VARIATIONS IN THE CHILLING SENSITIVITY OF SUSPENSION-CULTURED CELLS OF MUNG BEAN (VIGNA-RADIATA [L] WILCZEK) DURING THE GROWTH-CYCLE

Variations in the chilling sensitivity of mung bean (Vigna radiata [L.] Wilczek) cells in suspension culture were studied with reference to the growth cycle. At the initial stages after subculture, cells were relatively insensitive to chilling, but chilling sensitivity increased abruptly thereafter and reached a maximum at the early stage of exponential growth, namely 6 days after subculture. Upon further culturing, cells became chilling-tolerant, becoming most tolerant at the late stage of exponential growth, namely 14-15 days after subculture. The results of the reciprocal exchange of conditioned culture medium between the cells at the early and the late exponential phases of growth prior to cold incubation revealed that the change in sensitivity was due primarily to changes in physiological features of cells and not to changes in the chemical composition of the culture medium. In support of this possibility, we observed that the cold sensitivity of the tonoplast H+-ATPase in vivo changed markedly as a function of growth cycle: it was very sensitive at the early stage of exponential growth and less sensitive at the late stage.