Phytohormones in needles of healthy and declining silver fir (Abies alba Mill) .3. Ethylene: The immediate ethylene precursor 1-aminocyclopropane-1-carboxylic acid and its malonyl conjugate

Levels of the immediate ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and its conjugate N-malonyl-ACC (MACC) were determined in needles belonging to different age classes of healthy and declining silver fir trees (Abies alba Mill.) in the northern Black Forest, Germany. Levels of MACC in fir needles exhibited a seasonal trend with a maximum in winter and a minimum in spring just before budbreak, whereas no common seasonal trend was seen in levels of ACC. Levels of ACC and MACC were found to be elevated in declining trees several times, indicating that ethylene production in such trees is temporarily enhanced. During a prolonged period with restricted water availability from the soil, however, declining trees produced less ACC and MACC. As indicated by needle levels of abscisic acid (ABA; Christmann et al., 1995), these trees had suffered from severe water stress while healthy trees had experienced only moderate water stress. It is concluded that the formation of ACC is inhibited in trees that: experience severe water stress, and therefore, less MACC accumulates. Levels of MACC may thus serve as a measure of tree health status only when severe water stress is absent. Levels of ACC rose with increasing needle age, indicating that ethylene production is high in older needles. Since levels of indole-3-acetic acid were high in older needles of healthy trees but low in older needles of declining trees (Christmann et al., 1996), premature needle loss of declining trees is probably caused by an enhanced production of ethylene in needles that are not protected by high levels of IAA. The possible factors that temporarily enhance ethylene production in declining fir trees are discussed.