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Responses to environmental changes. What are plant hormones?. Plants need a method of responding to their environment to cope with predation and abiotic stresses, such as drought.
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What are plant hormones? Plants need a method of responding to their environment to cope with predation and abiotic stresses, such as drought. Plants do not have a nervous system, so a major way in which they respond to the environment is by using plant hormones, often called plant growth factors. Unlike animal hormones, plant growth factors are not made in specialized organs but in many tissues all over the plant. They may be used where they are made, or transported within the plant. They usually exert their influence by affecting growth.
Effects of plant growth factors Plant growth can be divided into three main types: • cell division • cell elongation / enlargement • cell differentiation (specialization). These types of growth can all be controlled by plant growth factors.
Tropisms Stimulus Tropism Examples A tropism is the movement of part of a plant in response to, and directed by, an external stimulus. The movement is by growth and is therefore slow. It is described as positive or negative depending on whether the growth is towards or away from the stimulus respectively. light phototropism shoots positive, roots negative gravity geotropism roots positive, shoots negative pollen tubes positive chemical chemotropism water hydrotropism roots positive air (oxygen) aerotropism pollen tubes negative
Difficulties in studying growth factors The effects of a plant growth factor can depend on: • the concentration of the growth factor • the tissue being acted on, e.g. stems or roots • the developmental stage of the plant • the species of plant • what other growth factors are present. Another growth factor can increase the effect of the growth factor being studied (synergism) or reduce it (antagonism). The fact that growth factors are only present in minute amounts adds to the difficulties in studying plant responses.
Photoreceptors light (stimulus) phototropin (receptor) cascade of reactions redistribution of auxin phototropism Photoreceptors are structures or pigments that are sensitive to light, often light of specific wavelengths. Phototropins are a group of photoreceptors primarily responsible for triggering phototropisms. When light of the right wavelength is present, changes in the phototropin molecules trigger a cascade of reactions in the cell. This ultimately results in the redistribution of auxin so that there is more on the shaded side of the plant.
IAA – an auxin Auxins are made continually in the shoot apex and young leaves (apical meristems). They can move by diffusion from cell to cell, or can be transported long distances via the phloem, e.g. from the shoots to the roots. The primary natural auxin is indoleacetic acid(IAA). It is involved in many important processes, including cell elongation and phototropism.
Phototropism in the classroom There are many ways phototropic responses can be shown in a school laboratory. How would you use this apparatus to investigate the effects of light on a growing plant? • What variable could you change? • What control experiment would you use? • How would you present your results if they were qualitative, rather than quantitative? • How might you extend this investigation to gain further information about how plants respond to light?
Apical dominance Apical dominance is the growth of the main central stem of a plant, with reduced production of lateral (side) shoots/branches. Apical dominanceis controlled by auxin. One piece of evidence for this is a lack of apical dominance when the top of a plant is pruned. lateral branches apex High concentrations of auxin are produced at the stem tip (apex), preventing the development of lateral branches near the apex. Pruning of the apical bud causes lateral branches higher up the plant. after pruning apical dominance
Stem elongation Gibberellinsare formed in young leaves and around growing tips. They stimulate the growth of shoots (stem elongation) and leaves. They are also involved in seed germination. Dwarf plants provide some evidence for the role of gibberellins in stem elongation because when gibberellins are applied artificially to dwarf plants, they can stimulate stem elongation, even in genetically determined dwarfism. This is because the dwarfism occurs due to the absence or mutation of the gene for gibberellin production.
Commercial applications: introduction Plant growth factors can be applied to a plant artificially for commercial reasons. The growth factors used are usually synthetically produced since this is cheaper and more efficient than extracting them from plants. The process is relatively inexpensive because only very low concentrations are needed to produce an effect. Synthetic growth factors can be used by farmers or gardeners to increase crop yield or quality.
