Starch and sugar accumulation in Sulla carnosa leaves upon Mg2+ starvation

In the present work, magnesium deficiency effects were studied in Sulla carnosa plants grown in nutrient solution containing 1.50, 0.05, 0.01, and 0.00 mM Mg2+. After 5 weeks of treatment, fully expanded leaves were harvested to study their morphological and ultrastructural changes, as well as their carbohydrate, pigment, and Mg2+ concentrations. In control plants, leaves were well developed with remarkable green color. Down to 0.05 mM Mg2+, no chlorosis symptom was recorded, but below this concentration, mature leaves showed an appearance of interveinal chlorosis that was much more pronounced at 0.00 mM Mg2+ with the development of necrotic spots. Optima of chlorophyll a, chlorophyll b, and carotenoid concentrations were observed at 0.05 and 1.50 mM Mg2+; leaf magnesium concentration was severely reduced at 0.05 mM Mg2+. A significant decrease in pigment concentrations was noticed at 0.01 mM Mg2+, but the lowest values were recorded at 0.00 mM Mg2+. Enzymatic assays showed an increase in the accumulation of soluble sugars and starch with decreasing Mg2+ concentration. These results were in accordance with those of ultrastructural studies that revealed a marked alteration of chloroplasts in leaves of deficient plants. These chloroplasts were round and bigger as a result of a massive accumulation of oversized starch grains with disrupted thylakoids. As a whole, 1.50, 0.05, and 0.01 mM Mg2+ were found optimal, suboptimal, and deficient concentrations, respectively, the latter showing no significant difference with absolute Mg2+ absence (0.00 mM Mg2+).