Cyanide-insensitive, Salicylhydroxamic Acid-sensitive Processes in Potentiation of Light-requiring Lettuce Seeds

Phytochrome-mediated germination of Lactuca sativa L. cv. Waldmann's Green seeds was inhibited strongly by 10 millimolar salicylhydroxamic acid (SHAM), but only slightly delayed by the same level of KCN. SHAM was most effective if applied within the 8-hour potentiation period (release from dormancy) following red light treatment, but much less effective with completely potentiated seeds. SHAM at 3 millimolar actually hastened completion of potentiation, whereas concentrations of 6.6 millimolar or higher retarded the process. A temporary upsurge of O(2) consumption was particularly evident during the period of most rapid potentiation (3 hours after red light), especially in the seed sections containing the embryonic axis. The embryonic axis obtained from dormant seeds also contained most of the SHAM-sensitive O(2) uptake. However, 8 hours of potentiation caused loss of SHAM sensitivity from axes and a simultaneous gain of SHAM sensitivity by cotyledons. Concomitant with this increased sensitivity to SHAM, O(2) uptake by cotyledonary tissues lost some sensitivity to KCN. Red light-stimulated metabolic processes leading to germination were blocked more effectively by SHAM than by KCN, but O(2) consumption by both dormant and nondormant seeds was much less sensitive to 10 millimolar SHAM than to the same concentration of KCN. This apparent contradiction between effects of SHAM on potentiation and O(2) uptake may be a result of: (a) compensatory electron flow through the cytochrome pathway at the expense of the alternate pathway; (b) a functional site of action of SHAM that differs from the organized, energy-coupled respiratory system; or (c) a combination of these possibilities.