What is responsible for the yellow-orange coloration of leaves in the fall?
a) rubp
b) chlorophyll a
c) carotenoids
d) chlorophyll b

The correct answer is - C) carotenoids.

The majority of the trees from the mid-latitude forests tend to shade their leaves in the autumn. This process occurs because of the seasonal changes in the climate, where the trees tend to be almost not-active during the winter, so in order to preserve as much energy and nutrients as possible they get read off their leaves.

It is not a sudden process, but it takes some time, where the leaves from the initial green color start to change to yellowish, than towards orange, reddish, or brownish before they fall.

The reason why this coloration appears in the leaves when the tree starts to process of shading them off are carotenoids. The carotenoids found in the leaves, the lutein and the beta-caroten, thorugh the xantophills and the flavonoids give a yellowish color to the leaves, thus we get the beautiful autumn colors and landscapes.