Some Physiological Changes Occurring during the Senescence of Auxin-Deprived Pear Cells in Culture.

Part of the changes in the hormonal balance involved in plant senescence is due to an auxin limitation. Some of its physiological consequences are studied using pear (Pyrus communis L.) cells cultured in a continuously renewed medium in which 2,4-dichlorophenoxyacetic acid (2,4-D) was absent. In these conditions, an assessment was made of the absence of nutrient deficiency.In the period preceding cell death, the rate of respiration and ethylene production remain low, and no major changes were observed in the total protein and RNA content of the cells. Beginning around day 9, an important efflux of three amino acids (serine, threonine, and aspartic acid) occurs among which serine represents more than 52%. However, exogenous serine supplied to the medium fails to show any senescence promoting effect. At the same time, leucine uptake and incorporation sharply and simultaneously increased. The presence of 2,4-D inhibits both these phenomena and prevents cell death. It is proposed that auxin deprivation is responsible for unmasking a program of synthesis of new proteins involved in cell death.