including free ethylene glycol and its glucose conjugate, plus oxalate and a number of unidentified prod-
ucts [98]. However, metabolism to volatile products and metabolism of nonvolatile products are inde-
pendent of each other. Pea, tomato, cotton, carnation, and morning glory tissues have also been observed
to metabolize ethylene [98]. Studies based on nonbiological systems suggest that copper (Cu) is in-
volved in ethylene oxidation.
C. Biological Response
Ethylene elicits biologically spectacular responses at very low concentrations. As a plant hormone, it is
unique in its structural simplicity and in being gaseous in nature. Whether the term hormoneshould be
applied to ethylene, in that its translocationin the gas phase seems nonspecific, is a subject of debate. But
being moderately water soluble, it moves rapidly between tissues, with minimum hindrance, in either the
gaseous or the liquid phase. Therefore, there can be no doubt that it is a natural mobile growth regulator.
Unlike other hormones, it is not transported directionally but accomplishes its integrative function by dif-
fusing rapidly through the tissues.
At physiological concentrations, ethylene inhibits stem and root extension growth, but there are in-
stances where it increases the growth rate in Callitriche platycarpastem, in Helianthuspetiole, and in rice
stems and roots [100]. The myriad of plant responses and functions, including seed germination, cell di-
vision, epicotyl curvature, seedling growth, flowering, fruit ripening, response to stress, and senescence,
are known to be influenced by ethylene. The diversity of the processes in a wide variety of plants makes
it difficult to assign the hormone a definitive role.
516 NAQVIFigure 4 Pathway of ethylene biosynthesis. (Modified from Ref. 97.)