Teste de tetrazólio em café: fisiologia e bioquímica de enzimas na estimação da germinação das sementes

Abstract

Due to the discrepancies between the results of the germination test and the tetrazolium test in the evaluation of the quality of coffee seeds, four studies were carried out aiming to investigate these discrepancies. The metabolic changes that occurred in coffee seeds with different levels of quality were evaluated in relation to the germination and tetrazolium tests. First, a statistical study of the historical results of the germination test and the tetrazolium test in coffee seed samples was carried out, analyzing the correlation between these results, using Generalized Regression Models, scale and form (GAMLSS). According to the results of the GAMLSS analysis, the germination estimation by the tetrazolium test is variable according to the percentage of germination classes. There are higher GAMLSS correlations between normal seedlings and viability percentages in the tetrazolium test, for germination values above 70% and low correlations below this value, showing that the evaluation of coffee seeds based only on the tetrazolium test may not correlate with the physiological performance. The second study evaluated the applicability of the tetrazolium test to estimate the viability of coffee seeds with different types and levels of stress. It was found that the tetrazolium test can overestimate the potential of coffee seeds to produce normal seedlings, especially in lowquality seed lots. In the third study, the physiological, respiratory and dehydrogenase enzymes alterations were evaluated in endosperms and embryos of coffee seeds, submitted to different types of stress. The tetrazolium test overestimates the germination of low quality seeds, especially when the seeds are subjected to drying stress at high temperature, exposure to subzero temperature and storage in an uncontrolled environment. The enzymes glucose-6-phosphate dehydrogenase, alcohol dehydrogenase and glutamate dehydrogenase have higher activity in embryos than in coffee seed endosperms. Changes occur in the activity of the isoforms of the xanthine dehydrogenase and diaphorase enzymes in the embryos and endosperms of the coffee seeds. In the fourth and last study, the physiological and antioxidant enzymatic system alterations were evaluated in endosperms and coffee embryos, after being subjected to different stress conditions. The discrepancies between the results of the germination test and the results of the tetrazolium test with overestimation of germination were confirmed in this study. The enzymes catalase and isocitrate lyase present greater activity in endosperms than in embryos, being this activity associated to the greater deterioration. The enzyme peroxidase has no activity in coffee seed embryos.