Comparison of the Effects of Gradual and Acute Treatment with Mn on Physiological Responses of Rumex hydrolapathum Plants

An understudied problem in plant heavy metal biology is the effects of acute versus gradual or chronic metal exposure. The aim of the present study was to compare the growth and physiological responses of Rumex hydrolapathum Huds. plants subjected to gradual or acute Mn stress treatment in controlled conditions. Heavy metal was applied to substrate either as one 1.00 g L−1 Mn dose (acute treatment) or the same dose in four steps of increasing amounts within 12 days (gradual treatment). Peroxidase activity in actively photosynthesizing leaves was used for monitoring induced biochemical changes resulting from Mn treatment. The number of leaves per plant significantly increased in the case of gradual treatment with Mn, but this effect was not statistically significant for acute treatment. Leaf fresh mass significantly decreased in both cases due to the decrease in leaf water content, but dry biomass of leaves was not affected, with no significant differences between the two types of treatments. A significantly lower chlorophyll fluorescence parameter Performance Index in large leaves of plants under the acute Mn treatment than in plants under the gradual treatment was evident. An increase in leaf peroxidase activity by Mn treatment was proportional to the metal dose received, but plants in the acute treatment with 1.00 g L−1 Mn had a significantly lower peroxidase response in comparison to the gradual treatment with 1.00 g L−1 Mn. In conclusion, under gradual treatment, biochemical changes related to the induction of tolerance to the heavy metal are expressed, as indicated by the continuous increase in leaf peroxidase activity after each treatment step.