Screening broad beans (Vicia faba) for magnesium deficiency. II. Photosynthetic performance and leaf bioelectrical responses

In search of rapid screening tools for magnesium (Mg) deficiency in crops at early stages of plant ontogeny, we studied the kinetics of leaf photosynthetic responses and changes in electrophysiological characteristics of broad bean leaves as affected by different levels of Mg in the nutrient solution (1-200 ppm). No apparent correlation between plant age, Mg supply level, leaf stomatal conductance (gs) and transpiration rate (E) were found. A significant difference in CO2 assimilation became obvious only at week 8. Chlorophyll fluorescence analysis, however, revealed a significant difference in the maximal quantum efficiency of PSII (Fv/F m ratio) between Mg-deficient and Mg-sufficient plants as early as 2 weeks after seedling emergence. The most sensitive measurements were of light-induced changes in the leaf surface electric potential, with an almost 2-fold difference in the magnitude of leaf bioelectric response between 10 ppm (deficient) and 50 ppm (optimal) treatments. Preliminary experiments in which net Mg2+ fluxes were measured using the non-invasive ion flux estimation (MIFE) technique showed that the electrical changes on the leaf surface might, to some extent, reflect the movement of Mg2+ across the plasma membrane.