The dynamics of Trifolium repens in a permanent pasture. II. Internode length and the movement of shoot axes

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Abstract
White clover moves through pastures as a result of its clonal growth. The rate at which each shoot axis moves is determined by the rate at which new modules are produced at the apex, the rate at which old modules die at the base, and the length of internode separating adjacent modules. Internode length was much more variable than the birth rate of modules, and was affected by more factors. A multiple regression on ten variables accounted for only 78% of the variation in internode length, with a residual coefficient of variation of 36%. For any given level of the explanatory variables, 5% of internodes were longer than 2.5-times or shorter than 0.4-times the length predicted from the fitted regression model. Internode length is therefore more sensitive than birth rate to environment, and is affected by environmental factors not measured in this study. The variables that affected internode length were, in order of the magnitude of their effects: number, length and proximity of roots supporting an internode as it elongates; maturity of the shoot axis; mean soil temperature during the period of elongation; differences between plants; flowering; duration of bright sunshine during the period of elongation; number of mature leaves supporting an internode as it elongates; maturity of the module; mean stocking rate before birth and during the period of elongation; partial burial of the shoot axis. Axes moved an average of 13.3 cm a$^{-1}$. During summer, the average extension rate of mature apices was 0.7 mm per day, ranging up to 2.5 mm per day. Extension rate varied with temperature through a response of both birth rate of modules and length of internodes. In all other respects, within-plant variation in extension rate resulted almost entirely from variation in internode length.