Plant hormones and tropisms explained
Homeostasis and response • Plant hormones (biology only)
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Definition and role of plant hormones
Plant hormones are chemical substances produced in specific tissues that control growth and development at distant sites. Hormones travel within plant tissues by diffusion and cell-to-cell transport, creating concentration gradients that change cell behaviour. Hormones coordinate responses across organs so that growth and development match environmental cues and internal programmes.
Auxin: production, transport and effect
Auxin (indoleacetic acid, IAA, and related molecules) is produced mainly in shoot and root tips and in young leaves. Auxin diffuses from the production sites and moves directionally through tissues, concentrating on particular sides of stems or roots. Higher auxin concentration causes increased cell elongation in shoot cells, producing faster growth on that side and causing curvature. Auxin therefore links a local sensory input (light or gravity detection) to a growth response through differential distribution of the hormone.
Phototropism (response to light)
Photoreceptor cells in the shoot tip detect directional light. Cause: directional light exposure shifts auxin distribution to the shaded side of the shoot. Effect: elevated auxin on the shaded side causes those cells to elongate more, bending the shoot toward the light (positive phototropism). Experiments that remove the shoot tip or cover it remove the photoreceptor or auxin source and prevent bending, demonstrating that the tip produces the signal and auxin moves away from the tip to cause curvature.
Gravitropism / geotropism (response to gravity)
Root and shoot cells sense gravity and redistribute auxin accordingly. Cause: when a root or shoot is reoriented, gravity-sensing cells trigger auxin to concentrate on a specific side. Effect in shoots: auxin accumulation on the lower side causes greater cell elongation there, so the shoot curves upward (negative gravitropism for shoots, positive phototropism often dominates shoots). Effect in roots: auxin accumulation on the lower side inhibits cell elongation in roots, causing the root to curve downward (positive gravitropism). The opposite effects in root and shoot result from tissue-specific sensitivity to auxin.
Hydrotropism and interaction of tropisms
Roots respond to water gradients (hydrotropism) and to gravity. Cause: a stronger water gradient produces a stronger hormonal or growth signal than gravity in many cases. Effect: root growth direction follows the dominant cue; hydrotropism can override gravitropism when water detection presents a stronger stimulus. Experimental systems with water droplets show roots bending toward moisture even when that direction opposes gravity.
Experimental evidence for hormone control
Tip removal stops phototropic bending because the auxin source and light-sensitive cells are in the tip. Cause: removing or covering the tip eliminates auxin production or light perception. Effect: no differential growth and no bending. Inserting an impermeable barrier (mica) on one side blocks diffusion and prevents bending on that side; a permeable barrier (gelatine) allows diffusion and bending resumes. These manipulations show that the tip produces a mobile chemical that travels down one side to cause curvature.
Other plant hormones and uses
Gibberellins control stem elongation, seed dormancy and germination. Ethene controls fruit ripening and some aspects of cell division and leaf drop. Agricultural and horticultural uses include rooting powders with auxins to promote root formation, selective weedkillers that exploit hormone imbalances to kill broad-leaved weeds, and ethene application to ripen fruit post-harvest. Hormone concentration and tissue sensitivity determine the effect and can limit usefulness or cause side effects.
Limiting factors and specificity
Hormone concentration, location of production, transport pathways and tissue sensitivity limit the response. Cause: too little auxin produces no effect; too much auxin can inhibit growth or cause abnormal development. Effect: optimal concentration ranges produce normal tropic bending; outside these ranges cells either fail to elongate correctly or show uncontrolled growth (as in some weedkiller effects). Environmental cues (light intensity, water availability) set the initial stimulus that changes hormone distribution.
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