THE DIFFERENCES OF SUBCELLULAR CA²⁺ DISTRIBUTION IN CUCUMBER SEEDLING LEAVES UNDER NO₃^- STRESS

The ultrastructural localization of calcium in cucumber seedling leaves was determined with the improved cytochemical method of calcium pyroantimonate precipitation, and the changes of calcium level in cells under NO₃^- stress were observed. After 7 d of treatment under optimum NO₃^- conditions, calcium antimonite deposits, which indicate calcium distribution, were mainly localized in intercellular spaces, cell wall and chloroplast, and smaller amounts of calcium precipitates randomly resided in plasma membrane and mitochondria. But under excessive NO₃^- stress, the distribution of Ca²⁺ in subcellular was changed greatly. Under 56, 98, 140 mmol•L^-1 of NO₃^- concentrations, Ca²⁺ level in cytoplasm and nucleus increased with the increases in NO₃^- concentration, due possibly to the opening of the channels in plasma membrane and tonoplast as well as the decrease or activity loss of the calcium pump. The increase of Ca²⁺ in the protoplasm might cause a series of disorders in metabolic processes. With the increasing NO₃^- concentrations, larger Ca²⁺ deposits localized on the envelope of plastid and Ca²⁺ were also deposited together in the intercellular spaces, while Ca²⁺ deposits were small and dense on the plasma membrane. In addition, Ca²⁺ accumulation in the mitochondria remained low and the shape and amount were also changed. Under 182 mmol•L^-1 NO₃^-, Ca²⁺ deposits in each compartment became smaller and denser, shacked tonoplast was not found, and Ca²⁺ in vacuoles was distributed closely to tonoplast. The possible relationship between the responsiveness of Ca²⁺ distribution in leaf cells and cucumber NO₃^- tolerance was discussed.