The key difference between plant hormones and plant growth regulators is that plant hormones are natural while plant growth regulators are artificial and are applied to plants by humans. Plant growth regulators mimic the function of natural plant hormones.
Hormones are produced naturally by plants, while plant growth regulators are applied to plants by humans. Plant hormones and growth regulators are chemicals that affect: Flowering.
Plant Growth Promoters – They promote cell division, cell enlargement, flowering, fruiting and seed formation. Examples are auxins, gibberellins and cytokinins. Plant Growth Inhibitors – These chemicals inhibit growth and promote dormancy and abscission in plants. An example is an abscisic acid.
Generally, there are five types of plant hormones, namely, auxin, gibberellins (GAs), cytokinins, abscisic acid (ABA) and ethylene. In addition to these, there are more derivative compounds, both natural and synthetic, which also act as plant growth regulators.
Auxins generally are responsible for maintaining apical dominance and root formation, whereas the cytokinins work in antagonism with the auxins to promote lateral bud and shoot development [37,45,46].
Plant Growth Regulators (PGRs): Types, Uses and Safety
There are several classes of plant growth regulators, including auxin, gibberellin, abscisic acid, cytokinins, salicylic, jasmonic acid and ethylene, as well as more recently investigated brassinosteroids, strigolactones, polyamine, etc.
For many years, most plant growth regulators have been considered low-toxicity or slight-toxicity pesticides. However, recent studies have found that many plant growth regulators and their degradation products in the environment are potentially harmful to humans, animals, and plants.
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Gibberellins: Gibberellins (GA) promote cell elongation, shoot growth, and are involved in regulating dormancy.
Plant growth regulators (PGRs) applied to greenhouse crops can suppress or promote growth, increase branching, or promote or delay flowering.
Auxin and cytokinin are critical growth hormones in plant development and are naturally present within the plant at variable concentrations throughout the season.
The top three plant growth promoters are: (1) Auxins (2) Gibberellins and (3) Cytokinins.
Florigen is known as the flowering hormone which is responsible for controlling the flowering in plants. The production of florigen is done in the leaves of the plants. It acts in the shoot of the plant and the growing tips of the apical meristem. It is considered graft transmissible and works between the species.
The residues of PGRs in agricultural products are seriously detrimental to human health because they have been found with hepatotoxicity, nephrotoxicity, genotoxicity, neurotoxicity, even carcinogenicity and teratogenicity.
Rooting Hormones are auxins, or plant growth regulators, that are involved in cell elongation and adventitious root formation.
Phytotoxicity. Phytotoxic effects have been noted on some plant species following application of PGRs, including foliage discolouration, leaf crinkling, growing point distortion and stunting. Often symptoms can be related to incorrect rates or water volumes, or poor application technique.
Apart from these, plants also need certain organic compounds to signal, regulate and control their growth. These are known as the Plant Growth Regulators or Hormones or Phytohormones. Plant growth regulators are the chemical substances that control the aspects of growth and development within the plants completely.
Plant hormones are the collective term for compounds produced in minute amounts by plants to regulate their own physiological functions. To date, seven natural plant hormones have been discovered: auxins, cytokinins, ethylene, jasmonates, abscisic acid, gibberillins and brassinosteroids (Fig. 1).
From the regulatory control perspective, plant growth regulators are classified under " pesticides ".
Since 1937, gibberellin (GA), ethylene, cytokinin, and ab- scisic acid (ABA) have joined auxin as phytohormones, and together, they are regarded as the “classical five” (Fig- ure 1).
Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture.
PGR is naturally present in every plant (phytohormones), although it is readily available but to accelerate the growth of roots and shoots, the addition of PGR is required. Root formation on cuttings is strongly influenced by auxin group PGR and shoot growth is influenced by cytokinin group.
The main difference between PGR cannabis and natural cannabis is that PGR cannabis is grown with synthetic hormones, while natural cannabis is not. Plant growth regulators in PGR cannabis also lend a different appearance to crops.
PGRs limit vertical grass growth within 3 to 4 weeks of application. Slower turf growth decreases the need to mow by up to 50%, reducing waste and making for more sustainable maintenance.
Common symptoms of plant injury due to exposure to PGR herbicides include stunting and shortened internodes on stems; twisted, calloused, cracked, or deformed stems and petioles; cupped, puckered, or crinkled leaves; leaf strapping, chlorosis, and mild mosaic pattern on leaves.