Impairment of Tonoplast H+-ATPase as an Initial Physiological Response of Cells to Chilling in Mung Bean (Vigna radiata [L.] Wilczek)

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Abstract
Biochemical alterations of cellular membranes in chilling-sensitive mung bean (Vigna radiata [L.] Wilczek) hypocotyls were investigated with reference to chilling injury. Reversible decreases in activities of tonoplast H+-ATPase and in vivo respiration became manifest within 24 hours of chilling when tissues suffered no permanent injury as assessed by electrolyte leakage and regrowth capacity. These changes were found to be the earliest cellular responses to chilling. A density-shift on a sucrose density gradient was observed in Golgi membranes early in the chilling treatment, suggesting that Golgi function and/or membrane biogenesis via the Golgi may have been altered upon chilling. After chilling more than 2 days, irreversible changes were generally produced in cellular membranes including the plasma membrane, endoplasmic reticulum, and mitochondria. Respiratory functions remained intact in mitochondria isolated from tissues prechilled for 24 hours, but were impaired after prechilling for 3 days. Given the important role of the tonoplast H+-ATPase in the active transport of ions and metabolites, the early decline in the tonoplast H+-ATPase activity may give rise to an alteration of the cytoplasmic environment and, consequently, trigger a series of degenerative reactions in the cells.