By finding the right balance of sugars created and sugars consumed, you can produce a more healthy and resilient crop while maximizing the plant’s use of available light.
Empowering Plants: Assimilate Balance
This article is part of a series (see other articles on the website). Growing by Plant Empowerment (GPE) starts with the natural behaviour of plants, as they are capable of coping with very different circumstances and climate conditions. The plants keep themselves alive by managing three balances: energy, water and assimilates (refer to 2019 Greenhouse Canada Mar/Apr, June, and Oct). In this article, we will focus on balancing the production and consumption of assimilates and how this knowledge can be used to produce higher crop yield and quality.
Starting with photosynthesis
Photosynthesis is the starting point for optimizing plant growth. In the process of photosynthesis, carbon dioxide (CO2) and water (H2O) are trans-formed into sugars or assimilates, with the help of energy from sunlight. These sugars can be used as building blocks for plant tissue production and to make new cells, for instance. The biochemical processes responsible for the production of new cells also need energy, which is indirectly supplied by sugars as well. So assimilates have two roles: 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 an assimilate shortage, the plant has to cut down on consumption, at the cost of development or quality. Surpluses, however, mean inefficient utilization of available assimilates, which is also undesirable.
Green plants contain a pigment called chlorophyll. Along with other secondary pigments, chlorophyll absorbs part of the solar spectrum that we call photosynthetically active radiation (PAR). This is light in the wavelength range of 400 to 700 nanometers – essentially the colour spectrum. Leaves that produce more assimilates than they consume are known as “sources” of the plant. All other parts of the plant which consume more assimilates than they produce are called “sinks.” The main sinks are the fruits or flower buds. Young leaves belong in the “sinks” category because they consume more assimilates than they can produce. But mature leaves, which don’t receive enough light because they are overgrown by younger leaves, also become sinks again when their assimilates consumption exceeds their production. The roots are typically the weakest sink of the plant; they are first to react to a shortage of assimilates (dying roots) or a surplus (quickly expanding roots). See also the section on root growth.
We need to take into account 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 then not only depends on the light-intercepting area of the leaves (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 big as its footprint on the green-house 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 leaf area per m2 greenhouse area). When the light penetrates more deeply into the crop canopy, the leaves keep their chlorophyll content at higher levels, which results in a higher light use efficiency. This is further improved with an open plant architecture combined with the use of diffuse light.
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