Carbohydrates and Defoliation

First of all, why do we even care about carbohydrates? Carbohydrates are compounds produced during photosynthesis. In plants, they have two main purposes. First, they provide building blocks for plant structural components, such as cellulose (important in building cell walls). Secondly, carbohydrates are molecules that deliver energy for plant growth.

Throughout the growing season, carbohydrates are constantly flowing through the plant going from locations that have an excess supply, called ‘sources’, to locations that have a need, called ‘sinks’. This is called the ‘source-sink relationship’.

The source-sink relationship can be compared to the relationship between a factory, a warehouse, and a retail store. Let’s use the hybrid car market as an example. A factory (source) manufactures hybrid cars and then ships them to the dealerships (sink) to sell. The dealership is selling as many cars as they receive from the factory. Then, the factory expands and produces twice as many cars as before. Meanwhile, the demand at the dealership is the same resulting in an excess of cars being produced at the factory. So, the factory starts sending the excess cars to the warehouse to store until they are needed. Later that year, the gas prices skyrocket and suddenly, there is a great demand for fuel-efficient hybrid cars. To keep up with the high demand, both the factory and the warehouse send cars to the dealership.

Now apply this to a perennial plant. The factory is the leaf material, where the carbohydrates are produced. The warehouse is the roots and crown where excess carbohydrates are stored until they are needed. And, the dealership is any part of the plant using the carbohydrates for building structure or for energy.

In early spring, there is no green leaf material for photosynthesis, so all carbohydrates used to start growth of the first leaves must come from storage in the roots and crown. In this case, the leaves are the ‘sink’, and the roots/crown are the ‘source’. Once the amount of leaf area becomes great enough, it is able to produce enough carbohydrates to supply the plants needs and carbohydrates no longer need to be taken from storage in the roots/crown. Eventually, later in the season, the leaf area will be great enough that it is producing more carbohydrates than the plant needs. The excess carbohydrates are then sent for storage in the roots/crown.