Magnesium deficiency or excess hinders tomato growth, potassium and calcium uptake

Despite accumulating evidence for the adverse effects of magnesium (Mg) deficiency or excess on grain crops, how Mg imbalance affects plant growth and potassium (K) and calcium (Ca) nutrition in vegetable crops is still unclear. The aim of this study was to ascertain the response of plant growth, nutrient uptake and Mg-K-Ca interactions in tomato (Solanum lycopersicum L.) to various levels of Mg supply. The growth parameters and nutrient contents of hydroponic plants were measured under the Mg levels of 0, 0.5, 1.0, 1.5 and 3.0 mmol/L Mg²⁺ from seedling to fruit ripening stage. Results showed that both Mg deficiency (0 mmol/L Mg²⁺) and excess (3.0 mmol/L Mg²⁺) negatively affected shoot and root growth, leading to a noticeable decrease in total plant biomass across different stages (41.2–52.8% and 17.7–38.3%, respectively). Mg imbalance additionally altered leaf morphology and disrupted chloroplast structure. As a consequence of increased Mg levels, the Mg contents in various plant organs increased, whereas the Ca contents decreased substantially. The trend of K contents under different Mg levels was dependent on the plant growth stage. Although Mg levels did not prominently affect plant K contents during the early growth stage, they were significantly negatively correlated in the leaves and positively correlated in the fruit during the late growth stage. When translocated from roots to aboveground organs, Mg and Ca were mainly distributed in the leaves, with K preferentially distributed in the fruit. The findings of this study underscore that the symptoms of Mg imbalance generally develop from middle leaves in vegetable crops, exemplified by tomato, which is different from the pattern in common grain crops. Vegetable production necessitates nutrient supply for the middle and upper parts of Mg-deficient plants, and attention should be paid to the nutritional imbalance of Ca and K in plants under excessive Mg supply.