Assimilate Balance

Starting with photosynthesis

Photosynthesis is the starting point for optimizing plant growth. During photosynthesis, carbon dioxide (CO₂) and water (H₂O) are transformed into sugars, or assimilates, with the help of energy from sunlight.

These sugars can be used as building blocks for plant tissue and the production of new cells. The biochemical processes responsible for producing new cells also require energy, which is indirectly supplied by sugars. Assimilates therefore have two roles: they are both building material and fuel for plant growth.

The plant will always balance its consumption and production of assimilates, as shown in Figure 1. If there is a shortage of assimilates, the plant has to reduce consumption, at the expense of development or quality. Surpluses, however, mean inefficient use of the available assimilates, which is also undesirable.

Source-sink relations

Green plants contain a pigment called chlorophyll. Along with other secondary pigments, chlorophyll absorbs part of the solar spectrum known as photosynthetically active radiation (PAR). This is light in the wavelength range of 400 to 700 nanometres, essentially the colour spectrum.

Leaves that produce more assimilates than they consume are known as the plant’s “sources”. All other parts of the plant that consume more assimilates than they produce are called “sinks”. The main sinks are fruits or flower buds.

Young leaves belong in the sink category because they consume more assimilates than they can produce. However, mature leaves that do not receive enough light because they are overgrown by younger leaves can also become sinks again when their assimilate consumption exceeds their production.

The roots are typically the weakest sink of the plant. They are the first to react to a shortage of assimilates, resulting in dying roots, or to a surplus, resulting in rapidly expanding roots. See also the section on root growth.

We need to take into account both the size and activity of the sources and sinks. For the production of assimilates, PAR light must first be absorbed by the green leaves. The level of assimilate production therefore depends not only on the light-intercepting area of the leaves, or source size, but also on the amount of light that penetrates through the crop canopy.

In general, when the total leaf area of the plant is about three times as large as its footprint on the greenhouse floor, all light intercepted by the plant is used for photosynthesis. In this case, the so-called LAI is 3 (Leaf Area Index, measured in m² of leaf area per m² of greenhouse area).

When light penetrates deeper into the crop canopy, the leaves maintain higher levels of chlorophyll, resulting in higher light use efficiency. This can be further improved with an open plant architecture combined with the use of diffuse light.

Read the whole article? Download the complete Plant Empowerment whitepaper series