Wilted Seedlings Under Colored Light
A student grows bean seedlings in three closed boxes, each lit by a different LED color: red, blue, and green. Each box holds 12 seedlings in separate pots with the same soil and watering schedule. The boxes sit on the same shelf, but the green box is nearest a sunny window and warms to 30 °C by afternoon, while the others stay near 22 °C. After three weeks, mean stem height is 18 cm (red), 21 cm (blue), and 26 cm (green). The student concludes green light is best for growth and plans to recommend green greenhouse lamps. A classmate notices many green-box seedlings are tall but thin and pale.
1. Explain why the conclusion about green light is not supported, citing the temperature difference and plant physiology.
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The green box was warmer (30 °C vs 22 °C) and near a window, so temperature and possibly natural light differ from the other boxes. Warmth can increase elongation, and extra window light adds energy. The green result is confounded.
Physiologically, chlorophyll absorbs red and blue light strongly and reflects green, so green light is generally the least effective for photosynthesis. The tall, thin, pale seedlings are consistent with etiolation - stem elongation driven by shade or weak usable light, not healthy growth. Height alone is a poor indicator; the plants may be spindly and low in biomass.
2. Propose better growth measures and explain why each is more informative than stem height.
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Use dry biomass (mass after drying) as the primary measure, since it reflects net photosynthesis rather than elongation. Add leaf area, chlorophyll content or leaf color score, stem diameter, and root mass.
Stem diameter and root mass distinguish sturdy growth from spindly stretching. Dry biomass integrates total carbon gained, directly tied to light quality. Measuring several traits prevents mistaking a stress response (elongation) for success.
3. Redesign the experiment to isolate light color, and state one remaining limitation.
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Place all boxes away from windows in a controlled room, use identical enclosures, and hold temperature at 22 °C in every box using vents or a fan, verified with sensors. Keep light intensity and photoperiod equal across colors by adjusting distance or using a meter. Randomize pot positions within each box and rotate them.
Limitation: LED spectra are not pure single wavelengths, and the experiment still uses one box per color, so box-level effects are not replicated. Multiple boxes per color would be needed for stronger inference.