Plant Physiology
A REVIEW OF PLANT GROWTH REGULATING CHEMICALS
I N SUGARCANE CULTIVATION
A. J. Vlitos
Tate & Lyle Limited, University of Reading,
PO Box 68, Reading, RG6 2BX, England
ABSTRACT
The future of sugarcane cultivation in many parts of the world may depend
on how well plant growth regulating chemicals can be incorporated into
cultivation systems. Economic factors have made it difficult to rely on handlabour to grow and harvest the crop, and competition from sugarbeet grown
in more highly developed and mechanized areas is bound to influence the
future of sugarcane.
Research has shown that certain types of growth regulants influence
germination, growth and sucrose storage in a wide range of cane varieties.
Field trials have indicated the feasibility of applying such compounds under
practical conditions at various times during the development of the crop.
There is little doubt that more research is required t o determine the long-range
consequences of applying new growth regulants on a large scale, but the
significance of the contribution of plant physiology to the future of a major
world crop is likely t o be a most important limiting factor.
INTRODUCTION
This paper deals with plant growth regulators and more specifically their
probable r81e in sugarcane cultivation in the future. This is a subject of topical
interest. I n view of severe competition from sugarbeet there is a need to improve
yields of sucrose from cane. The r8le of plant growth regulators can be vital
in this.
Over the past 10 years my colleagues, B. H. Most, H. Cutler, A. Yates,
and others have established that at least 4 groups of naturally-occurring plant
growth regulators occur in sugarcane a t various stages in its development.
Auxins, gibberellins, cytokinins and various growth inhibitors have been shown
to occur either as free acids or bases, and also as conjugates in which the acid
or base is associated with a sugar moiety such as glucose or ribose. The auxins
have been isolated from stem apical t i ~ s u e ,from
~
the rootsl1 and from the
seed.34 Gibberellins have been found in extracts and diffusates of immature
stem and leaf apical tissue~,lg-~~
in bleeding sap from cut stumps of sugarcane
stems,26in the roots and in dormant and developing lateral b ~ d s Cyto. ~
kinins in sugarcane occur in bleeding sap and in dormant lateral
Abscisic acid (ABA) has been isolated from immature stem and leaf apical
tissues together with several other unidentified growth inhibito~-s.z7
IAA and
abscisic acid have been found so far only as the free acids, while the gibberellins
and cytokinins have been found in the free and conjugated forms - but readily
water soluble.
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A . J. VLITOS
The occurrence of all 4 types of plant growth regulators in stem apical
tissues suggested that the regulatory mechanism in cane involves changes in
the balance between the 4 types. I t is characteristic of the hormonal regulation
of plant growth that more than one type of growth hormone is involved in
any one growth process and that their functions often overlap.32
Thus, auxins and gibberellins are both involved in promoting cell elongation, while auxins and cytokinins may both stimulate cell division. I n other
situations the actions of the same two hormones may be similar or opposed.
For example, the presence of cytokinin and auxin enhances growth of callus in
tissue culture30 while these same two hormones oppose one another in the
germination of lateral
THEORETICAL CONSIDERATIONS
The discovery by Most that sugarcane is rich in gibberellins and cytokinins
and that ABA occurs in apical stem and leaf tissues, led him to the hypothesis
that stem elongation in sugarcane is regulated by variations in the amounts of
gibberellin and cytokinin, translocated from the roots to the stem apex.2sThese
variations are induced by water-stress, changes in temperature and other
environmental pressures. I t has been known for quite a long time that elongation of sugarcane stems under field conditions is inhibited by drought or by
low temperature and that there is usually a corresponding increase in the percentage of sucrose extractable from the crop. I t has been shown that, when
the supply of gibberellins and cytokinins to the stem apex is plentiful, the growth
inhibitory effects of abscisic acid are counteracted. Growth is then rapid, and
sucrose levels drop. When the gibberellin supply is restricted, growth rate is
reduced -more sucrose is then stored, but also there is a tendency for more
cellulose to be formed.
In immature, stem apical tissues of fast-growing sugarcane there is sufficient
auxin to allow a response of the tissues to gibberellins and cytokinins. There
are also sufficient amounts of gibberellins and cytokinins to promote cell
elongation and cell division, and to counteract the premature ageing of
immature cells by ABA.
In slow-growing cane the supply of gibberellins and cytokinins to the
meristem is restricted. Cell division and cell elongation almost cease and the
synthesis of ABA is increased. There is then premature maturation of the cells.
I n this state more sucrose is stored but much of the carbohydrate is incorporated into cellulose and structural materials, but at different rates.
The so-called "suspended" state is one which represents an ideal and which
can serve as a criterion for selecting chemical ripening agents for practical use
in the field. The ideal chemical ripener would suspend all growth processes,
cell division, cell elongation and differentiation as well as the formation of
new cell wall material. All tissues would be maintained in an unchanged condition and all available carbohydrate would move into storage as sucrose. No
doubt there is a particular hormonal balance which could produce this effect.
I n practice it is necessary to settle for something less than the ideal. I t is possible
to get a gradual progression from a state of rapid vegetative through a phase
of suspended growth to one of slow growth, which can be fitted into the
harvesting programme.
I
934
PLANT PHYSIOLOGY
SYNTHETIC CHEMICALS AND THEIR EFFECTS ON SUCROSE STORAGE
In the early work on cane ripening, much attention was given to the
possible use of 2,4-D. Some authors reported increases in sucrose content of
cane when 2,4-D had been applied as a foliar pray;^?^ others were unable to
, ~ ~believe that the variable results might
obtain a similar r e ~ p o n s e . l ~ - l 7We
have been due to differences in environmental conditions.
Allen3 found that sub-lethal applications of 2,4-D to cowpeas increased
the activity of phosphofructokinase (PFK) activity in the light, but decreased
the activity of the enzyme in the dark. At a 12-hour photoperiod and constant
temperature of 25,6 C PFK is an important regulator of glycolytic respiration
in plants. A reduction in its activity probably indicates a lowered rate of
respiration. If 2,4-D has a similar effect on cane tissue then photoperiod and
temperature could influence its effectiveness as a ripener. Best results might
be expected under short day conditions with warm nights. I n fact one may find
that in those areas of the tropical world such as Cuba where 2,4-D has given
positive results, the photoperiod and night temperatures fit the ideal situation.
Another synthetic auxin, 2,3,6-trichlorobenzoic acid (TBA) has been
shown by Nickel1 and Tanimoto in Hawaiiz9 and by ourselves35 to be an
effective ripener. Also mixtures of 2,3,6TBA and MCPA35 have been found
effective by some and not effective by o t h e r ~ . l Our
~ > ~group has studied the
response of cane to this mixture in some detail, and Y a t e ~
published
~~
the
following results.
The chemical causes a n initial acceleration of stem elongation, followed
by a long suppression of growth. The initial growth response is largely confined
to one immature internode, but the growth of the younger internodes is completely suppressed. Similar results are obtained with applications of MCPA
or TBA alone. Senescence of the oldest leaves is accelerated, and the emergence
of new leaves can be prevented.
The response: of cane to MCPA-TBA mixtures may be interpreted in
terms of its possible effects on the internal hormonal balance. I n rapidly
elongating immature stem tissues the relative levels of the 4 types of hormones
are as follows:
- medium
auxin
high
gibberellin
high
cytokinin
ABA
low
Treatment with MCPA-TBA raises the auxin level and since this particular.
synthetic auxin is not readily degraded by cane tissues this increased auxin
level would persist. The higher auxin level would initially enhance the activities
of the gibberellins and cytokinins, thus promoting cell elongation and cell
division. Soon afterwards the higher level of auxin would tend to promote the
synthesis of ethylene. This could lead to auxin destruction, particularly of the
native auxin, and to the oxidation of metabolites and certain enzymes. The
growth-promoting activity of the gibberellins and cytokinins would be negated
and growth would be suspended. Premature senescence of the oldest leaves
may be due to ABA. Apart from its effect upon leaf tissues, the MCPA-TBA
mixture is an effective ripener. Complementary administration of a gibberellin
or a cytokinin might .overcome this effect by balancing the increased activity
of ABA.
A. J. VLITOS
The effect of GA on the accumulation of sucrose by cane has also been
studied in recent years. Villareal and Santos33 in the Philippines and Coleman1°
and his co-authors found no significant effect on sugar yields. But in Hawaii31
and Queensland12 applications of GA may increase sucrose yields, especially in
areas with relatively low (sub-tropical) temperatures.
Bull,7 in greenhouse experiments in Queensland found that weekly applications of GA over a 7- to 8-week period increased the length, fresh weight
sugar
- and fibre contents of the stalks, but decreased leaf area and weight. The
response of young (3-month-old) plants was greatest at low temperatures (1 7 C),
but older (6-month-old) plants gave the largest response a t higher temperatures (35 C).
I n terms of its possible effect upon the hormonal balance, treatment of
sugarcane with GA would be expected to stimulate stem elongation, particularly when the level of auxin is fairly high (i.e. in slow-growing cane). This
hormone might also tend to retard senescence by opposing the activity of
ABA. If this is so. the observed reduction in leaf area due to GA may to some
extent be compensated for by a delayed senescence of old leaves. I t is known
that GA does not increase photosynthetic activity per unit leaf area.13
Preliminary experiments37 with ABA and with 'Ethrel' (2-chloroethane
phosphonic acid), a compound which releases ethylene in plant tissues, indicate
that these chemicals may have potential as cane ripeners. Both retard stem
elongation for about two weeks and this is followed by a period of rapid 'compensatory' growth. As might be expected, ABA treatment hastens senescence
of the older leaves.
Treatment with Ethrel causes the loss of apical dominance as evidenced
by the development of side shoots, tillers and strut roots. This is consistent
with the finding that ethylene stimulates peroxidase activity in plant tissues,
leading to the destruction of auxin.14
Hormone transport is obviously an important step in the response of plants
to environmental stimuli. There is evidence that most hormones are translocated
in the plant as free acids (auxins) or readily water soluble conjugates (gibberellins and cytokinins). Several chemicals are known to inhibit auxin transport.18
One of these, 2,3,5-tri-iodobenzoic acid, has been tested on sugarcane but was
found to be a weak growth retardant.37
Chemicals which suppress the synthesis or activity of enzymes involved in
the utilisation of sucrose may prove useful as ripeners. Thus Alexander2 found
that treatment of sugarcane with 6-azauracil, a pyrimidine analogue which
may inhibit RNA synthesis, caused a decline in growth and invertase activity,
accompanied by an increase in sucrose content. The leaves of the plants were
not affected. but the chemical caused malformation or death of meristematic
tissue at the stem apex. Alexander2J also reports increased sucrose accumulation
as a result of treating sugarcane with molybdenum or tungsten, which inhibit
phosphatase activity, and with silicon, which inhibits invertase activity. I t is
interesting to note that co-administration of ,GA with 6-azauracil or with
silicon appears to enhance their ripening effect.
REFERENCES
1. Alexander, A. G. (1965). Introduction of varying sugar levels in the leaves of immature
sugar cane by use of acid phosphatase inhibitors, J Ag Univ P Rico, 49:35-59.
936
PLANT PHYSIOLOGY
2. Alexander, A. G. (1969). The use of chemicals for sucrose control in sugar cane, Sug
Azuc, 64:21-25.
3. Allen T . J. (1969). The effect of 2,4-dichlorophenoxyacetic acid 011 the activity of
phosphofructokinase in Vigna unguiculata (L.) Walp, leaf tissue, PhD dissertation, Texas
A & M Univ, USA.
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5. ~ e a i c h a m ~
C., E. (1950). Effects of 2,4-D on sugar content of sugar cane, Sug J,
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of vegetative buds to maturity, Physiologia P1, 15:27-42.
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14. Hall, W. C. and Morgan, P. W. (1964). Auxin-ethylene interrelationships, In: Regulateurs de la Croissance Vegetale, ed by J. P. Nitsch, CNRS, Paris, 727-745.
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Grow Q Bull, Queensland, 17 :52-53.
16. Loustalot, A. J., Cruzado, H. J. and Muzik, T. J. (1950). The effect of 2-4,D on sugar
content of sugar cane, Sugar J 13 (5) :78.
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P Rico, 37 :44-51.
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17 :283-294.
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A. J. VLITOS
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REVISTA DE LOS PRODUCTOS QUfMICOS REGULADORES
DEL CRECIMIENTO DE LA PLANTA EN EL CULTIVO DE LA
CARA DE AZUCAR
A. J. Vlitos
RESUMEN
El futuro del cultivo de la caiia de azljcar en muchas areas del mundo,
depende en gran medida en la forma eficiente en que se incorpore en las
sistemas de cultivo, productos quimicos reguladores del crecimiento de la
planta. Factores econdmicos hacen dificil depender de mano de obra para
cultivar y cosechar este cultivo, y la competencia de el azljcar de remolacha
que se siembra en areas altamente mechanizadas, influyen en el futuro de la
caiia de azljcar.
La investigacidn ha demostrado que ciertos tipos de reguladores de
crecimiento, influyen en la germinacidn, crecimiento y almacenamiento de
sucrosa en muchas variedades. Pruebas de campo indican la posibilidad de
aplicar estos compuestos bajo condiciones pricticas en varias ocaciones
durante el crecimeniento de este cultivo.
No hay duda, que hace falta mas investigacidn para determinar las
consecuencias a largo alcance de la aplicacidn en gran escala reguladores
nuevos de crecimiento.
La importancia de la contribucidn de la fisiologia de plantas para el
futuro de este importante cultivo mundial, es ciertamente un factor limitante.