Morfofisiologia foliar sob estresse causado pelo alumínio em cultivares de arroz (Oryza sativa L.) com tolerância diferencial

Abstract

Inhibition of radicular growing is the remarkable symptom of aluminum(Al3+) toxicity in plants. Several studies point out the root tip as the primary site of tolerance or sensitivity to Al. This occurs due to known mechanisms of exclusion or resistance, which operate outside of Casparian strips in endoderm. The conjunct action of physiological and biochemical processes associated to the physical barrier imposed by endoderm prevents the uptake of Al to the central cylinder and its translocation to the shoots. Evaluations of toxic effects caused by Al on the shoots, particularly in relation to the ultrastructure, only demonstrate the indirect effects caused by Al3+ in the leaves. This work proposed establish a model for studying the translocation of Al3+ to the shoot,from the removal of root apex as well as part of the roots of rice plants and the subsequent effect of this metal in leaves of plants with different tolerance to Al. To create this model, we used two rice cultivars with different tolerance to Al. Intact plants or plants that had part of its roots removed were used. The quantification of Al in shoots of both cultivars showed the success of the model. The photosynthetic analysis showed higher photoinhibition in sensitive treated with Al in relation to the tolerant. In light microscopy there was no tyloses production in vessel elements. In these tests, changes were seen in the mesophyll like necrosis in both cultivars treated with Al. Ultrastructural analysis showed changes in structure of chloroplasts and mitochondria, as well as a process of interaction between these organelles. Nitric oxide (NO) is directly involved in the Al stress response. In order to verify its possible effect on theshoot and the likely reversal of stress, the impact of this molecule for thephotosynthetic apparatus was evaluated. The results revealed differences in the rate of electron transport to photosystem II (PSII), as well as in the photosynthetic efficiency. This result demonstrates that the sensitive cultivar responds better than the tolerant one to NO presence in plants under Al stress. Therefore, NO supplied by nitric oxide-donor S-nitrosoglutathione (GSNO) was able to reverse some toxicity symptoms in Al-sensitive plants allowing them to maintain partially photosynthetic activity.