In the last article of this column, we discussed about the colored leaves on the trees. However, in autumn, these leaves turn brown and fall to the ground. Have you ever wondered why dead leaves turn brown every year?
When a fruit, for example, an apple is injured, the injured part slowly turns brown due to tannins. The injured cells release their contents, including the tannins, which come out of the cellular compartments where they are stored. The tannins react with the proteins that are also released (we have learnt in previous chronicle that tannins bind to proteins), and then slowly oxidize with the oxygen in the air, binding more closely to the proteins. Since these reactions change depending on the proteins they bind, the tannins take on different colors. The mixture of all these colors is brown, and the oxidized tannin-protein complexes are called “brown pigments”.
The same thing happens with the proteins and tannins remaining when the leaves die. As each cell breaks down, they mix and form brown pigments, coloring the dead leaf. All the nitrogen and phosphorus from the leaf remains are retained in these pigments, which play a major role in soil fertility. During winter, the large, insoluble brown pigments are not washed away by rainwater. When spring comes, fungi attack the brown leaves and release the nitrogen and phosphorus from the brown pigments. At this time of year, the roots become active again and can use these nutrients.
Without brown pigments, the fertility of soils would be lost during winter. Thanks to the tannins in the dead leaves, the nutrients remain trapped in the brown pigments on soils. Perennials accumulate more tannins before their leaves die than annuals, as they will grow on the same site the following year. They, therefore, benefit from the delayed fertilization provided by the brown pigments. It is in their interest to form them in abundance, which partly explains why trees in the forests accumulate so much tannin in their autumn leaves.
Thus, plants have the means to recover from the soil, after the death of the leaves, the resources, notably nitrogen and phosphorus, that they could not recycle in time. The brown color of the leaves hides the vital role of tannins after their death: fertilizing the soil.