First of all … what are assimilates? Assimilates, also known as glucose, are the product of the process of photosynthesis. The produced assimilates are used for plant growth, as they are converted into new plant tissue, for instance.
This means that the available amount of assimilates is directly related to the growth of the plant, the more it has available, the more steadily the plant can grow. This means that for good plant growth it is important to maximize the amount of available assimilates, by stimulating the photosynthetic process.
On top of making sure the production of assimilates is as high as possible through optimizing the photosynthesis process, and stimulating the use of these assimilates to maximize plant growth, it is important to keep a balance between vegetative and generative development within the plant. Combining these three things allows for optimum growth.
Optimizing assimilate production
One of the goals is to maximize assimilate production, and one of the most important elements for this is optimum utilization of PAR light. The more leaves are absorbing PAR light, the more leaves contribute to the total production of assimilates.
Another important element for optimizing assimilate production is the greenhouse climate. Besides PAR light, water availability, CO2 and humidity are other important factors that contribute to assimilate production. Light, water, and CO2 are all necessary for the process of photosynthesis, so no surprise there, but why humidity? Well, low humidity in combination with high radiation causes a higher evaporation rate, which tends to result in the stomata closing earlier, which in turn negatively impacts the uptake of CO2.
So for optimal assimilate production it is important to balance the availability of light, water, CO2 and humidity. Hoogendoorn states that “in many cases photosynthesis and assimilates production can be increased by simply keeping the ventilation windows of the greenhouse more closed, and sometimes applying additional misting.” By closing the windows you reduce CO2 loss, increase humidity, and increase temperature, all things that are desirable for assimilate production.
Optimizing assimilate consumption
Consumption should keep pace with the production of assimilates to maintain balance. Both the production and the usage of assimilates depend on temperature, the higher the temperature the faster these processes take place. If there is only little light, then the average temperatures should also be low, and vice versa. Maintaining a balance between temperature and light also allows the grower to keep a balance between vegetative and generative growth, creating optimal conditions for the plant.
Steps for creating the correct temperature-light ratio
In practice, however, it turns out that balancing temperature and lighting can be quite a challenge. Hoogendoorn states that often when there is a steady level of PAR light (assimilate production), the temperature averages, and thus the assimilate consumption, varies. So how do growers ensure a good temperature-light balance?
According to Hoogendoorn, it is best to prevent deviations of the balance by anticipatory control. What this means is keeping the PAR light sum and average 24-hour temperature in a steady ration on a daily basis. To do this first growers have to create a scatterplot of the average temperature and the PAR light sums. Then the grower has to work consciously on a better temperature light balance by adjusting elements to get the values closer to optimal. This can for instance be done by keeping the vents more closed during the day, to increase average temperatures, humidity and CO2.
Another element that is directly related to assimilate balance, is plant load. Plant load is the number of fruits per square meter greenhouse area. Higher plant load requires lower temperatures, because the assimilate demand would exceed the assimilate availability. According to Hoogendoorn it is preferable to maintain lower plant load combined with a higher temperature regime. “This offers more possibilities to realize steady and healthy crops.”