Physiological assessment of magnesium deficiency in broad beans (Vicia faba L.)

Despite the wide recognition of Mg2+ deficiency in several soil types, there has been little documentation of the relationship between Mg2+ concentration in the soil solution and plant growth, yield responses or the kinetics of development of deficiency symptoms in crops. Most of the literature deals with forest species, with only a handful of crops being characterized. Even in the latter case, these papers address very severe cases of Mg2+ deficiency. Meanwhile, in many crops, visual symptoms of Mg2+ deficiency are either virtually absent, or become obvious only at later stages of plant development. One of these species is broad bean (Vicia faba L). In this study, broad beans were used as a case study to characterize the development of Mg2+ deficiency symptoms in plants and make a comparative evaluation of the suitability of various physiological characteristics as prospective tools for early diagnosis of Mg2+ deficiency. The ultimate goal of this study was to suggest an efficient screening tool for early diagnosis of Mg2+ deficiency in crop species and to explain cellular mechanisms underlying observed changes at the whole-plant level. In a series of glasshouse experiments, growth characteristics were measured at regular intervals (on a monthly basis) from plants grown in a wide range of Mg 2+ concentrations (from 1 to 200 ppm) in soil solution. Those data were then correlated with plant yield responses, pigment composition and nutrient content in leaves as well as with visual deficiency symptoms. At the age of 4 weeks, no visual symptoms of deficiency were evident even for plants grown at 1 ppm (severe Mg2+ deficiency). Shoot growth characteristics were very similar for a wide range of treatments, although a pronounced difference in plant yield was observed at the end of the experiment. It appears that neither plant biomass nor leaf area is a good indicator for diagnosis of Mg2+ deficiency in broad beans. Although pigment analysis revealed some difference between treatments, at no age was it possible to distinguish between moderately Mg2+ deficient (10 or 20 ppm) and sufficient (50 to 80 ppm) treatments. Leaf elemental analysis for Mg2+ content remained the most sensitive and accurate indicator of Mg2+ deficiency in broad beans. However, its suitability for screening purposes is jeopardised by hight cost of analysis and a significant amount of time involved. In a search for rapid screening tools for Mg2+ deficiency in crops at early stages of plant ontogeny, kinetics of leaf photosynthetic responses and changes in electrophysiological characteristics of broad bean leaves were studied. No apparent correlation between plant age, Mg2+ supply level and 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 was more suitable for screening purposes, with significant (P=0.05) difference in the maximum quantum efficiency of PSII (Fv/Fm ratio) between Mg2+-deficient and sufficient plants as early as 2 weeks after seedling emergence. However, even in this case, only \extreme\" Mg2+ treatments were differentiated. The most sensitive potential screening tool was measurement of light-induced changes in the leaf surface electric potential (an integrated change in membrane potential of hundred of cells on the leaf surface). A significant difference in the magnitude of leaf electrical responses to light measured by means of the surface potential technique was found between optimal (Mg50) and deficient leaves as early as 2 weeks after the beginning of the experiment. Not only severe (Mg 1) but also moderately deficient (Mg10) leaves showed a significantly lower magnitude of response. At the age of 4 weeks it became possible to \"rank\" all treatments according to their predicted bean yield. Therefore it appears that leaf electric measurements warrants testing as a potential screening tool for Mg 2+ deficiency at least in broad bean plants. To establish a causal link between light-induced electrical events in bean leaves and membrane-transport processes at the plasma membrane of bean mesophyll cells a series of experiments were conducted using a non-invasive ion selective microelectrode (MIFE) technique. Netfluxes of Fr K+ Ca2+ and Mg2+ were measured from leaves grown at various Mg 2+ levels and light-induced kinetics of flux changes were characterised with high (5 sec) temporal resolution. To my knowledge this is the first report on net Mg2+ flux measurements from higher plants in physiological literature. Based on the stoichiometry of ion flux changes and results of pharmacological experiments it was suggested that at least two mechanisms are involved in Mg 2+ uptake across the plasma membrane of bean mesophyll cells. One of them is a nonselective cation channel (NSCC) also permeable to K + and Ca2+. The other mechanism operating at concentrations below 30 iiM was suggested to be H+7Mg2+ exchanger. Experiments performed on leaves grown at different levels of Mg2+ availability (from deficient to excessive) showed that Mg 2+ availability has a significant impact on the activity of plasma membrane transporters for Ca 2+ K+ and H+. Among other findings our data suggests that Mg 2+ availability during plant growth may significantly affect light-induced Ca 2+ flux \"signatures\" and thus regulate (directly or indirectly) signal transduction between light photoreceptors and plasma membrane effectors (ion channels). A possible causal link between light-induced changes in fluxes of other ions (Fr K+ and Mg2+) and whole-plant responses to Mg2+ deficiency is discussed."