RESPONSES OF THE PHOTOSYNTHETIC APPARATUS TO ALUMINUM STRESS IN TWO SORGHUM CULTIVARS

Abstract

This work focused on the effects of aluminum (Al) stress on photosynthetic apparatus using Al-tolerant and sensitive cultivars of sorghum (Sorghum bicolor (L.) Moench). Plants were grown in Clark's nutrient solution aerated continuously and adjusted daily to pH 4.0 and containing two Al concentrations (0 and 185 mmol m⁻³) supplied as Al₂(SO₄)₃ċ18H₂O. Measurements were performed on recently expanded leaves from the eighth to sixteenth day after imposing Al treatments. In general, chloroplastidic" pigment concentration, chlorophyll fluorescence parameters, and photosynthetic rates were initially affected by Al stress to a greater extent in the Al-tolerant cultivar than in the susceptible cultivar. However, the Al-tolerant genotype seemed to cope with high Al via progressive, sustained decreases in initial chlorophyll fluorescence and increases in chlorophyll a/b ratio, photosystem II photochemical efficiency, net carbon assimilation rate, and photosynthetic oxygen evolution. In contrast, the Al-sensitive cultivar did not show any apparent acclimation response to high Al application. The results suggest that the harmful effects of Al on photosynthesis might be partially alleviated through time-dependent mechanisms, particularly in the Al-tolerant sorghum cultivar.

ACKNOWLEDGMENTS

We gratefully acknowledge the Financiadora de Estudos e Projetos (FINEP, Brazil) and the Conselho Nacional de Desenvolvimento Cientifico e Tecnolögico (CNPq, Brazil) for financial support and fellowships. P.H.P.P. held a scholarship granted by the Coordenadoria de Aperfeïoamento de Pessoal de Nivel Superior (CAPES, Brazil). We are also indebted to Dr. James J. Griffith for helpful hints in English usage.