Plant Hormones in Turfgrass Management
By Tim Butler
Plant hormone application has been commonly practised in the turfgrass industry for many years. Many products, often called plant growth regulators containing plant hormones have appeared, however many people are still unclear as to what exactly plant hormones do and why are they needed.
Plant hormones may be classified as biostimulants and as a group of naturally occurring substances, which influence physiological processes within the plant. They may contain one or more of the following: cytokinins, gibberellins, auxins, abscisic acid and ethylene when growing under normal conditions, plants have adequate endogeneous levels of hormones for normal growth and development.
Most plant physiological processes involve either an interaction of several hormones or individual hormones with several functions. When produced exogeneously, plant hormones are sometimes known as plant growth regulators. Plant hormones are translocated within the plant, and influence the growth and differentiation of the tissues and organs with which they come in contact with, not only within the plant as a whole, but also within individual organs.
The five groups of hormones are auxins, cytokinin, gibberellins, abscisic acid and gibberellins. Auxins, cytokinins and gibberellins are the principle growth-promoting hormones found in plants. All three control, stimulate, inhibit or alter a plant's development to one degree or another, depending upon the external environment.
Each hormone has a specific function in the health and growth of plants, which is directly related to the concentration of the hormone, its sites of action, and the developmental stage of the plant. Changes in relative ratios of these hormones will signal certain processes to occur. For example, a higher ratio of auxins to cytokinins will promote adventitious rooting, while a higher ratio of cytokinins to auxins will promote tiller initiation and development.
However, it is well documented that each hormone also has the potential to inhibit the same growth responses. The key to whether a hormone promotes or inhibits growth depends primarily on its level, or endogenous concentration, in the plant. At natural levels in the plant, most of these substances have a promoting effect.
However, if levels increase significantly above those normally found in the plant, an inhibitory effect may result. Certain environmental and cultural stresses can limit the natural production of plant hormones. For example, flooding, drought and high temperatures may inhibit the production of cytokinin in roots and subsequent movement to shoots. Under these conditions, if other hormones in the plant are at proper levels, applications of a properly formulated cytokinin product could result in plant stimulation.
Auxins are present in all higher plants and are of fundamental importance in the physiology of growth and differentiation. IAA (indole -3- acetic acid) was the first auxin to be discovered in 1934. It is now believed to be the principle auxin in higher plants. Auxins cause several responses in plants including, (a) phototrophism, (b) geotrophism, (c) apical dominance, (d) cell elongation, (e) root initiation and elongation, (f) ethylene production, and (g) abscission. Auxins exhibit hormonal control of growth and development in plants. Auxins appear to be synthesized mainly in meristematic tissues such as those of young developing leaves, flowers and fruits.
Cytokinins occur as a bound form in the tRNA of most organisms, including plants, but plants also possess significant amounts of free cytokinins. While cytokinins are considered as hormones, which regulate cell division activities, their role in the plant has gradually been recognised as being so widespread that it includes some aspects of every part of growth.
Cytokinins are known to stimulate cell division, morphogenesis and leaf expansion resulting from cell enlargement and enhancement of stomotal opening. Several studies have also shown that cytokinins can also influence germination, particularly when conditions are suboptimal for germination. Cytokinins are primarily synthesised in root tips and transported via the xylem to the shoots, where they exert regulatory effects on the aerial parts of plants. They may play an important role in photoperiodic improvement of plant growth under environmental stresses.
Exogeneous cytokinins have been used to alleviate heat stress injury, increase drought resistance (related to higher antioxidant levels), maintain photochemical efficiency, increase root mass and cell expansion and stimulate cotyledon growth.
Applications of cytokinin to the rootzone of creeping bentgrass inhibited the decline of growth and turf quality under heat stress and alleviated heat stress injury by maintaining active antioxidants and reducing lipid peroxidations, when applied to the leaves of bentgrass plants. Cytokinins not only serve to promote tillering, but also function as antioxidants to help preserve chlorophyll integrity and photosynthetic function during stress.
In recent years, it has been demonstrated that seaweed extracts contain phytohormones and the stimulatory effects of seaweed extract, particularly for turfgrasses growing under stresses have been attributed to its hormonal activity, especially that of cytokinins and auxins. Auxin and cytokinin-like activities of humic acids have been reported.
Gibberellins are a class of plant growth substances that stimulate cell division and/or cell elongation and other regulatory functions. Gibberellins are noted as the most powerful of the growth promoters because they affect internode growth (Wright, 1993). They also promote uniform growth, and cause plants to elongate and grow tall, with light green leaves. GA3 is the naturally occurring hormone in many plants.
Plant growth and development are regulated by internal signals and by external environmental conditions. One important regulator that co-ordinates growth and development with responses to the environment is the sesquiterpenoid hormone abscisic acid (ABA). ABA effects stomatal opening and closing, abscission, seed germination, growth, geotropism and protects against cold stress. An increase in ABA levels within the plant signifies plant stress.
Ethylene causes inhibitory effects on growth. Treatment of plants with very low concentrations of exogenous ethylene has many profound effects on physiological and metabolic activities. Endogenously synthesised ethylene is involved in the normal control of many aspects of plant growth, differentiation and responses to the environment. Ethylene produces an inhibitory effect on auxin translocation in plants.
Once ethylene has become present in plant tissue, the tissue will produce greater quantities of ethylene because of ethylene's autocatalytic properties.
In turfgrass commonly used plant growth regulators include primo (trinexapac ethyl), proxy (ethephon), Bonzi (paclobutrazol) and Embark (Mefluidide). Such products are regularly used on many golf courses and sports pitches.
These products have many beneficial functions in turfgrass management including reducing stem elongation, which is a common problem with turfgrass grown under shade, increasing tillering, rooting and stress tolerance and also enhancing turfgrass density. Plant growth regulators are and will continue to play a vital role in turfgrass management into the future.
Tim Butler is currently studying for a doctorate degree in turfgrass nutrition at both University College Dublin, Ireland and Michigan State University, USA. Contact: firstname.lastname@example.org
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