Department of Floriculture and Landscape Architecture, Dr. YS Parmar University of Horticulture and Forestry, Nauni, Solan, H.P., India
Received Date: June 24, 2021; Accepted Date: July 07, 2021; Published Date: July 14, 2021
Citation: Vanlalhruaii R, Kamal S, Pangtu S (2021) Role of Plant Growth Regulators in Flower Crops. Periodon Prosthodon Vol.12 No.7:34.
Plant growth regulators consist of a large group of naturally occurring or synthetically produced organic chemicals and considered as a helping tool in the modern production system of ornamentals. Their exogenous application helps to improve the different economically important and market desirable characteristics of ornamental plants. The effects of PGRs in plants depend on various factors which play an important role to achieve expected results. These factors include the application method, time of application, concentration of PGRs, plant species and also the environmental conditions in which plants are grown. The intensity of applications is also considered an important factor affecting the efficacy of PGRs, as some plants respond well to a single application, but in most cases, multiple applications are beneficial to attain good results. This review briefly discusses the different areas in which PGRs are used in flower crops.
Growth regulators; Plants, Environment; Crops; Flowers
Plant growth regulators (PGRs) consist of organic molecules produced synthetically and used to alter the growth of plants or plant parts. They have the ability to accelerate or retard the plant growth. The hormone which is produced in plants is called as plant hormone and also known as phytohormone. Phytohormone is defined as, an organic substance produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of production, and active in minute amounts. Plant growth regulators are organic compound, natural or synthetic, organic molecules which when present or apply at low concentration, results in a change in plant growth or development .
Plant growth regulators consist of a large group of naturally occurring or synthetically produced organic chemicals and considered as a helping tool in the modern production system of ornamentals. Their exogenous application helps to improve the different economically important and market desirable characteristics of ornamental plants. The effects of PGRs in plants depend on various factors which play an important role to achieve expected results. These factors include the application method, time of application, concentration of PGRs, plant species and also the environmental conditions in which plants are grown [2,3]. The intensity of applications is also considered an important factor affecting the efficacy of PGRs, as some plants respond well to a single application, but in most cases, multiple applications are beneficial to attain good results .
The plant growth regulators represent various categories as American Society for Horticultural Science also divides the plant growth regulators into six classes including gibberellins, auxins, cytokinins, ethylene generators, growth inhibitors and growth retardants. Plant growth regulator includes synthetic compounds as well as naturally occurring hormones.
Auxins: Ruppert reported that some species of plants produce rooting over a wide range of different concentrations of IBA . Lower concentrations of IBA produced superior results than higher concentration while their effectiveness decreases with higher concentration . Auxin treatment significantly reduced time-to rooting in carnation, and early rooting (18.69 days), high rate of rooting (58.70%) and high number of roots per cutting (13.18) with NAA 500 mg/l . Verma reported that the effect of 0.5 mg/l NAA s on in-vitro rooting in chrysanthemum induced maximum rooting . Meshram et al. reported foliar application of NAA 50 ppm recorded significant maximum weight of flower (6.10 g), diameter of fully opened flower marigold. It also recorded significantly maximum plant height, number of branches plant, spread of plant, stem diameter .
Gibberellins: Gibberellic acid is known to increase the plant height and number of leaves that might have led to increased rate of photosynthesis. As a result of this, availability of metabolites to the developing corm and cormels might have increased, thereby leading to increase in the weight of corm in gladiolus . Sharifuzzaman et al.  reported that foliar application of 150 ppm GA3 produced higher number of suckers, maximum number of cut blooms with longer stalk and bigger flower size in chrysanthemum. Maximum flower size and flower yield were observed in China aster cv. ‘Kamini’ with foliar spray of GA3 200 ppm by Gupta et al. . Kasturi and Chandrasekhar reported that spray of GA3 recorded minimum number of days to bud sprout in carnation . Longest vase life of cut tuberose spike was recorded with the foliar application of GA3 200 ppm as reported by Kurve et al. .
Cytokinins: Kasturi and Chandra Sekhar reported that spray of BA significantly increased the number of flower stalks harvested per plant in carnation and recorded maximum vase life of cut flowers. Singh et al. recorded maximum number of scales/plant and maximum number of leaves/plant with foliar spray of BA 150 ppm (Double dose) in gladiolus . Khan et al. reported multiple shooting enhanced by BA 125 ppm dipped corms in gladiolus . Sajjad et al. reported that pre-soaking of corms in 150 ppm benzyladenine solution for 24 hrs before planting in the field enhanced the number of sprouting per corm in gladiolus .
Abscisic acid: Endogenous levels of ABA decrease during flower development from bud to the fully open stage, while it rapidly increases during senescence [18,19]. Shibuya et al. reported that treatments with exogenous ABA speed up flower senescence in many cut flowers. Cut carnations treated with ABA showed a reduced vase life . In certain flowers, ABA causes senescence through ethylene and using ethylene inhibitors ABA-induced senescence can be prevented as reported by Müller and Stummann . The application of a 500 mgL-1 s-ABA spray or drench to drought stressed plants delayed visible wilting in the treated plants and resulted in a subsequent increase in shelf life of 1.7 to 4.3 days depending on the species .
Ethylene: Reid et al. reported that a very low concentration of ethylene (0.5 μl L-1) markedly inhibited the opening of cut rose flowers . Kumar et al. observed that foliar spray of ethrel at 100 and 200 ppm increased the number of flowers per plant, diameter of flower, fresh weight of flower and flower yield per plant than control in marigold cv . ‘PusaNarangiGainda’. Sethy et al. reported significant improvement in flower size and number of ray florets in sunflower due to application of Ethrel (250 ppm) .
Plant growth retardants/inhibitors
Growth retardants: The term growth retarding or growth retardant is that the chemical slows cell division and cell elongation of shoot tissue and regulates plant height physiologically without formative effects. Eg. Phosphon-D, CCC and Alar. These do not occur naturally in plants and act in retardation of stem elongation, preventing cell division.
Inhibitors: Suppress the growth of plants. There are phenolic inhibitors and synthetic inhibitors and Abscissic acid. Benzoic acid, Salicylic acid are examples for phenolic inhibitors, while maleic hydrazide (MH) and Triiodo benzoic acid (TIBA) are examples of synthetic inhibitors.
Maleic hydrazide: Maleic hydrazide (MH) is one of the first plant growth inhibitors to be used commercially. It has herbicidal effects when applied at higher concentrations. Since it is a general inhibitor of meristematic activity, it retards stem elongation, preventing leaf and flower induction. Moond and Gehlot reported in chrysanthemum, MH increase in vase life of flowers with increase in levels of MH (250, 500, 750, 1000 and 1250 ppm) . Sethy et al. reported that the application of MH at 500 ppm and 1000 ppm in sunflower significantly increased the number of flowers per plant and the size of flowers was significantly decreased as compared to control .
Cycocel: Cycocel is a growth retardant known to reduce the level of endogenous gibberellins, which could help in reducing vegetative growth and enhance flowering. The most conspicuous effect of Cycocel is increasing the number of laterals and earliness in flowering. Prashanth recorded minimum shoot length and internodal length with Cycocel at 3000 ppm in floribunda rose cv. ‘Iceberg’ and significantly more number laterals and leaves at Cycocel 1500 ppm . Moond and Gehlot reported that CCC at 4000 ppm recorded significantly more number of flowers per plant compared to other concentrations (2000, 6000, 8000 and 10000 ppm) in chrysanthemum . Kumar et al. reported that application of 5000ppm CCC in tuberose exhibited superiority with respect to duration of flowering (37.13 days), length of spike (76.68 cm) and number of florets per spike (30.84) .
Ancymidol, Flurprimidol, Paclobutrazol and Uniconazole: These are all listed together due to their similar chemical structures. They all inhibit GA production at similar sites in the GA production process. These PGRs have the strongest efficacy relative to others, so are typically applied at lower concentrations.
Use of plant growth regulators
Plant growth regulators are not highly specific in their action and affect a variety of growth and developmental processes in the plant. Sometimes there are many overlapping and interacting effects of growth regulators in cut flower and foliage plants. However, the uses of some plant growth regulators in cut flower and foliage plants production are described below.
Dormancy: Plant growth regulators can be successfully used in breaking dormancy. Dormancy is more pronounced in cormels and bulblets than corms and bulbs. Pradhan recorded that corm dipping of gladiolus in GA3 200 ppm took a minimum number of days (5.53) for 50 percentages sprouting . Aier et al. observed minimum days to emergence of shoot, days to initiation of spike in gladiolus with GA3 at 200 ppm whereas maximum number of cormels was produced by dipping corms in kinetin at 500 ppm concentration [30,31]. Jamil et al.  reported that bulbs of hippeastrum treated with ethrel at 100 ppm enhanced early emergence of flower scape and flowering. Kurve et al. reported that the earliest first spike emergence in tuberose by dipping in GA3 200 ppm followed by GA3 150 ppm .
Propagation: Auxins are extensively used in the rooting of cuttings. Exogenous auxin application improves rooting efficiency and quality of stem cuttings, while IBA and NAA stimulate adventitious rooting in cuttings . Bhatt and Chauhan reported maximum average number of roots per cutting under the treatments at IBA + NAA 150 mg/l and maximum average length of root per cutting under NAA 200 mg/l after 20 days in marigold . Dawa et al. reported dipping basal portion of rose cuttings in IBA (1000 ppm) for 2 seconds recorded minimum days to root initiation (23.33), maximum rooting (72.22%) and root length (6.42 cm). In chrysanthemum, the maximum root length (8.89 cm) was recorded when the leaf cuttings were treated with IBA 100ppm by dipping the basal portion of the cuttings for 15 minutes [35,36].
Micro propagation: Cytokinins and auxins are widely used as supplements for induction of shoots and roots, respectively in tissue cultured plantlets. Sedaghat et al. observed that in Anthurium andraeanum tissue culture the best result for callus induction was obtained in mMS medium containing 0.12 mg/l 2, 4-D, 1 mg/l BA and 3% sucrose . Bhatia et al. conducted in vitro multiplication of gerbera cv. ‘Cabana’ using capitulum as explant and obtained best establishment of immature capitulum explants on modified MS medium supplemented with 10 mg/l BAP and 1 mg/l IAA and maximum numbers of quality shoots were obtained on MS medium containing 1 mg/l BAP and 0.1 mg/l IAA . Kanwar and Kumar studied in-vitro shoot bud formation from callus in Dianthus caryophyllus L  and reported highest callus induction with 2 mg/l 2,4-D and 1 mg/l BA and highest average number of shoots was observed with 2 mg/l TDZ and 1 mg/l IAA .
Growth promotion: GA3 is the most popular growth promoting regulator which has extensive use in various flower crops. Apart from GA3, there are some other plant regulators that are responsible for growth promotion. Singh et al. reported bulb dipping of tuberose for 12 hours in GA3 200 ppm produced maximum plant height, spike/clump, and spike length and bulb and bulblet production . Mishra recorded that GA3 300 ppm resulted in the maximum plant height and spread and Ethrel 400 ppm resulted in maximum number of branches in marigold. In China aster, the of GA3 at 100 ppm recorded maximum plant height, number of primary branches, number of leaves, number of flowers and duration of flowering as reported by Mishra et al . Khan et al. noticed multiple shooting enhanced by BA 125 ppm treated corms of gladiolus.
Regulation of flowering: Growth regulators can be used for advancing or delaying the flowering and to induce uniform and more number of flowering. Singh et al. observed that bulb dipping of tuberose cv. ‘Single’ in GA3 200 ppm was found to have the best floret number/spike, inflorescence length and width, spike weight, for early emergence of spike, increased flowering duration and vase life . Mishra reported that GA3 300 ppm resulted in the early flower bud initiation, opening of the first flower and duration of flowering, number of flowers per plant, flower weight and flower diameter in marigold . Malik et al. stated that the highest flower number was recorded with MH 500 ppm while maximum flower bud diameter with MH 1000 ppm in dahlia cv . ‘Charmit’. Khan et al. reported more number of florets per spike gladiolus in first year with cormels and next year with the corms produced in the previous year influenced by BA and GA3 with control .
Plant height control: Inhibitors of gibberellin (GA) biosynthesis are widely accepted for use on ornamental plants to restrict shoot growth and plant development . Pradhan reported that CCC 750 ppm (corm dipping + foliar spray) measured the minimum height of the plant (66.99 cm) and took maximum numbers of days (81.85 days) for spike emergence in gladiolus . Malik et al. reported that foliar spray of MH 1000 ppm recorded minimum plant height, maximum leaf number, stem diameter, primary and secondary branch number in dahlia cv. ‘Charmit’ . Kumar et al. reported that ethrel 400 ppm significantly reduced plant height (66.12 cm) of marigold . Abrol reported minimum plant height of chrysanthemum in plants sprayed with paclobutrazol 90 ppm in cultivar ‘Ajay’ and daminozide 2500 ppm in cultivar ‘UHFSChr-Collection 1’ .
Vase life: The growth regulators can be used for prolonging the life of cut flowers and loose flowers. Kumar and Gupta observed that vase life of gladiolus was significantly increased by pre-soaking and foliar spray of gibberellic acid 100 ppm (16.70 days) over control (12.28 days). Parmar et al. recorded that GA₃ 200 ppm showed the maximum vase life of rose (12.23) and maximum shelf life (8.23 days) was recorded with CCC 4000 ppm . Sangma et al. observed maximum vase life of gerbera with treatment of GA3 200 ppm (14.80) as compared to control (9.67) .
The exogenous application of plant growth regulators (PGRs) can alter various characteristics of flower crops including growth promotion and retardation, number of flowers, flowering, flower life, breaking dormancy, improving seed germination and vase life. However, the use of different PGRs depends on different objectives for quality flower production.