SENSITIVITY OF THE CYTOCHROME AND ALTERNATIVE PATHWAYS TO OSMOTIC-STRESS IN LEAF-SLICES OF SAXIFRAGA-CERNUA L

Saxifraga cernua leaf slices were exposed to a range of sorbitol osmot ic potentials from 0.0 MPa to - 4.0 MPa to determine the relative sens itivities of the cytochrome and alternative respiratory pathways to os motic stress. Incubation of leaf tissue in osmotic potentials more neg ative than - 1.5 MPa significantly reduced oxygen=uptake along both th e cytochrome and alternative pathways. A 65 % decrease in cytochrome p athway activity was observed between - 1.5 MPa and - 2.5 MPa. No furth er significant reductions in cytochrome activity occurred at more nega tive osmotic potentials. Similarly, alternative pathway activity was s ignificantly inhibited at potentials more negative than - 1.5 MPa and was reduced by approximately 60 % at - 2.5 MPa. Further significant re ductions in alternative pathway activity occurred at more negative pot entials such that pathway activity was virtually undetectable by - 4.0 MPa. The decreases in the activity of both pathways paralleled the re duction in the respiratory capacity of each pathway. Rehydration of st ressed tissue returned respiration levels to control values indicating that osmotic stress did not cause permanent mitochondrial damage. The data did not support any differential stress sensitivity between the two respiratory pathways.