This study analysed the dynamics of cell production and extension, and how these were aected by applied gibberellic acid (GA 3 ), during internode development in dwarf peas (Pisum sativum L. cv. Meteor). Image analysis was used to obtain cell number and length data for entire cell columns along the epidermis, the two outermost cortical layers, and the pith, from internode 7, over a time period covering the whole of the internode's growth phase. For a few days following the inception of an internode at the shoot apex, little further growth occurred, and there was no signi®cant eect of GA 3 on cell division or cell extension. The subsequent growth of the internode was stimulated more than fourfold by GA 3 as a result of the production of more than twice the number of cells, which were twice as long. At least 96.5% of the cells of the mature internode were actually formed within the internode itself during this period of growth, demonstrating that the internode cells themselves represent the morphogenetic site of response to GA 3 . Mitoses and cell extension occurred along the full length of the internode throughout its development. The daily changes in cell numbers were modelled by the Richards function, and manipulations of the ®tted functions to reveal time trends of absolute and speci®c cell production rates were performed for each stem tissue. The increase in cell numbers in the +GA 3 plants was brought about by an increase in the rate of cell production, over a shorter time interval; speci®c cell production rates declined continuously from initial rapid rates in the +GA 3 epidermis and pith, but declined more slowly in the cortex. The control (AGA 3 ) epidermis and cortex cells exhibited a constant speci®c cell production rate (i.e. purely exponential) for several days. Cell extension rates were calculated so as to compensate for the size-reduction eects of concurrent cell division. These calculations con®rmed that`real' cell extension rates were higher in the +GA 3 internodes. Models of the cellular controls of internode growth, based on the estimated dynamics of cell division and extension, are discussed.Abbreviations: ANOVA = analysis of variance; CDT = cell doubling time; GA 3 = gibberellic acid Correspondence to: I.M. Scott;

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