TRANSIENT CHANGES IN LENGTH AND GROWTH OF WHEAT COLEOPTILE SEGMENTS FOLLOWING TREATMENTS WITH OSMOTICA AND AUXIN

The dependence of elongation on the osmotic potential of the medium was investigated, using coleoptile segments (CS) of Triticum aestivum L. (cv. Hartri) and an optoelectronic device. The study aimed at separating the osmoelastic response from the irreversible growth response when an osmoticum (mannitol) was added, and to compare both processes in order to consider the possibility of growth-induced reduction in turgor pressure. The prompt inhibition of elongation registered just after addition of 50 mM mannitol as well as the subsequent resumption of the original elongation rate could be quantitatively explained by the extent and the kinetics of the osmoelastic relaxation. An initial reduction in the irreversible elongation component by mild osmotic stress could not be demonstrated. Above a critical value, the irreversible growth was insensitive to a further increase in water potential. The minimum turgor pressure required to drive steady growth was not far from zero in both the presence and absence of auxin. The rate (r) of osmotically caused shortening per unit change of water potential r = 1/1 X dl/dt X 1/DELTApsi was determined from the kinetics of CS shortening induced by addition of mannitol at nearly isotonic concentration (300 mM). This parameter relates a fractional change in length to the difference in water potential between inside and outside, and was assumed to depend largely on the hydraulic resistance of the tissue and cuticle. It was found to be independent of IAA. The relatively low value of r suggests significant reduction of turgor at high growth rates. In accordance with this conclusion, the extent of osmoelastic shortening after a transfer to 300 mM mannitol (dependent on wall strain) was significantly decreased in the presence of IAA. Addition of 100 muM IAA to CS growing at a constant rate induced pronounced oscillations in the rate of elongation, which may be connected with the change in elastic cell wall strain. Whereas the steady state growth rate before the addition of IAA was the same in the presence and in the absence of 50 mM mannitol, the maximum growth rate found after addition of IAA was substantially reduced in the mannitol variant.