Effect of potassium, magnesium, and calcium deficiencies on nitrogen constituents and chloroplast components in Citrus leaves

The chlorotic appearance of mineral-deficient citrus leaves presumably refl ects degradative changes in chloroplast components, most of which have nitr ogen as a principal constituent. To examine this assumption the size of som e major nitrogen pools, the SDS-PACE pattern of soluble and chloroplast mem branal proteins, and the activities of nitrate reductase and ribulose bisph osphate carboxylase (RuBPcase) were determined in leaves of rough-lemon (Ci trus volkameriana Ten. & Pasq) plants grown hydroponically for 3 to 10 mont hs under K, Mg, and Ca deficiencies. Plants grown under minerally deficient conditions produced less biomass, Leaves developing under K, Mg, and Ca de ficient conditions had significantly reduced concentrations of the respecti ve elements. Chlorophyll levels of the chlorotic Mg and Ca-deficient leaves were lower than those of control leaves but chlorophyll am ratios were not markedly different, Calcium deficiency caused significant decreases in tot al nitrogen, nitrates, and the free amino acid pool. Proline which is the m ajor component of the free amino acid pool decreased by 82.5%. Calcium-defi cient leaves had significantly lower nitrate reductase and RuBPcase activit ies. The level of RuBPcase holoenzyme and its subunits were also reduced. P rotein levels of K, Mg, and Ca-deficient leaves were not significantly alte red. The SDS-PAGE patterns of soluble and chloroplast membranal proteins di d not reveal major qualitative changes. In conclusion, the data do not demo nstrate a general close link between chlorosis of minerally deficient citru s leaves and nitrogen metabolism. Calcium deficiency seems to specifically interfere with early stages of nitrogen assimilation and free amino acid ac cumulation but the metabolic integrity of the leaf is apparently maintained even under severe nutritional stress conditions.