Investigating light and gravity in seedlings
Homeostasis and response • Plant hormones (biology only)
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Key concepts
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Required practical setup and procedure
Seeds are placed on moist cotton wool in shallow containers and covered to control light exposure. Different covers allow no light, top-only light, or side-only light so the only variable between treatments is the direction of incoming light . Observations occur after several days. Comparison of shoot direction between treatments identifies the effect of light direction. For gravity investigations, seedlings are grown vertically then re-oriented or tilted; root and shoot curvature indicate gravitropic responses.
Phototropism - definition and mechanism
Phototropism describes growth of plant shoots towards or away from a light source; positive phototropism means growth toward light. Light-sensitive cells in the shoot tip detect light direction and cause uneven distribution of growth signals, producing differential cell elongation and bending of the shoot toward the light . Cause → effect: light exposure on one side reduces auxin concentration there; higher auxin concentration on the shaded side causes increased cell elongation on that side, so the shoot curves toward the light.
Gravitropism (geotropism) - definition and mechanism
Gravitropism describes growth in response to gravity; positive gravitropism means growth towards gravity (roots), and negative gravitropism means growth away from gravity (shoots). Gravity perception in root and shoot tips leads to redistribution of growth signals and unequal cell elongation, producing curvature in the appropriate direction . Cause → effect: when a seedling is reoriented, auxin and other signals redistribute; in shoots higher auxin on the lower side causes cell elongation and upward bending, while in roots high auxin on the lower side inhibits elongation and causes downward bending.
Role of auxin in tropisms
Auxin is a plant hormone that controls cell elongation and moves from shoot tips to other tissues. Unequal auxin distribution causes asymmetric cell elongation and bending in both phototropism and gravitropism . Experimental evidence shows auxins are produced in tips and diffuse down the shaded side in phototropic responses; barriers that prevent diffusion stop bending, while permeable barriers allow it, linking hormone movement to the observed directional growth .
Variables, controls and measurement
Independent variable examples: light direction (top, side, none) or seedling orientation (upright, tilted, inverted). Dependent variables: shoot curvature angle, direction of growth, root direction. Controls: same seed type, same moisture, same temperature, identical timing and container type to isolate the effect of the stimulus . Measurement methods: record qualitative sketches and quantitative angles of curvature using a protractor, count days to visible response, and repeat with replicates to assess variation. Data presentation uses tables and line/bar charts to compare treatments.
Limitations and limiting factors
Limiting factors for the practical include seed quality, temperature, moisture level and light intensity. Any uncontrolled variation in these factors can alter growth rates and confound interpretation of the effect of light or gravity. Cause → effect examples: low temperature slows metabolism and elongation, reducing observable curvature; uneven moisture reduces germination and introduces variability. Replication and careful control of these factors improve reliability.
Key notes
Important points to keep in mind