An Association Between Acid Invertase Activity and Cell Growth during Leaf Expansion inPhaseolus vulgarisL.

Abstract

Changes in lamina area, dimensions of epidermal and palisade cells, acid invertase activity and content of sucrose and hexose in the primary leaves of Phaseolus vulgaris L. were determined between emergence of the hypocotyl hook and the completion of leaf expansion. Growth in area and thickness of the primary leaf after emergence was attributable to the expansion of cells already present in the lamina at emergence. The major invertase in the expanding leaf was a readily soluble acid invertase; little insoluble invertase activity was detected. Soluble and insoluble fractions of leaf homogenates contained little neutral invertase activity. The specific activity of the soluble acid invertase increased rapidly during the early stages of leaf expansion, reaching a peak at the time of most rapid cell enlargement (5 d after emergence) and then declining as the leaf matured. Highly significant positive correlations were found between enzyme specific activity and the rates of cell and leaf enlargement. The early, rapid phase of lamina expansion was characterized by high concentrations of hexose sugar and low concentrations of sucrose. As the rates of leaf cell enlargement declined the concentration of hexose fell and that of sucrose increased. Between 5 d and 11 d after hypocotyl emergence, the hexose/sucrose ratio in the primary leaf decreased approximately 10-fold as the specific activity of acid invertase decreased. The results are discussed with reference to sources of carbon substrates for cell growth and to the sink/source transition during leaf development.