The following points highlight the eight main commercial applications of plant growth regulator. The applications are: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene-Releasing Compounds 5. Abscisic Acid.
1. Commercial Applications of Auxins:
i. Auxins as Herbicides, Ripeners, and Rooting Inducers:
The most widespread commercial application of auxins as herbicide is well known. Auxins like (2,4- dichlorophenoxy) acetic acid (2,4-D) and (2,4,5-trichlorophenoxy) acetic acid (2,4,5-T), which act as herbicides when used in large doses.
Dicotyledonous weeds are particularly sensitive to very high auxin level. Some of the most valuable and widely used selective herbicides in the weed control are auxins, particularly the phenoxyacetic acid analogous (e.g., 2, 4-D, 2, 4, 5-T, and McPA).
2, 4-D is one of the most widely used herbicides. It is highly selectively, non-corrosive, effective at low concentrations, safe to handle, relatively easy to formulate, and economical to use. Several benzoic acid analogs (e.g., dicamba, chloramben, and substituted picolinic acid, picloram [Tandon]) are also important herbicides.
R. J. Weaver (1972) proposed another use of auxin in agriculture by using the principle of inhibition of abscission layer formation, auxin (e.g., NAA, 2, 4- D) are effective to control fruit drop in apple and pear. Auxin including 2, 4-D, induced ethylene formation and fruit set in pineapple.
Biennial bearing (light and heavy crops in alternate year) is common to many tree crops. This is a great problem for agriculturists. By corrected and timely thinning in the heavy years and timely application of NAA or other auxins can solve the problem.
2, 4-D enhances fruit set and fruit ripening particularly in pineapple cultivation. Induction or forcing of flowering is the major problem in pineapple. This was achieved initially in the field by 1-naphthyl-acetic acid (NAA), which is also used as a rooting agent. Ethephon is also preferred for this purpose.
Commercial use of rooting auxin compounds is very common. They promote callus and root formation, which can improve establishment from cuttings. Dipping the cut surface into rooting compounds enhances species and cultivars difficult to root. Auxins are also used in the prevention of sprouting of stored potatoes by dipping them in NAA solution or by sprinkling with talc or fuller’s earth containing auxin, or stored with strip of paper impregnated with auxin solutions.
Indolyl butyric acid (IBA) is also used in inducing the rooting of cuttings. In vitro rooting of shoots of antonovka 313 apples was achieved by using a medium containing 0.25 mM indolyl butyric acid and 1.5% sucrose. Promotion of rooting by auxins is sometimes enhanced by the presence of some phenolic compounds, which have a synergistic effect.
More roots were produced in cuttings of Phaseolus vulgaris L. in the presence of both indole butyric acid and 4-chlororesorcinol, an effect explained by the inhibition of polyphenol oxidase (PPO) activity.
Glyphosine [N, N-bis (phosphonomethyl) glycine], an auxin-like substance is also used as sugarcane ripeners. Its action, primarily, is as herbicides, to prevent late growth of the cane so as to cause a major part of the assimilate to be stored as sugar.
The compound has enabled a marked increase in the yield of sugar. Other important sugarcane ripeners are ethephon, 2-chloroethyl-phosphonic acid, and glyphosate N-(phosphonomethyl) glycine.
The use of ethephon has enabled an increase in the yield of rubber from Hevea brasiliensis by 200% to 300%.
Synthetic auxins or auxin-synergists like 2,4-D, 4-CPA (4- chlorophenoxyacetic acid), and N- naphthylphthalamic acid are used widely to increase fruit set in vegetables, for example, with tomatoes grown in glasshouses or under plastic tunnels, or with aubergines. Highly effective synthetic auxins, which are derivatives of benzotriazoles, were recently synthesized, using a design approach based on QSARs.
ii. Menadione Sodium Bisulfite (MSB):
Since the presence of auxin is necessary for plant growth, an inhibition of the degradation of this hormone may enhance growth. Decreased activities of the enzymes involved in IAA oxidation were observed after the application of Menadione Sodium Bisulfite (MSB).
Following the addition of MSB to tomato, cucumber, corn, and capsicum plants, the concentrations of IAA in the plant cells were increased by up to four fold relative to the untreated plants.
Enzymatic studies in tomato plants sprayed with 10-5 M MSB indicated significant decreases in the activities of indole acetic acid oxidase and of peroxidase, but had almost no effect on ascorbic acid oxidase and of peroxidase, but had almost no effect on ascorbic acid oxidase and polyphenol oxidase.
The application of MSB also resulted in remarkable increases in the growth of tomato plants, alfalfa callus, and the rooting of mung bean cuttings. Spraying tomato plants twice during their growth with a 10-5 M solution of this compound caused an increase of 40-80% in fruit yield per plot, relative to the untreated plants.
The water-soluble part of MSB is the sodium sulfite adduct of menadione (vitamin K3, 2-methyl-1, 4-naphthaquinone). Soil application of a powder containing a mixture of menadione bisulfite. Tween 20 and diatomaceous earth enhanced the growth of roots and aerial parts of rice by 32% and 22%, relative to the control.
2. Commercial Applications of Gibberellins:
Since the discovery of GAs in the 1920s a considerable body of information has emerged regarding the roles these hormones play in plant growth and development. As these compounds and analogs became available, either by fermentation or chemical synthesis, a number of potential practical applications were identified.
Gibberellins have been primarily used for manipulating production practices and insuring the quality of high value specialty crops such as grapes, citrus, cherries, and apples.
Increasing market demands for a greater variety of quality fruit may provide new opportunities for the expanded use of Gas (i.e., tropical fruits, thinning and sizing wine and new table grape varieties, and as components of integrated pest management programs for postharvest quality of fruits).
Despite considerable research, GAs have not found utility in major agronomic crops. However, increased understanding of the interrelationship between GAs and other PGRs, concurrent with the development of new GAs or GA-mimics, may offer the unique activities necessary to penetrate agronomic markets.
However, despite extensive research with GAs the number of commercial uses has fallen short of their predicted impact and growth in agriculture. Unique opportunities may still result from new research and development with GAs to meet the future demands for increased food quality and production.
Although over 97 different GAs have been identified in plants, only three (GA3 and a mixture of GA4+7) are available via fermentation by the fungus Fusarium moniliforme (= Gibberella fujikuroi) in reasonable quantity for extensive applied research. The complexity of GA chemistry has precluded the entirely synthetic routes of production. With the diversity of physiological processes in which GAs are involved, it is not surprising that their practical application spans the entire range of plant growth and development.
The majority of these uses however, represent specially niche markets in high cash value horticultural commodities. Despite the extensive research with GA3 over the past 30 years, no defined GA product has been registered for use on a major agronomic crop. This may be due in part to a lack of specificity of response where multiple effects, often detrimental (i.e., lodging), are manifested from exogenous application of GA.
i. Floriculture:
GA3 is also used to thin approximately 60% of the flowers because almost every flower tends to set, resulting in a tightly packed cluster. The physiological basis for grape flower thinning by GA is unknown, but could be due to pollenicidal activity. The result of adequate thinning almost appears excessive, but with two subsequent applications of GA, berry size is increased by 60%. The final product is a loose cluster that does not require hand thinning and is less susceptible to Botrytis bunch rot.
GA3 also inhibits flowering on several stone fruits. In young sweet and tart cherries, GA3 is applied to prevent excessive flowering to minimize the competitive effect of early fruiting on vegetation growth. Since cherry has true flower buds, the node following flowering will not support vegetative growth. If flowering is excessive, fewer spurs develop, thereby resulting in lower yield potential during the production life of the tree.
The manipulation of flowering by GA3 has also been integrated into a management program to suppress a pollen transmitted ring spot virus infection as well as the maintenance of the desired ratio of vegetative to flower bud formation in mature tart cherries.
The consequence of continuous GA3 applications over a five years period has been a 22% increase in yield. A mixture of GA4+7 is commercially available. It effects on varying structure in the flowering of Lolium temulentum, a LD-requiring plant and Pharbitis nil, a SD-requiring plant, as well as for flowering of Cupressaceae and Pinaceae conifers. While it is too early to predict commercial implications of such specificities in flowering response for GAs.
ii. Fruit:
An additional application of GA3 to sweet cherry when fruit is light green to straw coloured will delay harvest, increase fruit size and firmness. While this application is not new, the recent development of an export market to Asia for cherries has significantly increased this use.
a. Fruit Reset:
The other commercially used GAs produced by Fusarium moniliforme cultures are a mixture of GA4 and GA7. An application of GA4+7 of increasing importance is the prevention of fruit russet, a superficial disorder in which the exocarp is interrupted by raised corky outgrowths over the surface of green and yellow apple cultivars.
Although somewhat obscure, a number of factors have been implicated for russet, such as high humidity, presence of free water, light frosts, pesticides and powdery mildew. The multiple applications on ‘Golden Delicious’ effectively increase the fruit finish quality and this corresponds with higher packout gardens.
b. Grape Industry:
GA3 has been used in the seedless table grape industry since 1960 to manipulate three physiological events; rachis cell elongation, flower thinning, and berry enlargement. The first is a pre-bloom spray to promote elongation of the rachis to provide a larger and looser framework for the grape cluster.
In wine grapes, there is a need for cluster loosening by either rachis elongation and/or flower thinning to prevent Botrytis bunch rot. To date, there is no chemical thinning agent for wine grapes. Results with GA3 have been variable with some varieties exhibiting a dramatic reduction of return bloom the year following application; others are relatively insensitive to increasing rates of GA3. The response of wine grapes to other GAs certainly needs to be examined.
c. Citrus:
The effect of delayed fruit senescence by GA3 has also been exploited in the citrus industry. GA3 delays maturity of citrus rind without altering the maturation process of the internal fruit, thus preventing several rind disorders associated with fruit ripening. In lemons, this facility scheduling of harvesting to synchronize with market demands.
As harvest date is delayed in grapefruit, there is added benefit in combining 2, 4-D to retain the fruit on the tree. Recent findings suggest that the delay in citrus rind maturity is associated with the quantity and quality of epicuticular wax deposition and its influence on respiratory gas exchange.
d. Apple:
More recent results suggest that GA4 may promote apple flowering. The differential effect of GA4 and GA7 in apple serves to illustrate the specificity associated with a specific GA. However, separation of these compounds on a large scale is presently cost prohibitive.
iii. Seed Production:
There are also minor practical uses of GAs in seed production of commercially important conifers. GA3 can be used to promote flowering, and thereby significantly increase female cones and seed per tree in seed production orchards of Cupressaceae and Taxodiaceae species.
iv. Protection against Plant Pathogens:
The use of GA to modify growth for protection against plant pathogens in cherries and grapes and is becoming an increasingly recognized potential benefit of PGRs in a number of crops.
v. Insect Control:
Recent laboratory studies have shown that GA3 application significantly reduces the number of probes and egg fecundity of the fruit fly due to increased exposure to toxic peel oils contained in the juvenile rind. If this effect is manifested in the field, a high level of resistance may preclude the need for post-harvest treatment with an insecticide/larvicide.
vi. Applications on Agronomic Crops:
The successful introduction of a GA product in an agronomic crop is dependent upon the specificity of a physiological response over a wide range of environmental/cultural conditions, and is complicated by the inherent genetic variability amongst cultivars.
For example, the genetic reduction of plant height has led to dramatic increases in cereal grain yield, due principally to increased reduction of lodging. Consequently, these varieties have poor emergence rates and poor seedling vigor. In rice, GA application promotes seedling establishment in the principle semi-dwarf commercial varieties utilized.
3. Commercial Applications of Cytokinins:
i. Mixture of BA and GA4+7:
The synergistic combination of BA with other GAs (especially GA5) has also been demonstrated since 1977. A mixture of BA and GA4+7 is used for commercial purpose of a cytokinin in agriculture. Shortly thereafter, the tetrahydropyranyl derivative of BA (PBA) was registered on ornamentals for induction of lateral branching.
The mixture of GA4+7 and BA is used to induce the extension of apple calyx lobes for the increased size and length/diameter ratio associated with high calyx lobes for the increased size and length/diameter ratio associated with a high quality ‘Red Delicious’ apple fruit.
Additional research with this mixture demonstrated that applications to non- bearing apple trees increased lateral bud break and improved branch angles to provide a better framework and bearing surface for early cropping.
Bud break and branch angles were primarily due to the BA component, but the combination with GA4+7 promoted shoot growth to a much greater extent than either of the two compounds alone. The mixture is used in the promotion of flowering of Chrysanthemum under non-inductive conditions.
ii. Fascicular Bud Break in Conifers:
BA is registered for use in promoting fascicular bud break in conifers to reduce the required mechanical shearing and altering the rotation period required to grow marketable Christmas trees. While this use has not been commercially significant, the compound’s availability has encouraged research in a number of areas.
iii. Extension of Vegetable Shelf Life:
Potential applications such as the extension of vegetable shelf life, prevention of crop senescence, and increased soybean pod set have not been pursued due to cultivar differences or lack of field efficacy.
Commercial Uses of Cytokinin-Like Activity Compounds:
In contrast to GAs, a number of compounds have been made synthetically which exhibit cytokinin-like activity including N6-substituted adenine analogues, like, pyridyl urea and thiadiazolyl ureas and pyrimidines. Of these the highly active thiadiazolyl urea, thidiazuron (TDZ) was found to have cotton defoliant properties and was subsequently the first cytokinin-like product to be successfully developed for widespread major crop use.
TDZ is unique as a harvest aid in cotton because it promotes green leaf abscission and prevents regrowth, thus allowing growers more flexible harvesting schedules. Its mode of action in the cotton leaf abscission process appears to be associated with a sustained increase in ethylene production coupled with the disruption of auxin transport.
i. Thinning of Apples:
Recently, promising results were reported for thinning of apples with BA alone or in combination with carbaryl. Another cytokinin-like compound currently under development, which also thins apples, is 2-chloropyridylphenylurea (CPPU).
A number of potential applications have been examined with CPPU which overlaps with several GA effects including grape berry enlargement, and increased fruit size and weight in apple. The future of this compound as a commercial product is yet to be determined and may require an identified use in a major agronomic crop to justify further development.
ii. Agronomic Crops:
The successful introduction of a CK product in an agronomic crop is dependent upon the specificity of a physiological response over a wide range of environmental/cultural conditions, and is complicated by the inherent genetic variability amongst cultivars. For example, the genetic reduction of plant height has led to dramatic increases in cereal grain yield, due principally to increased reduction of lodging. The current characterization of endogenous CKs in correlation with developmental events in major crop plants will certainly aid in the identification of application timings for candidate compounds.
A key factor in the successful development of thidiazuron (TDZ) in cotton was the availability of a large number of synthetic cytokinin-active compounds. With different classes of cytokinin chemistry, a systematic approach toward structure activity relationships could be employed.
4. Commercial Applications of Ethylene-Releasing Compounds:
The diversity of present commercial uses of ethephon and other ethylene- releasing compounds indicates the substantial contribution that stimulation of ethylene production is making to agricultural productivity. For many years ethylene itself has been used commercially to elicit some of these effects, for example, fruit ripening. Compounds, which initiate ethylene responses by inducing the plant to produce its own ethylene, (a wound response) have been used in the field.
i. Growth and Development:
a. Prevention of Lodging:
Neljubow observed prevention of lodging. It is the basis of one of the most important uses of ethephon. The regulator treatment not only reduces length of the stems, but also stiffens the straw.
b. Carbohydrate Content of the Straw:
Ethephon-treated plants contained Production Technology of Fruit Crops more water soluble carbohydrates, however, anticipated increase in structural carbohydrates was not observed.
c. Reduction in Scape Length:
The floral axis in bulb-type flowers is often too long for their use as potted flowers. Application of ethephon to hyacinth and narcissus plants when the leaves were 10 cm long reduced the length of the floral scape by 20-30%. The reduction in scape length was associated not only with a reduction in cell size, but, surprisingly, also with an increase in cell number, indicating that ethephon application had actually stimulated cell division.
d. Reduced Yield and Mean Tuber Size:
Hollow heart and brown centre of potatoes are disorders that accompany rapid plant and tuber growth. Although application of ethephon at tuber initiation reduced yield and mean tuber size, it also substantially reduced the incidence of these disorders, leading to increased yield of marketable tubers, relative to control fields which had high disorder incidence.
ii. Flowering Control:
a. Stimulation of Flowering in Pineapples:
For many years it has been known that ethylene stimulates flowering of Bromeliads, and treatment of pineapple plants with calcium carbide (which releases acetylene on hydration) was widely used commercially to stimulate flowering of pineapples. This response is now elicited in pineapples and flowering bromeliads by application of ethephon.
A comparison of the effectiveness of ACC and ethephon in eliciting this response suggested that ACC was the preferred ethylene-releasing compound, but because of differences in application method and concentrations, this interpretation needs confirmation.
b. Other Tropical Fruit Crops:
Flowering is also stimulated in many other fruit crops after ethylene treatment. Smoky fires are, for example, commonly lit in mango orchards to stimulate flowering. Application of ethephon has also been shown to stimulate flowering in Litchi, although the results vary widely depending on application time and cultivar. Ethylene stimulates or accelerate flowering of many geophytes.
In Iris, for example, a short treatment of harvested bulbs with ethylene will stimulate flowering in small-sized bulbs that would not normally flower, e.g., Iris and Freesia. Ethephon application has been used to extend the flowering season of plantations of Leucospermum, a woody proteaceous ornamental.
iii. Leaf and Branch Abscission:
The effects of ethylene in stimulating abscission are the basis of important commercial uses of ethylene-releasing compounds. Enhancement of fruit abscission in sour cherry, nut fall in macadamia, the removal of leaves from potato plants, cotton, and nursery stock, and the dehiscence of the shucks of walnuts and pecans are but a few examples of the potential uses of these materials in agriculture.
Defoliation of rutabaga required very high concentrations of ethephon; addition of 1% ammonium peroxydisulfate greatly increased the effectiveness of the ethephon treatment, perhaps due to accelerated oxidation of IAA in the abscising organ.
iv. Fruit Ripening:
Ethylene gas is widely used commercially for ripening a variety of climacteric fruits and decolouring non-climacteric citrus fruits. However, the use of ethylene-releasing compounds to affect this response is confined to relatively new crops, and normally when they are used as a preharvest application.
This is probably due to the ease with which ethylene gas can be applied to harvested fruit, and the dangers of applying aqueous solutions to these disease- susceptible organs.
v. Fruit Abscission:
Promotion of abscission as an aid in thinning or harvesting fruit crops is an important horticultural use of ethylene-releasing compounds. A recent study by Takeda and Peterson on mechanical harvesting of blackberries provides a good example of the benefits ethylene-releasing compounds provide in stimulating this response.
vi. Combination with Other Chemicals:
The rapid production of ethylene from the neutralized ethephon solution stimulated fruit abscission, but elevated ethylene concentrations were not presents for long enough to cause leaf abscission. A mixture of 1% glycerin + ethephon is the most successful ethephon formulation for abscission uses. Combination with other PGRs has also proved to be useful in modifying plant responses to ethephon.
Application to pecans of a spray combining 6 mM NAA with 3 mM ethephon resulted in accelerated shuck dehiscence without major leaflet abscission. Similarly, bloom-time sprays of ethephon combined with an auxin have achieved successful thinning of apples without undesirable leaf drop.
5. Commercial Applications of Abscisic Acid:
i. Stomatal Closure and Water Relations:
The drop in water potential is detected by the chloroplasts of the mesophyll cells of leaf, whose photosynthetic activities are thought to be the most sensitive of all the plant’s metabolic processes to water stress.
As the water potential decreases there is marked increase in permeability of chloroplast membranes to ABA. The ABA synthesized by and stored in chloroplast then diffuses into mesophyll cytoplasm and moves from cell to cell through plasmodesmata to the guard cells where it induces stomatal closure.
The drop in the level of stored ABA in the mesophyll chloroplasts induce fresh biosynthesis of more ABA, which continues to be translocated to the guard cells as long as the water potential remains low. The release of ABA from the mesophyll chloroplast stops when the water potential is restored to normal and its rate of synthesis are markedly reduced.
The evidences for this are as follows:
1. The exogenous application of low concentrations (e.g. 1mM) of ABA to leaves causes stomatal closure within 3-9 minutes.
2. The concentration of ABA in water stressed leaves rises markedly.
3. Mesophyll chloroplasts can synthesize ABA but guard cell chloroplasts cannot, and
4. The ABA of leaves of well-watered spinach was found to be contained in the chloroplasts but after 4 h of water stress the total ABA level was found to have risen 11 fold whilst that of the chloroplast had risen only 2 fold; this suggests that the newly synthesized ABA is rapidly exported from the chloroplasts.
ii. Guard Cells:
Guard cells showed equal closure when exposed to ABA solutions over the pH range 5.0-8.0, while no significant uptake of the hormone took place at pH 8.0. This indicates that the sites of ABA action are on the outer surface of the plasmalemma.
ABA prevents stomatal opening by rapidly blocking H+ extrusion and K– influx and it initiates closure by the rapid release of osmotica, in particular K+, CI– and malate. The latter phenomenon can also be observed in the shrinking of guard cell protoplasts.
iii. Chemical Signals for Stomatal Closure:
Saab and Sharp (1990) have shown that the roots send signals to the shoot and causes stomatal closure and inhibition of leaf growth in Maize when there is even slight reduction in soil water potential with no detectable change in leaf water relations.
They observed no effect of soil drying on stomata. In bean plants with oxygen-deficient roots, stomatal closure is influenced by changes in leaf water relations and by increased fluxes of ABA.
iv. ABA and Potassium Uptake:
The primary action of ABA on guard cells seems to be its inhibition of potassium uptake by the guard cells. Certain substances, such as a fungal toxin fusicoccin, are known to overcome the effect of ABA by stimulating potassium uptake and proton release. The flow of other solutes also seems to be influenced by ABA during the stimulated closing of stomata.
ABA and other hormones can alter the response of stomata to CO2. The increase in stomatal sensitivity to CO2, occurring as an early reaction to the arrival of a chemical signal from the roots, could be important in curtailing transpiration in wind.
v. Photosynthesis:
There are several reports that ABA applied via the transpiration stream to leaves affected photosynthesis both via stomatal closure and via a direct effect on carbon fixation. As a possible mechanism, inhibition of the carboxylation of ribulose -1, 5-bisphosphate has been suggested, but extractable ribulose-1, 5- bisphosphate carboxylase activity from ABA-treated leaves was as high as that of control leaves.
vi. Root Growth:
Reports on the effects of ABA on root growth are contradictory. Since exogenous ABA can inhibit root growth as well as promote growth of this organ.
vii. Elongation of Maize Coleoptile:
ABA decreased the elongation rate in maize coleoptile and also inhibited growth-induced by auxin, fusicoccin or acid. In all cases, this effect was due to an inhibition of cell wall loosening.
viii. Root Gravitropism:
The root cap responds to light and gravity by synthesizing or accumulating growth inhibitors. It is believed that the inhibitors are produced in the lower portion of root cap in response to gravity and are translocated into the zone of cell enlargement. They may inhibit the cell enlargement of cells on the lower side of the root. Differential growth results in geotropism.
ix. Bud Dormancy:
Innate or true dormancy (i.e., temporarily suspended growth due to internal conditions) is exhibited by the buds of temperate-zone trees during winter, stem tubers (e.g., potato), root tubers (e.g., Dahlia), Corms, bulbs and rhizomes. Hemberg (1949) found that the dormancy of buds of ash (Fraxinus) and potato tubers was positively correlated with the concentration in them of growth inhibitors, identified as ABA.
The external application of ABA to buds of growing seedlings caused them to become dormant. According to these findings ABA was synthesized in the plant, in the response of some environmental trigger and this ABA travelled to the buds and induced them to go dormant.
x. Seed Dormancy:
ABA builds up in embryos of developing seeds. It is believed that increased ABA level comes primarily from de novo synthesis. However, there may be translocation of ABA or its precursors from the leaves since the radioactivity from exogenously applied 14C-ABA to leaves can be recovered in developing seeds.
ABA in developing ovules inhibits the formation of germination enzymes in the embryo. Thus, ABA plays an important role in developing seeds by inhibiting vivipary. In the case of immature seeds, several evidences indicate that ABA prevents precocious germination (vivipary) of the developing embryo.
xi. Heterophylly:
Aquatic plants often produce two distinct types of leaves on the same plant. Submerged leaves are highly dissected or linear and they have undifferentiated mesophyll and an epidermis with few stomata, if present.
In contrast, aerial leaves are entire and broad and they have differentiated mesophyll and a stomata epidermis. It is interesting that the ABA treatment can induce the formation of aerial leaves on submerged shoots. Such changes in leaf morphology due to ABA have been reported for a fern, a monocot, and several dicots.
xii. Response Mutants:
Several mutants have been isolated that exhibit a decreased response to exogenous ABA. A viviparous mutant of maize, vp 1, has green seedlings and normal ABA and carotenoid levels. The embryos of this mutant are insensitive to ABA. Therefore, it has been classified as a response mutant.
xiii. Tuberization:
ABA appears to be involved in the process of tuberization. Wareing has shown that ABA formed by the leaf of a one-node cutting taken from a short day- induced plant of Solatium andigena (a wild species of potato which requires about 20 short day cycles to fully induce tuberization) is exported to the axillary bud and causes it to develop rapidly into a small tuber. Externally applied ABA has also been shown to promote the formation of these tubers in Dahlia and Jerusalem artichoke.
xiv. Senescence:
ABA may also be involved in senescence and ripening processes. Externally applied ABA accelerates the senescence of detached leaves; however, its effect on the senescence of leaves still attached to a healthy plant is minimal. Externally applied ABA accelerates the ripening of young fruit; moreover, there is a rise in the level of endogenous ABA during the ripening of strawberries and grapes.
xv. Resistance to Frost Damage:
ABA may also be involved in increasing the resistance of temperate-zone plants to frost damage; the external application of ABA has been shown to increase the frost hardiness of seedlings of box elder (Acer negundo), apple and alfalfa (Medicago sativa) and to ameliorate chilling injury in cucumber seedlings.
xvi. Barley Aleurone Cells:
Among the most studied effects of hormones on gene expression is the GA induced production of hydrolytic enzymes and its inhibition by ABA in barley aleurone layers. When ABA is applied 25-fold excess of GA the synthesis of a- amylase, protease b-glucanase and ribonuclease is suppressed.
xvii. Developing Embryo:
ABA not only prevents premature germination during embryogenesis, but it has also been implicated in the regulation of storage protein synthesis. In embryos of rapeseed, soybean and wheat cultured in vitro, exogenous ABA promoted the synthesis of embryo-specific proteins and mRNAs.
xviii. Protein Synthesis:
ABA has been called as stress hormone since it enhances adaptation to various stresses. Freezing tolerance could be increased by ABA both in plants and in cell cultures. Increased cold tolerance was not induced in the presence of cycloheximide, which indicates that synthesis of new proteins is involved with stress adaptation.