Plant Growth Regulators Used in Ornamental Geophytes: History
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Contributor:
Beata Janowska
,
Roman Andrzejak

Geophytes are a very important group among ornamental plants, for which more and more plant growth regulators (PGRs) are being used to improve the plant quality, flowering intensity, and vase life of flowers and leaves. PGRs constitute a large group of naturally occurring or synthetically produced organic chemical compounds. There are many factors that influence the efficiency of PGRs, and the method of their application plays a key role in determining their success. In the case of geophytes, the most common method of application is spraying and soaking the storage organs before planting. 

  • PGRs
  • storage organs
  • ornamental plants
  • flowering
  • vase life

1. Gibberellins (GAs)

Among the PGRs, the most numerous are gibberellins (GAs), which were discovered and isolated by Japanese researchers from the fungus Giberella fujikuroi, which belongs to Ascomycetes. This pathogen infects rice seedlings, causing their excessive elongation. The plants growing from such seedlings are thinner and more fragile. The disease was named bakanae—“crazy seedling”. In 1926, Kurosawa observed that, in sterile extracts from cultures of Giberella fujikuroi (the perfect sexually reproducing stage) and Fusarium moniliforme (the imperfect conidial stage), there is a compound that causes excessive elongation of the internodes of rice. In 1935, Yabuta, Hayashi, and Sumiki isolated chemical compounds from the metabolic products of the fungus, which they named GAs [1][2]. The research on GAs was interrupted by World War II. It continued in the early 1950s.
GAs are acids with a structure based on a giban ring. They are denoted by the symbol GA followed by the appropriate sequence number. GAs are formed in the youngest parts of plants [3]. They are translocated by the xylem and phloem [4]. GAs interrupt hereditary plant dwarfism [5]. Under their influence, shoots elongate, and plants flower more abundantly [6]. When GAs is applied, plant dormancy can be interrupted [7], seed germination can be improved [8], flowering can be changed [9], and flower and leaf vase life can be improved [6], as GAs inhibit chlorophyll and protein degradation in leaves [10][11]. Gibberellic acid (GA3) is the most commonly used GA [5].

1.1. Mechanism of GA Action

Through the use of biochemical, genetic, and molecular research techniques, the mechanism of GA signal perception and transduction in plants has been elucidated. In the first step, the GA signal is received by GA-insensitive dwarf 1 (GID1) located in the cytoplasm and nucleus. In the Oryza sativa genome, a single GID1 gene was identified, while in the Arabidopsis thaliana genome, three were discovered—GID1a, GID1b, and GID1c—with partially overlapping functions. The binding of bioactive GAs to the GID1 receptor promotes interactions between GID1 and the DELLA domain present in DELLA proteins, which are the main repressors of the GA pathway. The rapid degradation of DELLA proteins involving ubiquitin-protein ligases is a universal mode of GA function. Gas in a non-dissociated form can penetrate into the cell, as the cell membrane does not provide a barrier to limit their diffusion. An increase in the concentration of bioactive Gas in cells results in the formation of the GA–GID1 complex. Interactions between these proteins occur through hydrophilic (direct hydrogen bonds or indirect via water molecules), hydrophobic, and van der Waals interactions. This allows one of the DELLA proteins to attach via the highly conserved N-terminal DELLA and TVHYNP motifs. Eventually, a polyubiquitin chain is attached to the DELLA proteins, which is the signal for their degradation in proteasomes. The entire cycle of events leads to the release of specific transcription factors that activate or inhibit target genes [12].

1.2. The Effect of Gas on Flowering and Quality of Geophytes

As in the case of other species of ornamental plants, the most commonly used GA in geophytes is GA3 (Table 1).
Table 1. The effect of PGRs on flowering and quality of geophytes and on the vase life of flowers and leaves.
Species PGR Effect Source
Alstroemeria aurantiaca GA3 better vase life of leaves [13][14][15]
Amaryllis belladonna ‘Zephyranthes’ GA3 earlier flowering, larger flowers, more leaves, better vase life of flowers [16]
Anemone coronaria GA3

BA		 longer flowering shoots, earlier flowering, more chlorophyll, and carotenoids
no effect on flowering intensity, shorter flower stalks, smaller flowers		 [17][18][19]

[20]
Arum italicum GA3
BA		 better vase life of leaves [20]
[21]
Convallaria majalis [Gib][Ach] better vase life of leaves [22]
Crocosmia × crocosmiiflora GA3 more kaempferol, quercetin, quercetin 3-O-glucoside, kaempferol 3-O-rhamnosylglucoside and β-carotene [23]
Cyklamen persicum GA3 longer flower stalks, earlier flowering, more flowers [24][25][26]
Freesia reflacta GA

BA		 taller plants, more leaves, more inflorescences shoots
longer inflorescences, larger leaves		 [27]

[28]
Gladiolus hybridus
Gladiolus grandiflorus






Gladiolus sp.		 GA3




BA


IAA
NAA		 more adventitious corms; shorter spikes and fewer flowers on a short day, but longer spikes and more flowers on a long day
earlier and better flowering
shorter inflorescence shoots but longer flower clusters and more flowers
more adventitious corms		 [9][29][30][31][32]



[33]
[29]


[34]
[35][36]
Hemerocallis × hybrida MemT
MemTR
[Chol][Gib] [Q-C2][Gib] [Gib][Ach]		 better vase life of leaves [37]
Hippeastrum × hybridum GA3 better vase life of leaves [38]
Hyacinthus orientalis GA3 intensive growth of leaves, inhibition of the formation of adventitious bulbs [39][40]
Iris × hollandica GA3 no impact [41]
Lilium sp. GA3 better vase life of leaves [42]
Muscari armeniacum GA3 longer leaves and inflorescence stalks [43]
Tulipa sp. GA3 longer flower shoots [44][45]
×Amarine tubergenii ‘Zwanenburg’ GA3 more leaves, more bulb weight, more daughter bulbs, better carbon dioxide (CO2) assimilation, more sugars and proteins, earlier flowering, better flowering [46]
Zantedeschia aethiopica BA
GA3 + BA
GA3		 more inflorescences

better vase life of leaves		 [47][48]

[49]
Zantedeschia with colorful spathes GA3
BA
GA3 + BA
BA
GA3
MemT
MemTR		 later flowering, more inflorescences


better vase life of flowers
better vase life of leaves		 [50][51][52][53][54][55][56][57][58]

[11]
[10][11][59][60]
The beneficial effects of GA3 have been used in the field cultivation of Anemone coronaria, inter alia. Piskornik and Piskornik [17] showed that, in the case of that species, the effect of GA3 depends not only on the concentration but also on the timing of application, which the authors explained by the different thermal conditions prevailing after the application of this PGR. According to the authors, with respect to that species, the use of GA3 at a concentration of 100 mg·dm−3 effectively improves the quality of flowers expressed by the weight and length of flowering shoots. Later studies by Janowska et al. [18] indicated that GA3 is also worth using in Anemone coronaria when grown while covered. According to the authors, in the case of the ‘Sylphide’ cultivar, the use of GA3 at 50–150 mg·dm−3 speeds up flowering by 11–16 days and stimulates the elongation of flowering shoots (Figure 1).
Agriculture 13 00855 g003 550
Figure 1. Anemone coronaria ‘Sylphide’: from the left, a control plant and plants grown from tubers soaked in GA3 at a concentration of 50 mg·dm−3, 100 mg·dm−3, and 150 mg·dm−3.
However, when GA3 is applied at concentrations of 100 or 150 mg·dm−3, the flower yield doubles. Furthermore, according to a study by Janowska et al. [19], GA3 in this cultivar stimulates the formation of chlorophyll and carotenoids and the accumulation of sugars in the leaves. GA3 is also useful in the cultivation of Cyclamen persicum. Spraying the leaves with GA3 at a concentration of 10 or 50 mgˑdm−3 not only initiates the growth of flower stalks but also significantly accelerates flowering and increases the number of flowers [24][25][26]. GA3 is used in the cultivation of Zantedeschia with colorful spathes cultivars. The results of the tests carried out indicated a differential response of the cultivars, which was closely related to the concentrations of GA3 used. In Zantedeschia, the intensity of flowering depends not only on the cultivar but also on the size of the rhizomes and the length of their storage [61]. However, the size of the rhizomes is not correlated with intensive flowering, as even from the largest ones, without the use of GA3, a very good yield of flowers was not obtained. The research conducted showed that GA3 could be used at concentrations from 25 to 500 mg∙dm−3 [50][54][55][56][57][62][63][64][65]. However, too high a concentration of GA3 causes inflorescence deformation [57][62]. The use of GA3 in colored Zantedeschia cultivars delays flowering but also prolongs it [50][57]. In addition to soaking rhizomes in water–GA3 solutions, the spraying of leaves and rhizomes is used [51][53]. Such methods protect Zantedeschia from Pectobacterium carotovorum subsp. carotovorum [66]. Jerzy and Janowska [67] conducted a study in which they evaluated the subsequent effect of GA3 used at the in vitro propagation stage on the flowering and quality of two Zantedeschia cultivars. GA3 was applied at the final stage of in vitro plant micropropagation [68] by introducing it at 50 mg·dm−3 into pre-sterilized rooting medium. Prior sterilization of the medium was necessary because the GA3 activity in the autoclave could be drastically reduced to as low as 10% [69]. In regenerated plants from GA3-treated cuttings, the authors [67] observed an altered leaf shape and a reduced rhizome size and weight. In contrast, Andrzejak and Janowska [6] demonstrated that in Z. albomaculata ‘Albomaculata’, GA3 could be replaced by mycorrhiza. The addition of arbuscular mycorrhizal fungi (AMF) stimulated flowering in this cultivar, probably because AMF produces PGRs, including GAs [66][70].
Attempts were made to improve the quality of Iris × hollandica with GA3. In the cultivars Wedgewood and ‘Prof. Blaauw’, however, no large effect of GA3 on flower shoot elongation was found [41]. In Tulipa sp., on the other hand, GA3 was shown to have a stimulating effect on flower shoot elongation [44][50], the inhibition of flower aging, and earlier flowering [71].
However, the use of GAs does not always have the desired effect. For example, in Hyacinthus orientalis, there was an inhibitory effect of GA3 on the formation of adventitious bulbs in hollowed-out parent bulbs, while at the same time, there was very intensive leaf growth [39][40]. However, soaking Gladiolus corms in GA3 at 100 or 500 mgˑ·dm−3 had a beneficial effect on the number of adventitious corms. GA3 also stimulated photosynthetic intensity as the chlorophyll levels increased. Moreover, plants grown under short-day conditions with GA3 flowered; however, the forming spikes were shorter and had fewer flowers than control plants grown under natural day-length conditions. Consequently, short days and low light levels are limiting factors for GAs [30]. Janowska et al. [9] reported, however, that in G. hybridus ‘Black Velvet’, GA3 at a concentration of 100–600 mg·dm−3 inhibited inflorescence shoot elongation but stimulated spike elongation. Moreover, it had a beneficial effect on the uptake of calcium (Ca) and manganese (Mn). Sajid et al. [28] reported that spraying G. grandiflorus leaves with GA3 at a concentration of 25, 50, or 100 mg∙dm−3 stimulated inflorescence shoot growth. The longest inflorescence stems were recorded by the authors for the treatment in which GA3 was applied at a concentration of 100 mg∙dm−3. Moreover, GA3 at 50 or 100 mg∙dm−3 stimulated spike elongation and flower development. Shoot and inflorescence elongation after GA3 application in Gladiolus ‘H.B.Pitt’ was reported by Sable et al. [32]. The authors further reported that, in this cultivar, GA3 at 100–200 mg∙dm−3 stimulated flower development, and at 200 mg∙dm−3, it caused significant inflorescence elongation. The beneficial effect of GA3 on the shoot length, inflorescence development, and flower development in Gladiolus ‘White Prosperity’ was reported by Sajjad et al. [29]. On the other hand, in Muscari armeniacum, GA3 not only accelerated flowering but also stimulated inflorescence stalk and leaf growth in partially and fully cooled bulbs [43]. GA3 was also used for Freesia reflacta. Żurawik and Placek [27] reported that, in three Freesia cultivars from the Easy Pot group, soaking corms in a GA3 solution with a concentration of 10, 20, 40, 80, or 160 mg·dm−3 for 24 h stimulated inflorescence shoot elongation and leaf development, but reduced flower diameter. The application of GA3 increased the weight of offspring corms. The highest effect among the concentrations assessed was recorded at 160 mg·dm−3. In contrast, the GA3 used in the experiment had no effect on the number of new corms obtained. An interesting study was conducted by Janowska et al. [23]. This study assessed the effect of GA3 on the content of biologically active substances in the corms of Crocosmia × crocosmiiflora ‘Lucifer.’ Four groups of biologically active substances with antioxidant properties were extracted from C. × crocosmiiflora ‘Lucifer’ corms: saponins (medicagenic acid, medicagenic acid 3-O-triglucoside, and polygalic acid), phenolic acids (caffeic acid, p-coumaric acid, and gallic acid), flavonoids (kaempferol, kaempferol 3-O-rhamnosylglucoside, quercetin, and quercetin 3-O-glucoside) and carotenoids (crocin and β-carotene). The corms of the ‘Lucifer’ cultivar proved to be a rich source of antioxidants. After treatment with GA3, the antioxidative activity increased in proportion to the concentration of GA3 used in the experiment. GA3 increased the content of medicagenic acid, polygalic acid, caffeic acid, p-coumaric acid, gallic acid, kaempferol, quercetin, quercetin 3-O-glucoside, kaempferol 3-O-rhamnosylglucoside, and β-carotene without affecting the content of medicagenic acid 3-O-triglucoside and crocin.

2. Cytokinins (CKs)

Naturally occurring cytokinins (CKs) are derivatives of the adenine purine base, with an alkyl chain or aryl group attached to the amino group. In plants, CKs with aliphatic substituents are common, especially zeatin and dihydrozeatin. CKs with an aromatic benzyl substituent are less common. They are responsible for cell division and differentiation [69]. In horticulture, they are especially used to extend the vase life of cut flowers and florists’ greens [52][72][73]. CKs are produced by a variety of creative tissues, but the main sites of their synthesis include the apical meristems of the root system, young fruits and seeds during intensive growth, and callus tissue [74].

3. Effect of a Mixture of CKs and GAs on Flowering and Plant Quality of Geophytes

In the West, ready-made preparations containing PGRs of various compositions are often used in floricultural production. These include Promalin (100 mg·dm−3 GA4+7 + 100 mg·dm−3 BA) [75][76]. Unfortunately, this preparation is expensive due to the costly synthesis of GA4+7. Therefore, e.g., in nursery production, it is replaced by the cheaper GA3- and BA-containing Arbolin [77]. However, it should be mentioned that these preparations are not registered in Poland, and consequently, combined PGRs from different groups are used in studies (Table 1). Janowska and Stanecki [73] found that based on studies evaluating the effect of the combined application of GA3 and BA on the flowering of Zantedeschia with colorful spathes, soaking the rhizomes in a mixture of those PGRs increased the inflorescence yield in the cultivars ‘Black Magic’ and ‘Albomaculata,’ which confirmed an earlier study by Funnell et al. [56], who found an increase in the cut inflorescence yield of up to 469% in Zantedeschia ‘Galaxy’ after treatment with Promalin compared to control plants. In this cultivar, GA3 also caused an increase in yield, but only by half as much. Similarly, in Z. aethiopica ‘Green Goddess,’ the cut flower yield increased after the application of the BA + GA3 mixture [48]. According to Janowska and Stanecki [72], in the Zantedeschia with colorful spathes cultivar, the application of the BA + GA3 mixture influenced the growth of inflorescences with shorter stalks from the rhizomes. On the contrary, Ngamau [48] claimed that slightly longer stalks were obtained in Z. aethiopica ‘Green Goddess’ after an application of BA + GA3; however, these differences were not statistically significant. Interesting results were obtained by Janowska et al. [78]. The authors evaluated the effect of a mixture of BA and GA3 on the number and size of stomata in the epidermis of Zantedeschia leaves. They found that in the cultivar ‘Albomaculata’, after an application of the BA + GA3 mixture (100 + 100 or 350 + 350 mg·dm−3), the stomata in the upper leaf epidermis were larger, and their number decreased. In the lower epidermis, the BA + GA3 at the concentrations used affected the formation of larger stomata, with their abundance decreasing when the mixture was applied at concentrations of 350 + 350 mg·dm−3.

4. Auxins

Auxins are synthesized in shoot and root apices. The natural auxin is indole acetic acid (IAA). Synthetic auxins include indole butyric acid (IBA), naphthalene acetic acid (NAA), methyl ester of naphthalene acetic acid (MENA), 2-methyl-4-chlorophenoxyacetic acid (MCPA), 2,3,5-triiodobenzoic acid (TIBA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Natural auxins can be in the form of free auxins, which move freely or diffuse readily from plant tissues, or bound auxins, which are only released from plant tissues after hydrolysis, autolysis, or enzymolysis. Auxins promote growth at low concentrations, while they inhibit growth at high concentrations. [79]. The rooting of cuttings is the most important role played by auxins [80]. IBA [81] and IAA [82] are most commonly used for this purpose. Auxins influence the formation of primary, secondary, and adventitious roots [83]. They are used for the rooting of many ornamental plant species from different groups [84][85][86][87][88]. The stimulation of the rooting of cuttings involves reducing the number of days required for rooting [85], increasing the percentage of rooting, and increasing the length, number, and weight of roots [86].

This entry is adapted from the peer-reviewed paper 10.3390/agriculture13040855

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