Enhanced Ethylene Emissions from Red and Norway

Acidic cloudwater is believed to cause needle injury and to decrease winter hardiness in conifers. During simulations of these adverse conditions, rates of ethylene emissions from and levels of 1-aminocyclopropane-1-carboxylic acid (ACC) in both red and Norway spruce needles increased as a result of treatment with acidic mists but amounts of 1-malonyl(amino)cyclopropane-1carboxylic acid remained unchanged. However, release of significant quantities of ethylene by another mechanism independent of ACC was also detected from brown needles. Application of exogenous plant growth regulators such as auxin, kinetin, abscisic acid and gibberellic acid (each 0.1 millimolar) had no obvious effects on the rates of basal or stress ethylene production from Norway spruce needles. The kinetics of ethylene formation by acidic mist-stressed needles suggest that there is no active inhibitive mechanism in spruce to prevent stress ethylene being released once ACC has been formed. Environmental stresses, such as wounding, chilling, air pollution, drought, infection by fungi or insect attack, are all known to induce the formation of stress ethylene. This gas may be derived from methionine, via S-adenosylmethionine and l-aminocyclopropane-l-carboxylic acid (ACC') as shown below. Synthesis of ACC is the key regulatory step and 1malonyl(amino)cyclopropane-l-carboxylic acid (MACC) is formed as a side reaction (2, 3, 36). During the wilting of wheat leaves, for example, there is a sharp rise followed by a decline in the levels of endogenous ACC and the rates of production of ethylene, although the levels ofMACC rise and remain high throughout wilting (9). Similar changes in the rates of ethylene emission and levels ofACC and MACC have been reported in S02-fumigated wheat seedlings (18). The rapid decline ofACC levels and consequential fall in ethylene production in both cases can be attributed, in part, to the efficient conjugation of ACC to MACC. Thus, the malonylation of ACC has been suggested (9, 36) to be a regulative