Suitability of the antioxidative system as marker of magnesium deficiencyin Capsicum annuum L. plants under controlled conditions

To explore the viability of using enzyme activities and their substrates as an alternative tool for the determination of mineral (i.e., Mg) critical values, a detailed characterization of the response of the antioxidative system of Capsicum annuum L. leaves under Mg deficiency was carried out. The response of each selected enzyme activity and substrate [i.e., superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR); ascorbate and glutathione pool; protein and chlorophyll concentration] was subjected to mathematical modelling in order to calculate Mg critical values (x(c)). Our x(c) values ranged from 0.70 to 0.14%, on a dry weight (DW) basis, for GR activity and total glutathione concentration, respectively. Our results suggest that, under Mg deficiency, cells enhance their antioxidative defence system by initially increasing their SOD and GR activities. Subsequently, higher GSH/GSSG ratios were observed, probably due to a greater increase in GR activity (x(c) = 0.70% DW) than in total glutathione concentration (x(c) = 0.14% DW). In contrast, x(c) values for total ascorbate concentrations (x(c) = 0.29% DW) were higher than those for DHAR activities (x(c) = 0.19% DW). In an attempt to study the limitations regarding the utilization of these enzymes and substrates as markers of Mg critical values in pepper, the x(c) values here obtained were compared to those based on growth parameters that have been reported in the literature. Overall, the results indicate that some enzymes and substrates, such as total ascorbate concentration, 1/protein ratio, and DHAR activity, might be suitable markers for the determination of Mg critical values in pepper plants under controlled conditions.