Its production is regulated by a number of developmental and environmental factors. Ethylene pro-
duction is induced at germination, ripening of fruits, and senescence (abscission) by auxins and by
wounding and other chemical stress. It is also produced autocatalytically in the climactaric fruits. Many
of the effects of IAA, such as apical dominance and stomatal movement, are attributed to IAA-induced
ethylene production [58,101]. However, evidence with transgene-mediated auxin/or ethylene deficiencies
and mutants insensitive to either of the hormones has ruled out the notion that auxin-induced ethylene was
involved in the inhibitory influence that the apical bud exerts on the growth of lateral buds [58].
- Emergence and Seedling Growth
A seed is considered to have germinated when its radicle emerges through the outer covering. Cell divi-
sion and elongation start at about this time, and to most seed scientists it is the completion of germina-
tion. But from the crop physiologists’ point of view it is extended to include the processes that ensure
seedling establishment. Several fungal species and some bacteria are known to produce ethylene, includ-
ing those growing in the soil [98]. The ethylene released by these soilborne microorganisms is suspected
to influence seed germination, retard soilborne diseases, and regulate seedling growth. Thus, these stages
can be separated in two phases: radicle protrusion (phase I) and the subsequent events related to seedling
growth, dependent on seed reserves (phase II).
Two types of germination have been recorded in plants and crops: epigean, whereby the cotyledons
emerge above ground (Phaseolus vulgaris; garden bean), and hypogean(Pisum sativum; garden pea),
where the cotyledon remains below the soil surface. In the former case, the cotyledons emerge above
ground with the growing tip, due to the elongation of hypocotyl forming a hook or arch. With the emer-
gence of hypocotyl hook and exposure to light, symmetrical growth takes place and the hook straightens
up. In the second case, the plumule is arched or recurved near the apex, to protect the shoot tip, and when
it is pushed through the soil, exposure to light causes the epicotyl to straighten up. As it turned out, lo-
calized production of ethylene, at a rate of about 6 L/kg per hour (etiolated pea seedlings), was found
responsible for the formation and maintenance of the hook [17]. As the etiolated seedling emerges from
the soil or is exposed to white light, a transient decrease in ethylene is observed with concomitant straight-
ening of the shoot [17]. A similar explanation applies to seedlings that show epigeangermination (gar-
den bean) and develop a hypocotyl hook. It has further been observed that green tissues of seedlings are
not as sensitive to ethylene as are etiolated tissues. Ethylene responses of emerging dicot seedlings have
a survival value for the crop. Shortly after germination the hook is formed, in response to endogenous
ethylene, which helps the cotyledons or young leaves to emerge safely out of the normal soil. Under com-
pacted soils the hook and the primary root become unusually thick (i.e., they grow in diameter). This re-
sponse is elicited by the organs, perhaps due to enhanced ethylene synthesis by imposed mechanical
stress.
However, plants growing with their roots and stems submerged respond to ethylene by eliciting en-
hanced growth. This occurs due to accumulation of ethylene because of slower diffusion of the gas out of
the tissue and through the water. Among the species are Regnellidium diphyllum(water fern),
Nymphoides peltata,Rananculus sceleratus, and Callitriche platycarpa(star wort), which experience
submergence at least part of the time during their growth. During submergence the stems elongate rapidly
to keep leaves and upper stem parts buoyant. Submergence causes ethylene accumulation, which causes
stems or petioles to grow rapidly [102,103]. Deepwater rice exhibits similar phenomena, and internode
lengths of up to 0.6 m have been recorded and the plant completes its life cycle in several meters of wa-
ter [102]. These contrasting responses support the notion that depending on the condition, similar cells re-
spond differently to the same hormone [104].
- Stress Ethylene
A low level of ethylene is produced by plant species, but when tissues are injured by a variety of stresses
(wounding, pathogens, mechanical, chemical, temperature extremes, etc.), ethylene production increases
severalfold. This enhanced production, frequently referred to as wound- or stress-induced ethylene, in-
jures not only the tissues but also the site of ethylene production [105]. The intensity of enhancement in
stress ethylene has been related to gamma radiation dosage, and it has also been suggested as a rapid as-
say method for bacterial toxins [106]. The enhancement in its production has also been observed to cor-
relate with the number and size of foliar lesions induced by tobacco mosaic virus [106]. Visual injury was
suggested as the most sensitive and rapid technique to evaluate the plant response to acute pollution
PLANT GROWTH HORMONES 517