Light is essential for photosynthesis. The photograph shows thale cress, a small flowering plant found in Europe, Asia and Africa. (Source: © blickwinkel / Alamy Stock Photo) The effect of wavelength of light on thale cress was investigated. Three groups of plants were treated with a different wavelength of light. One group of plants was treated with blue light, one group with amber light and one group with red light. The following were measured: - rate of photosynthesis - leaf biomass - pigment content of the leaves - changes in transcription of genes in the leaves. Another group of these plants had a control light shone on them. Which would have been the most appropriate light to use as a positive control? (1)

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