Gai mutation in Arabidopsis results in a dwarf phenotype like that of Little Marvel peas. However, unlike the Little Marvels, elongation of the internodes does not occur when the Gai mutants are sprayed with gibberellin. This suggests that the mutation occurs in the synthesis of gibberellins. Light stimulates seed germination, inhibits stem elongation, promotes chlorophyll synthesis and the development of chloroplasts and promotion of leaf expansion. These responses induced by light are examples of photoperiodism.
Phototropism is a growth of a plant toward a unidirectional light source.
Phototropism involves detection of a light gradient across a coleoptile and a subsequent redistribution of IAA from lighted to shaded side of the coleoptile. The photoreceptor for phototropism is phytochrome since only near UVA and blue light are affective in causing the response. Different degrees of cell elongation across the width of the stem results in the stem bending characteristic of phototropism. One portion of the data curve depicts a linear relationship between these two variables. Provide one physical mechanistic explanation and one biochemical mechanistic explanation in the space below.