Abstract
The sequential growth response of excised wheat coleoptiles during growth and cellular differentiation to solutions of gibberellic acid, kinetin, and indolyl-3-acetic acid is further analysed in this paper. Excised yound coleoptiles which possess a low potential growth-rate (controls) respond to gibberellic acid and kinetin, whereas excised older coleoptiles which have a high potential growth-rate respond to indolyl-3-acetic acid. The difference between these results and those reported for pea-stem sections is discussed. To obtain a well-defined separation of the sequential growth response to gibberellic acid and kinetin it is necessary to grow the wheat coleoptiles at 22.5°C. At a higher temperature (25°C) the two optimal responses tend to overlap. It is suggested that this difference is due to a delayed onset of cell division at the lower of the two temperatures. Substances diffusing from the primary leaves contained within the excised coleoptile do not interfere with the sequential growth response of the coleoptile. Differential uptake of the growth regulators into the older coleoptiles does not explain the lower growth response stimulated in these coleoptiles by gibberellic acid and kinetin as compared to the high activity of indolyl-3-acetic acid. The possible importance of gibberllic acid in primary cell-wall growth is discussed.
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