Ascorbic acid redox metabolism in 'Paluma' guava under nitrogen fertilization
2018
Lima, R. P. | Silva, G. C. | Dantas, A. L. | Dantas, R. L. | Sousa, A. S. B. | Melo, R. S. | Silva, S. M.
Guava (Psidium guajava L.) has high levels of reduced total ascorbate (TAsA), L-ascorbic acid (AsA), and oxidized dehydroascorbic acid (DHAsA), providing high antioxidant capacity to this fruit. Guava trees demand high levels of N fertilization in order to obtain high yields. However, in general, high levels of nitrogen fertilization tend to decrease the TAsA content in many fruits and vegetables. Based on that, this study aimed to evaluate the effect of nitrogen fertilization on the contents of ascorbic acid and the activity of enzymes related to the oxidation and recycling of ascorbic acid in guava. Fruits were harvested from 'Paluma' guava trees, fertilized with different nitrogen levels (50, 100, 150, and 200 g N plant-1) and evaluated at the green yellowish maturity stage. The increase in nitrogen levels up to 130 g plant-1 increased by more than 28% the content of AsA and 24% the TasA in 'Paluma' guava. However, from this dose on the content of AsA and TAsA decreased. In addition, an increase in the activity of the enzymes ascorbic acid peroxidase (APX) and ascorbate oxidase (AO) was observed by the application of higher doses of N. Moreover, the activity of the monodehydroascorbate reductase (MDHAR) decreased with increasing levels of N, which, in turn, favored reducing the AsA levels. Furthermore, a reduction was observed in the levels of H2O2, simultaneously with the increased activity of APX. Thus, it is presumed that high doses of N can provide an unbalanced redox system of the ascorbic acid in 'Paluma' guava since it favors an increased rate of oxidation and a low recycling rate of AsA. Therefore, in 'Paluma' guava, the estimated dose of 130 g plant-1 of N favors the contents of AsA and DHAsA, and consequently the higher total ascorbate content. Under this same condition, the H2O2 content was the lowest.
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