How Do Plant Hormones Influence Plant Growth?
Hormones are organic chemicals produced in one part of the plant and transported to another part where they initiate a response.
Charles Darwin was the first to document the effect of the plant hormone auxin in his book The Power of Movement in Plants, where he noted that plants tended to bend towards light sources. Chemical signals are, more specifically, hormones and signaling molecules that are a vital component of plant sensory systems.
This article is a bit science-heavy, so we tried to put the most technical detail towards the end if you are skimming. Having an understanding of how plant hormones work is very useful when it comes to cloning and using stress training on plants, and if you are just the type of person who is super curious about how things work. Understanding and manipulating plant hormones is also essential for anyone who is interested in micropropagation, where plants can be cloned and grown in a laboratory medium (petri dish/test tube).
What Are Plant Hormones?
Plant hormones (phytohormones) are small chemical molecules that communicate signals from one part of the plant to the responding plant part which alters physiological processes. These signals generally travel through a chain or cascade with multiple molecules involved. The five main classes of plant hormones are auxins, abscisic acid, cytokinins, ethylene, and gibberellins. Other molecules like steroid and peptide hormones play a lesser role.
How Do Plant Hormones Work?
The best way to explain how plant hormones work is with an example like calcium.
Calcium (Ca2+) is a common signaling molecule that is normally kept at low concentrations in the plant. When an immediate large amount is released, the change is sensed, and genes are triggered to produce proteins leading to a physical response. Along with calcium, many other compounds send signals to trigger stress responses. These compounds may work alone, or together, or simultaneously depending on the specific instance of mechanical stimulus.
Types of Plant Hormones and Signal Molecules
Auxins: abscission (natural detachment of parts of a plant) deterrent, apical dominance, cell elongation, root formation in cuttings, fruit maturation, tropisms, xylem differentiation
Calcium: diverse stimulus responses, thigmomorphogenesis (plant response to mechanical/touch stimulus)
Cytokinin: bud activation, cell division, fruit and embryo development, mimicry of phytochrome and red light, prevents leaf senescence
Gibberellin: maturation from juvenile to adult (in some plants), converts from an adult back to juvenile (in other plants), involved in flowering, may release seeds and buds from dormancy, stem elongation, pollen tube growth
Abscisic Acid: initiates dormancy, stress response regulation, in very low dose may stimulate growth, stomatal closure
Ethylene: fruit ripening and abscission (natural detachment of parts of a plant), initiation of root hairs, latex production
Jasmonates: chlorophyll alterations, stunted shoots, increased biomass, herbivory and fungi defense, alkaloid synthesis for digestive disruption, and antifungal peptide synthesis
Brassinosteroids: vegetative growth regulation, seed germination, leaf morphogenesis, root and stem growth, vascular differentiation
Salicylic Acid: pathogen resistance
Nitric Oxide: developmental and physiological process regulation (Germination, cell differentiation, transitioning to flowering and death), pathogen defense
Reactive Oxygen Species (ROS): mechanical and environmental stress, accumulates hydrogen peroxide and superoxide to use as signals and work with calcium signaling mechanisms
The Role of Auxin in Pruning
Signaling molecules and hormones can have different effects depending on the amount present. For most cascades of reactions to occur, a critical dose level must be present. While auxin can be a growth promoter in plants, it can also be used as an herbicide (2,4-D is very common). Since Darwin’s observations, scientists have identified three main kinds of auxin- indole-3-acetic acid (IAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and 1-naphthaleneacetic acid (NAA). For most plants, IAA is the most common auxin. It is vital to embryonic development and regulates many growth processes.
The natural responses plants have to stress from herbivory and competition for light both play a role in manipulating plant growth and yield through stress induction. Naturally, plants are pruned by grazing herbivores. This grazing, like manual pruning, damages the apical meristem of the plant. When damage to the apical meristem occurs, it triggers growth in the lateral shoots. The scientific name for this response is apical dominance.
The Phenomenon of Apical Dominance and Pruning
Pruning causes local stimulation of the buds below the pruned region. This results in the growth of the lower side branches when the top of the plant is pruned. When the new growth region of a branch is pinched off or cut off, the tip of the branch will release the hormone auxin down the stem. Lateral branches that have become dormant are then stimulated to grow.
Shoot tips can influence each other’s growth because they must compete for auxin. A special hormone called strigolactone interferes with auxin transport amongst shoot tips and reduces branching by strengthening competition between auxin sources. If auxin levels are high in the main stem, the tips of plants cannot export more down the stem. The inability to export auxin causes stems to go dormant. Stronger branches that are more able to export auxin will dominate the plant. The main plant tip often has an advantage only because it emerges first, but has no benefit from its central position.
This is why it is so important to take caution when pruning. Proper pruning is great for plants and for yields, but when it is not done correctly, it can result in damage, plant illnesses, and plant death.
Benefits and Precautions for Stress Training
Stress responses are naturally present to one extent or another in plants. Stress induction (low-stress training or high-stress training) can take advantage of these abilities to increase the growth and yield of crops. As plants are brushed, tied, bent, or cut, the hormone auxin triggers the plant to grow in a shorter and more branching way. The overall increase in floral yield significantly increases the profitability of the crop. This must be weighed and considered against the risk of damage and potential infection. Much caution must be utilized when employing high-stress methods and methods such as monster cropping which require great skill.