Abstract: The changes of calcium level in rootmeristem region of cucumber seedling (‘XintaiMici’) under different NO₃^- concentrations stress were determined by the imp roved cytochemical method of calcium pyroantimonate p recip itation. After 7 days of treatment, under op timum NO₃^- (14 mmol·L ^-1) conditions,
calcium antimonite deposits, which indicates calcium distribution, were mainly localized in cellmembrane and vacuoles, as a storeroom of Ca²⁺, and smaller amounts of calcium p recip itates randomly resided in mitochon2dria, cytop lasm and nucleus. But under excessive NO₃^-stress, the distribution of Ca²⁺ in subcellular was changed greatly. After 7 days treated with 56 and 140 mmol·L^-1 NO₃^-, Ca²⁺level in the intercellular spaces, cytop lasm and mitochondria elevated. With the increasing NO₃^-concentrations, the shape and amount of mitochondria and vacuoleswere changed greatly. Higher NO₃^-concentrations (182 mmol·L^-1) caused a series of disorders of metabolic p rocesses and serious damages of membrane system of organelles in cucumber seedling roots, thus, Ca²⁺deposits in each compartments became smaller significantly. The result indicated that the change of Ca²⁺ from vacuoles to hyalop lasm can reduce the NO₃^-stress in cucumber seedlings.

Key words: cucumber seedling, NO₃^-stress, calcium pyroantimonate precipitation, cytochemistry, Ca²⁺