EFFECT OF OSMOTIC-STRESS ON THE RESPIRATORY PROPERTIES OF SHOOTS AND ROOTS OF ARNICA-ALPINA

Arnica alpina plants were exposed to 0.3 or 0.6 M sorbitol for 5 days to investigate the effect of long-term osmotic stress on the cytochrom e arid alternative respiratory pathways in both shoots and roots. Expo sure to 0.3 M (-0.72 MPa) sorbitol significantly reduced growth in bot h shoots and roots. Increased alternative pathway activity was observe d in shoots, exposed to 0.3 M sorbitol whereas increased cytochrome pa thway activity, with a concomitant increase in the theoretical rate of ATP production, was found in the roots. Despite a reduction in shoot and root ethanol-soluble sugars, respiration was never substrate limit ed. These results suggest that the reduced demand for shoot growth ene rgy was offset by an equally increased demand for maintenance energy. In roots, however, additional ATP production was required to meet the metabolic demands of exposure to 0.3 M sorbitol. Exposure to 0.6 M (-1 .44 MPa) sorbitol completely stopped shoot and root growth and decreas ed respiration after only 1 day . In shoots, cytochrome pathway activi ty was inhibited, whereas in roots both pathways were transiently stim ulated. By day 5, respiration along both pathways was significantly in hibited. The possible role of the alternative pathway as an energy ove rflow and an energy overcharge during osmotic stress is discussed.