Why do the leaves of trees in the northern latitude change their colour in autumn?
S. BHAGWATI, Chennai
The major determinants of leaf colour are the type and amount of pigments present in the leaf. The primary function of leaves during warm seasons is photosynthesis (production of sugars from atmospheric carbon-dioxide using water and solar energy).
Therefore, leaves of deciduous trees produce abundant levels of the green pigment chlorophyll (the major pigment involved in photosynthesis) during the warm seasons rendering the leaves appear green. During the cooler seasons, water in the leaves freezes; causing foliar damage and inhibition of photosynthesis.
Therefore, during fall/autumn season, deciduous trees prepare for winter by reducing chlorophyll synthesis and shedding leaves. When chlorophyll is depleted, other minor pigments such as the orange/yellow carotenoids become prominent making the leaves change color. Additionally, leaves produce the red/purple anthocyanin pigments resulting in additional color change.
Does this color change benefit the trees? Yes. There are multiple theories on why trees spend energy to produce these pigments in leaves that they are going to shed. Trees might delay leaf shedding by producing these pigments. This would enable them to reabsorb a number of nutrients (e.g. nitrogen) before shedding.
These pigments might deter insect damage. For example, apple trees produce red pigments in the leaves to deter aphids (sap-sucking insects) from laying over-wintering eggs. The pigments in these leaves are released in the soil close to the trees when they are shed and get decomposed. This might help the trees to influence the types of microbes and other plant species that grow in proximity.
Plant Science Department
South Dakota State University