ETHYLENE BIOSYNTHESIS DURING AERENCHYMA FORMATION IN ROOTS OF MAIZE SUBJECTED TO MECHANICAL IMPEDANCE AND HYPOXIA

Germinated maize (Zea mays L.) seedlings were enclosed in modified tri axial cells in an artificial substrate and exposed to oxygen deficienc y stress (4% oxygen, hypoxia) or to mechanical resistance to elongatio n growth (mechanical impedance) achieved by external pressure on the a rtificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mech anical impedance exhibited increased rates of ethylene evolution, grea ter activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase , ACC oxidase, and cellulase, and more cell death and aerenchyma forma tion in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activ ity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onse t of stress and increases in both ACC synthase activity and ethylene p roduction was shortened by 2 to 3 h when mechanical impedance or imped ance plus hypoxia was applied compared with hypoxia alone. The synergi stic effects of hypoxia and mechanical impedance and the earlier respo nses to mechanical impedance than to hypoxia suggest that different me chanisms are involved in the promotive effects of these stresses on ma ize root ethylene biosynthesis.