Hi There


I was just reading about the old classic experiment of growing cress in the dark vs. the light. Cress grown in the dark grows much taller (in an attempt to reach light I presume) and also has much paler, yellow leaves.


Why are these leaves yellow? Are there not enough resources in the seed upon germination for it to synthesise sufficient chlorophyll? That means that the light allows the chlorophyll to be synthesised. Is it purely an energy thing?


Thanks as always.


Nick

Chlorophyll needs light for photosynthesis. In the presence of light it absorbs red light and reflects green light and carries out photosynthesis.

So the chlorophyll is present but by keeping the plant in the dark you're making it work harder to:
a) photosynthesise
b) grow taller to reach the light

But why does that make the leaves yellow?

Because with little or no sunlight, plants do not produce chlorophyll.

The organelles which contain chlorophyll, called plastids, need light in order to complete the synthesis and activation of chlorophyll, which together with other pigments then absorb most wavelengths of light excepting those in the green wave bands - this results in the light reflected from leaves being relatively enriched in green wavelengths and therefore appear to be green to us. 

In the absence of chlorophyll or another pigment light reflected off the surface of a leaf is not enriched in any particular wavelengths and thus appears to be white Or nearly so (often cream or pale yellow). Leaves can appear white when they lack a gene to synthesise the chlorophyll pigment, which may be apparent only in part of a so-called variegated leaf.  Leaf miners which eat the chlorophyll-containing tissues inside a leaf may also leave white trails across the surface of a leaf.  Lack of critical nutrients may prevent a leaf from making chlorophyll, which can result in a leaf with yellow or white blotches on it, known as chlorosis.  As you mention in the question, keeping a plant in the dark will also prevent the leaves from turning green.