|MadSci Network: Botany|
Sorry for the delay in answering. Sun leaves are also usually thicker with a better developed palisade layer, a thicker cuticle and contain more chlorophyll per unit area than shade leaves. Most of the differences seem to be triggered by light but more water stress in sun versus shade might have an effect on cuticle development and leaf expansion. Plants have several pigments that they use to sense light level. For example, plants have several phytochrome pigments that allows the plant to determine whether it is in light, shade or dark based on the relative amount of red and far-red light it receives. In sun, there is a lot of red and little far-red, so a high R/FR ratio. In shade, there is just the opposite, little red and lots of far-red, so low R/FR ratio. The plant response to shade is to elongate its growing stems and expand its growing leaves. This may be beneficial because taller stems may carry the leaves above surrounding plants and out of the shade. As you mentioned, increased leaf area increases light interception. Botanists have been studying the genes that are affected by phytochromes. See the first website for a discussion. It gets very complicated when looking at all the genes and the proteins they code for. Changes in plant shape due to light is termed photomorphogenesis, which is a good search word. Also, try putting "shade leaf" or "sun leaf" in parentheses when searching with google.com or other search engine to give more hits. There seem to be many websites discussing sun leaves and shade leaves. References Photomorphogenesis Photomorphogenesis lecture Sun leaf versus shade leaf Developmental Plasticity in Oak Leaves The shade avoidance syndrome: multiple responses mediated by multiple phytochromes
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