Invertase Activity, Carbohydrate Metabolism and Cell Expansion in the Stem ofPhaseolus vulgarisL.

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Abstract
In the stem of Phaseolus vulgaris L. the specific activity of acid invertase was highest in the most rapidly elongating internode. Activity of the enzyme was very low in internodes which had completed their elongation, in young internodes before the onset of rapid elongation, and in the apical bud. From shortly after its emergence from the apical bud the elongation of internode 3 was attributable mainly to cell expansion. Total and specific activities of acid invertase in this internode rose to a maximum at the time of most rapid elongation and then declined. Transfer of plants to complete darkness, or treatment of plants with gibberellic acid (GA3), increased the rate of internode elongation and final internode length by stimulating cell expansion. Both treatments rapidly increased the total and specific activities of acid invertase in the responding internodes; peak activities of the enzyme occurred at the time of most rapid cell expansion. In light-grown plants, including those treated with GA3, rapid cell and internode elongation and high specific activities of acid invertase were associated with high concentrations of hexose sugar and low concentrations of sucrose. As cell growth rates and invertase activities declined, the concentration of hexose fell and that of sucrose rose. In plants transferred to darkness, stimulated cell elongation was accompanied by a rapid decrease in hexose concentration and the disappearance of sucrose, indicating rapid utilization of hexose. No sucrose was detected in the apical tissues of light-grown plants. The results are discussed in relation to the role of acid invertase in the provision of carbon substrates for cell growth.