adenosine ribohydrolase (adenosine nucleosidase) [72]. The ribosides are considered to be the transloca-
tion form, while ribotides are associated with uptake and transport across the cell membrane forms of cy-
tokinins. Thus, enzymatic regulation of bases, ribosides, and ribotides plays an important role in keeping
adequate levels of free and active forms of cytokinins in plants and crops.
C. Transport
It is paradoxical that plant parts that are meristematic or that otherwise have growth potential (young
leaves, buds and internodes, and developing fruits and seeds) are known to be the primary center of pro-
duction as well as the main sink for the endogenous cytokinins or its metabolites.
The detection of cytokinin, from the xylem exudate and phloem sap of a large number of plants and
crops, clearly indicates that nonliving as well as living tissues are involved in translocation. The polarity
of cytokinin movement is acropetal in the xylem, whereas it moves bidirectionally in the phloem. Thus,
in phloem, cytokinins move not only from organ to organ in the aerial portion but also from shoot to root
and vice versa [79]. Thus, the velocity of its movement is the same as that of other assimilates. However,
under in vitro conditions (donor-tissue-receiver system), 6-benzylaminopurine (BAP) is very poorly
translocated [80].
D. Biological Response
Cytokinins are known to evoke a diversity of responses when applied exogenously to whole plants, plant
tissues, or plant organs. Like other hormones, cytokinins have been used as a tool to investigate their role
as endogenous controllers of plant growth and development. Like other hormones, cytokinins influence
a multitude of morphological and physiological processes, among them seed germination, cell division
and cell elongation, promotion of cotyledonary and leaf growth, control of apical dominance, delayed
senescence, and morphology of cultured tissues.
As pointed out earlier, deficiency of a hormone must exist either experimentally or genetically to
show that adding hormone has an effect. Because cytokinins occur in all meristematic as well as in po-
tential growing tissues and organs, it is not possible to create experimental deficiency. Genetically engi-
neered cytokinin-overproducing tobacco and Arabidopsisplants have been used to study the phenomena
of apical dominance [81]. But in the absence of comparison with cytokinin-deficient mutants, the con-
clusion is equivocal. Thus, clear evidence is yet lacking which shows that specific physiological processes
in plants and crops are under the control of endogenous cytokinins.
- Bioassay
The physicochemical methods for isolation, purification, and identification of endogenous cytokinins
have improved rapidly, but it appears that bioassays will always be an integral part of the identification
process. Therefore, a number of bioassays have been developed and used by various investigators to test
the bioactivities of endogenous as well as synthetic cytokinins. They include (1) lettuce seed germination,
(2) radish (Raphanus sativus) leaf disk expansion, (3) Xanthiumleaf disk (chlorophyll preservation), (4)
soybean callus, (5) carrot phloem, (6) tobacco pith callus, (7) cucumber (Cucumis sativus) cotyledon
greening, (8) radish cotyledon expansion, (9) Amaranthusbetacyanin, (10) barley leaf senescence, and
(11) oat leaf senescence [82,83].
- Germination
Seeds that require preexposure to light for germination are called photodormant. Red-light exposure stim-
ulates, and far-red exposure inhibits, germination of a number of species, including lettuce (cv. Grand
Rapids) seeds. But the seeds of most crops do not require light because of natural selection against such
a requirement. Cytokinin-imbibed seeds germinate better in dark than do unimbibed lettuce seeds. Simi-
larly, cytokinin together with gibberellin effectively breaks the photodormancy of celery (Apium grave-
olens) seeds, but it was not as effective alone [84]. This indicates that red-light exposure may cause en-
hancement in the hormone level either by biosynthesis or by release from a bound form. However, such
information is lacking because the level of hormones in the radicle or hypocotyl cells, responsible for
germination, has not yet been determined.
PLANT GROWTH HORMONES 513