Cadmium stress affects seed germination and seedling growth in Sorghum bicolor (L.) Moench by changing the activities of hydrolyzing enzymes

Seed germination, one of the most important phases in the life cycle of a plant, is highly responsive to existing environment. Hydrolyzing enzymes play a major role in the mobilization of food reserves by hydrolyzing carbohydrates, proteins and fats. This paper reports on the effect of Cd toxicity on seed germination and the activities of hydrolyzing enzymes, like acid phosphatases (ACPs), proteases and alpha-amylases in Sorghum bicolor (L.) Moench. The metal uptake by embryonic axes and seeds was quantified. We found that sorghum could tolerate up to 0.5 mM Cd. At concentrations above 3.0 mM, seed germination was adversely affected with a complete cessation of seedling growth. All investigated hydrolyzing enzymes exhibited a significant decrease in activity with increasing Cd concentrations. The isozyme profiles indicated the loss of one or two isozymes of ACP, induction of a new isozyme for total protease (at 3.0 mM Cd) and a decline in the intensity of alpha-amylase isozymes. SEM studies revealed that Cd affected a change in root hair density. SEM investigations also confirmed the assay results of the inhibition of starch mobilization from endosperm. This suggested an inhibition of the hydrolysis of reserve carbohydrates and translocation of hydrolyzed sugars, ultimately resulting in decreased germination and disruption of seedling growth. Because sorghum is an important dryland crop, its response to the presence of Cd in agro-ecosystems and Cd-induced phytotoxicity during seed germination and seedling growth needs critical investigation.