RELATION BETWEEN GROWTH AND OSMOREGULATION IN WHEAT COLEOPTILES

Osmoregulation of isolated wheat coleoptiles [Triticum aestivum] was investigated during the course of their elongation growth. Results concern the concentration of osmotically active substances in the tissue (OC, mosmol per kg fresh weight), their total amount in the sample (OS, .mu.osmol per 30 segments) and the difference is osmotic potentials of cell sap and medium solution (.DELTA..psi..pi., bar). The favourable effect of glucose (50 mM) on elongation growth of isolated coleoptile segments may be explained by an improved ability for osmoregulation. After the tissue concentration OC had been strongly diminished by growth in sugar free medium the rate of auxin-stimulated growth was also decreased and remained below that of segments grown in glucose medium. An additional decrease of osmotic potential of the glucose medium by 50 mM mannitol (-1.3 bar) induced a significant increase in the accumulation of intracellular OS. Higher mannitol concentrations (up to 250 mM) did not amplify this effect. IAA (1 and 5 .times. 10-6 M) stimulated the production of OS in glucose medium irrespective of reductions in water potential by additional mannitol up to 150 mM. After a growth period of 24 h the total amount of OS reached the same value at the different mannitol concentrations (0-150 mM), whilst elongation was strongly inhibited by osmotic stress. The promotion of OS accumulation of IAA was stronger than the promotion induced by mannitol in the absence of IAA. Both IAA and stress induced stimulations of OS accumulation in growing coleoptile segments were not found in the absence of glucose. More severe osmotic stress (200 and 250 mM mannitol) inhibited the OS accumulation in the presence of auxin. The role of osmoregulation in the elongation growth of coleoptile segments is discussed.