THE EFFECTS OF OXYGEN, CARBON-DIOXIDE AND ETHYLENE ON ETHYLENE BIOSYNTHESIS IN RELATION TO SHOOT EXTENSION IN SEEDLINGS OF RICE (ORYZA-SATIVA) AND BARNYARD GRASS (ECHINOCHLOA-ORYZOIDES)

Exposing dark-grown seedlings for 3 d to oxygen deficiency (0 or 5 kPa) or to additions of carbon dioxide (10 kPa) or ethylene (01 Pa) slowed shoot extension in Echinochloa oryzoides, while in rice it was promoted by these treatments, except that 5 kPa oxygen was without effect. In E. oryzoides this was due to reduced growth of the mesocotyl, and in rice to enhanced growth of the coleoptile. These responses to carbon dioxide and oxygen deficiency were not consequences of increased ethylene production, since this remained unchanged by carbon dioxide and depressed by oxygen shortage in both species. Furthermore, exogenous ethylene and the ethylene action inhibitor 2,5- norbomadiene each failed to influence extension in anoxic seedlings, indicating no regulatory role for ethylene in the absence of oxygen. However, concentrations of the ethylene precursor 1 -aminocyclopropane-1 -carboxylic acid (ACC) were increased by carbon dioxide and 0 kPa or 5 kPa oxygen, although after 72 h without oxygen total ACC production (i.e. changes in ethylene + ACC + МАCC) was suppressed in both species. There was little effect on bound ACC [putative malonyl-ACC (МАCC)] formation. Transferring anaerobic (0 kPa) seedlings to oxygenated conditions (21 kPa) resulted in abnormally fast rates of ethylene formation, possibly due to the accumulation of ACC under anoxia. This post-anoxic ethylene may have contributed to the faster extension by rice coleoptiles and slower extension by mesocotyls of E. oryzoides compared with those of seedlings maintained continuously in air. © 1992 Annals of Botany Company.