The mechanism of depth accommodation in Nymphoides Peltata and other water plants

Malone, M (1984). The mechanism of depth accommodation in Nymphoides Peltata and other water plants. PhD thesis The Open University.



Auxin and ethylene are shown to be important mediating factors in depth accommodation in Nymphoides, as in other water plants. Control of growth by both these regulators is shown to depend on their capacity to affect extensibility of the cell wall. Neutral sugar composition in the fraction of the cell wall most likely to be the site of the wall- loosening reactions is assayed, but no change in the proportions of the constituent monosaccharides was detected even where large increases in length were induced by auxin and ethylene. Thus no light was shed on this aspect of cell growth.

Using a variety of techniques, the effects of ethylene and auxin are assessed in the light of current theories on the mode of action of plant growth regulators; both auxin and ethylene are shown to stimulate proton excretion when they promote extension. Acid alone also stimulates extension. It is thus concluded that auxin and ethylene promote growth by an "acid-growth" mechanism i.e. they stimulate hydrogen ion efflux from the cytoplasm into the cell wall, the resultant pH drop in the wall causes an increase in WE and thereby leads to a substantial increase in the rate of cell expansion.

This is the first reported finding that ethylene works by an acid- growth mechanism when it promotes growth. The extension of "acid-growth" to this novel situation of rapid-growth promotion is important support for the theory. In a second species, Regnellidium diphyllum, predictions of the "acid-growth" theory were tested but despite repeated attempts, were not fulfilled. This is the only known tissue which shows rapid growth in response to auxin, but not to acid. Implications for the "acid-growth" theory, of the findings on both Nymphoides and Regnellidium are discussed.

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