I have properly googled this and got various answers. However, those answers do not provide an answer to the "Why" part of my question. Why is it that the upper surface of leaves is more green than the lower one? Some sites suggest that it is because of the thick cuticle. Yet, that sounds weird and doesn't fully convince me. What other reasons to why the upper surface of leaves is more green and shiny than the lower one, could there be?
Photosynthesis is the process of trapping sunlight and converting the energy into chemical energy which is then provided as a food source.
So the sunlight is trapped through chlorophylls. This is a pigment in the organelle called the chloroplast. The upper surface of the leaf is open to sunlight where the plants have more chloroplasts containing more chlorophylls to trap the most amount of sunlight.
All these chlorophylls are green coloured and since there is an increased amount of chlorophylls in the upper surface than in the lower surface (to trap more and more sunlight), the upper surface appears to be greener than the lower surface.
Source: Grade 12 Science student.
You can find two chlorophylls in most plant leaves, chlorophyll a and b. We'll use as a reference Leaf characteristics and chlorophyll concentration of Schyzolobium parahybum and Hymenaea stilbocarpa seedlings grown in different light regimes. The most important observation is that in shade-tolerant leaves, chlorophyll b dominates chlorophyll a. And the opposite is true of shade-intolerant leaves. I'll argue now that the portion of a given leaf that must tolerate more shade will have the darker green pigment due to the presence of more cholorphyll b, and that the side of the leaf that must tolerate lots of light will have the lighter pigment due to the presence of more chlorophyll a.
The light-tolerant side of the leaf is also shiny, and this is because of a thick waxy cuticle. The shade-tolerant side of the leaf also has a cuticle but if we follow it logically, the light-tolerant side of the leaf needs more protection from heat evaporation because it's always being hit by light.
So what I'd do, instead of thinking of the leaf as having a top or bottom, think of them as having a side that must tolerate excessive shade, and a side that must tolerate excessive light.
With ref to wax it is called epicuticular wax, it can be called as a protective mechanism of light absorbing side of leaf, from uv rays, anti climbing agent for small insects , etc wikipedia has a nice definition https://en.wikipedia.org/wiki/Epicuticular_wax
i will reproduce the same for future reference.
Epicuticular wax is a coating of wax covering the outer surface of the plant cuticle in land plants. It may form a whitish film or bloom on leaves, fruits and other plant organs. Chemically, it consists of hydrophobic organic compounds, mainly straight-chain aliphatic hydrocarbons with a variety of substituted functional groups. The main functions of the epicuticular wax are to decrease surface wetting and moisture loss. Other functions include reflection of ultraviolet light, assisting in the formation of an ultrahydrophobic and self-cleaning surface and acting as an anti-climb surface.
with ref to more green part, accumulation of chlorophyll over ventral side is a necessity to absorb more sunlight. a distant and probably not so correct analogy can be, the tanning of the skin surface.
protected by Community♦ Aug 2 '18 at 20:31
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