The effects of IAA and ethylene have been compared on the development of epidermal cells in intact shoots of etiolated pea plants. In the expanding sub-apical region ethylene has little effect on cell volume over 24 h but between 12 and 24 h rapid increases occur in cell wall thickening, part of which is due to the deposition of longitudinally orientated microfibrils adjacent to the plasmalemma. During this period, there is a rise in the levels of extractable cytoplasmic peroxidase. Two distinct phases of growth occur in response to IAA: an initial stimulation of cell expansion which causes the wall to stretch and decrease inthickness relative to the control (this phase is considered to be the true auxin response), followed some 12 h later by a decline in the rate of expansion and a thickening of the cell walls. During the first phase, peroxidase levels are depressed by IAA but a stimulation occurs after prolonged treatment. The effects observed in the second phase are believed to be mediated by ethylene which is synthesized at a high rate following treatment with IAA. Epidermal cells of mature internodes show a slight first phase expansion in response to IAA and their walls become a little thinner. Ethylene, however, has no effect on either expansion or wall thickness of mature cells even though the activity of peroxidase and the level of hydroxyproline-rich protein in the wall increases. These findings are discussed in relation to the dual regulation of cell growth by auxin and ethylene and the biphasic nature of the auxin response.
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