. The tolerant cultivar IRGA 424 showed higher photosynthetic efficiency with longer exposure time to Fe and did not alter its photosynthetic parameters, and showed photoprotective mechanisms such as non-photochemical extinction and photorespiration. The sensitive cultivar IRGA 417 showed greater oxidative damage, reduced quantum yields and efficiencies, and specific energy fluxes of PSII, in which increased photoinhibitory processes. Fe concentrations reduced xylem sap flow and leaf water potential. Exposure of the cultivars to increased temperature, resulted in higher Fe concentration in the aerial part of the tolerant cultivar and lower carboxylation rate of rubisco in both cultivars. Stomatal limitation occurred only as a late response, reducing Fe accumulation with increasing temperature in the sensitive cultivar IRGA 417. Activation of photorespiration as an electron drain under excess Fe was also efficient with increasing temperature during heat waves and prevented further membrane damage. Sensitivity to iron toxicity and heat stress is associated with inefficient non-photochemical dissipation. Thus, compression of the events involved in the role of Fe and temperature increase will facilitate the development of new cultivars tolerant to climate change.