the pronounced seasonal changes unfavorable for their normal growth and development. Walton [143]
concluded that “a role for ABA on the induction of bud and seed dormancy has been neither unequivo-
cally demonstrated nor disproven.” This is still valid with regard to bud dormancy in woody species. A
seasonal change in the ABA content of leaves, stem apices, and xylem sap of Salix viminalis[144] and in
the buds and stems of Acer saccharum[145] was observed. But these workers concluded that ABA did
not play a role in the photoperiodic control of bud dormancy. These works have received further support
from the evidence that cessation of seedling growth in Salixspp. is not regulated through the effect of day
length on ABA levels [146,147]. It is, however, possible that short-day conditions may have altered tis-
sue sensitivity to ABA [146]. These studies indicate further that in the control of bud dormancy, factors
other than ABA are possibly involved.
The results in the case of seeds are, however, different. Several studies indicate that ABA treatment
prevents vivipary (precocious germination of the developing embryo) in immature seeds. In vitro studies
have shown that a high percentage of germination was obtained when the ABA content of immature soy-
bean embryo was less than 4 g/g fresh weight [148]. Similar studies with cultured immature embryos of
wheat [149], soybean [148,150,151], cotton [152], rapeseed [153], and maize [154] have shown that ex-
ogenous ABA not only prevented precocious germination but often caused embryo growth and storage
protein accumulation. However, with the maturity of embryos, the endogenous level of ABA and the sen-
sitivity to exogenous hormone declined. Convincing evidence for the control of seed dormancy by ABA
has been provided by a reciprocal cross between wild-type and ABA-deficient Arabidopsismutants as
well as with the treatments of wild-type young maize kernels with fluridone. The reciprocal crosses indi-
cated that maternal ABA had a minor role [155], while treatment with fluridone induced precocious ger-
mination [156].
- Stomatal Control
The discovery that ABA plays a leading role in the regulation of stomatal movement generated the inter-
est of many workers [157,158]. Abscisic acid–deficient mutants are known in tomato, potato, pea, and
Arabidopsis[125], which reverts phenotypically to the wild types when treated with ABA. The response
is quite rapid, and after exogenous ABA application to the cut leaf bases, it takes 3 to 9 min to close stom-
ata in maize, sugar beet (Beta vulgaris), and Rumex obtusifolia[17]. The magnitude of stomatal response
to ABA is, however, dependent on the concentration of Kin the incubation media [159]. It has been es-
timated that when the stomates are closed, Kconcentration of the epidermal cells ranges from 250 to
450 mol/m^3 , but when Kconcentration falls to about 100 mol/m^3 , it opens. Harris and Outlaw [160] have
measured ABA levels in isolated guard cells using an enzyme-amplified immunoassay and observed that
water stress caused at least a 20-fold increase (up to 8 fg per cell). This may suggest that ABA causes sto-
mates to close by inhibiting an energy-dependent (ATP/cAMP) proton pump in the guard cell plasma
membrane. Thus ABA exerts two major biochemical effects. One is its effect on altering plasma mem-
branes, which by shutting off the proton pump stops influx of K, causing Kand water to leak out. This
reduces guard cell turgor, causing the stomates to close. However, the evidence of ABA role in stomatal
regulation discussed above is not unequivocal and there is evidence that suggests the involvement of other
factor(s), including hormone(s) and/or modification in tissue sensitivity [6,161].
VII. CONCLUSIONS
Currently, five classes of hormone—auxin, gibberellin, cytokinin, ethylene, and abscisic acid—are
known to be ubiquitous in higher plants and crops. Some of them have also been found to be produced by
bacteria, fungi, bryophytes, and pteridophytes. They influence a myriad of plant functions and responses,
and presumably any one process is influenced by the balance of the existing complement of hormones.
Hormone physiologists generally classify auxin, gibberellin, and cytokinin as growth promotors and ethy-
lene and abscisic acid as growth inhibitors. Although plant and crop hormones regulate a wide range of
growth and developmental processes, their diversity makes it difficult to assign a definitive role to them
from observations on plant responses. They influence each other’s level and thus play important roles in
a network of feedback control mechanisms modulating normal growth and development and thus pre-
venting odd overgrowths. At times, each of them can act as a promote or inhibitor, or vice versa, in this
network of feedback control mechanisms. Therefore, the categorization seems rather conjectural.
New research in molecular biology and biotechnology/genetic engineering has opened the door to
exciting approaches. Mutants are available that are either synthesis or response mutants, and genetically
PLANT GROWTH HORMONES 521