D. Biological Activity
It is reasonable to assume that if growth and developmental processes proceeded uncontrolled, the result
could be a distinct disadvantage. Plants have localized areas of cell division, the meristem, which is usu-
ally located at the tips of the growing organ(s). There are obvious structural limitations to an unlimited
elongation of stem internodes, leaves, or in fact nearly any plant organ. Thus, the programmed plasticity
exhibited by growth and developmental processes is certainly advantageous for plant survival, as shown
by their ability to become dormant or otherwise restrain their growth or reproductive activities to match
the alterations in their external environment. Plants do this by producing ABA to adjust their shoot and
root growth accordingly. Like other hormones, abscisic acid also has multiple physiological effects in in-
fluencing plant growth and development. The concentrations of ABA varies widely, from 3 to 5 g/kg in
aquatic plants to 10 g/kg in avocado (Persea americana) fruit mesocarp, and in the leaves of temperate
crop plants it is usually between 50 and 500 g/kg [130].
- Bioassay
The main impact of bioassays has been to study the growth-regulating properties of endogenous sub-
stances and aid in their isolation and identification in a pure form by existing physicochemical methods.
Despite the superiority of the physicochemical methodologies, the sheer simplicity of the bioassay is
likely to continue indefinitely to be used as an analytical tool. An enzyme-amplified immunoassay, with
a sensitivity of 0.05 to 2.5 pg of ABA, has been developed to measure the ABA content of mesophyll
and/or guard cells [131]. The majority of the bioassays have exploited the growth-inhibiting properties of
the ABA, and such diverse materials as Lemna, oat first internode or wheat coleoptile, lettuce and cu-
cumber hypocotyls, and rice seedlings were used. Stomatal closure response of Commelina communisand
barley and inhibition of hydrolases in barley aleurone layers have also been used effectively to detect the
presence or absence of ABA [132,133]. Depending on the test employed, these bioassays have been able
to detect ABA levels ranging from 10^6 to 10^11 M.
- Growth
Abscisic acid was the first inhibitory hormone known to be involved in the regulation of growth along
with growth promotors. At a concentration of 10^7 to 10^5 M it inhibits the growth of wheat coleoptiles,
barley shoots, bean axes, and the second leaf sheaths of rice seedlings [134]. It is generally accepted that
abscisic acid inhibits shoot growth, but its effect on root growth is contradictory and needs satisfactory
resolution. Exogenous ABA has been observed to promote as well as inhibit root growth [135,136], and
endogenous ABA has been shown to elicit similar responses. Under water stress conditions, the endoge-
nous ABA level increases manyfold, which in turn has been implicated to affect shoot and root growth
differentially (i.e., reducing shoot while maintaining root growth) [137].
Mulkey et al. [135] observed a triphasic response to ABA treatment: a period of promotion lasting
12 hr, followed by a similar period of inhibition (12 hr) and gradual recovery to about 80% of the normal
growth rate after 24 hr Robertson et al. [138] dried sunflower roots to enhance the endogenous ABA level
or treated them with exogenous ABA and observed a similar triphasic response. The initial transitory in-
crease in length was related to the initial rise in water potential in the root apices, followed by an inhibi-
tion and then a partial recovery in root elongation. On the other hand, Saab et al. [137], using fluridone
(an inhibitor of carotenoid biosynthesis) and a mutant deficient in carotenoid biosynthesis (vp5) to reduce
the endogenous ABA level in maize seedlings, concluded that ABA played a direct role in the inhibition
of shoot growth and in the maintainance of root elongation. However, Creelman et al. [139] concluded
that under water-deficit conditions, at all the internal ABA concentrations tested, root growth was inhib-
ited less than hypocotyl growth. But Plaut and Carmi [140] attributed the root response to the hydrotropic
nature of the organ, which induces it to reach to the wet soil rather than to any other factor(s).
Abscisic acid was also projected as causing differential growth in gravitropic responses of roots. But
later work using norflurazon or fluridone (inhibitors of carotenoid biosynthesis) and viviparous maize
mutantvp-9 (lacking ABA biosynthesis) showed that a drastic reduction in endogenous ABA level did
not alter root gravitropic response [141,142].
- Dormancy
Dormancy can be considered as the ability to retain viability while having minimal metabolic activity and
no visible growth. Plants and crops have evolved this strategy as a mechanism of survival to cope with
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