A Complicated balance
Plants must be able to counteract environmental changes in order to stay healthy and reproduce. Altering one environmental condition can lead to limitations in other factors. These interactions are highly complex and triggered by environmental stimuli, which is then relayed as gene expression. Some of the more straightforward interactions as they relate to light and controlled environment agriculture are explained below.
Low or high PAR Light
Under light limiting conditions, the plant will respond morphologically to try to grow into a better light environment. This is sometimes referred to as a “stretching” or “shade-avoidance” response. Optimal light levels for a plant vary with species, and even too much light can cause serious injury to the photosynthetic “machinery” and even death to the plant because the plant can not handle an excess of light energy. Other environmental factors that can affect a plant’s ability to use light effectively include CO2, nutrients, temperature, and humidity.
Adding CO2 to the environment is a common practice in order to improve plant growth, especially under high light conditions. This is because plants are not very efficient at pulling CO2 out of ambient air. The chart to the left shows recommended CO2 supplementation levels by growth stage. This applies to light intensive crops, such as cannabis, tomato, pepper, and cucumber.
Nutrient Solution (EC & pH)
The electrical conductivity (EC) is a good measure of the concentration of micro/macro-nutrients in the media solution. Recommended ranges for different growth stages are shown in the chart to the left. To make sure these nutrients are available in their proper chemical form, the pH should be maintained between 5.2 – 6.8. Growers should test the leachate (media not taken up by the plant) to determine if build up is occurring.
Ambient temperature is a critical factor in plant growth and development, with each crop species growing best under a specific temperature range. Often, the ideal temperature is slightly different for day vs night cycles, and over the growth stages. A typical range for light intensive crops, such as cannabis, tomato, pepper, and cucumber, can be seen in the chart to the right. Ambient temperature is not the same as leaf temperature, which is further affected by humidity, air flow and evapotranspiration rates (see below).
The relative humidity (RH) is the percentage of the amount of water vapor in the air to the amount of water vapor at the point of saturation. It is important to provide enough relative humidity during the plant establishment stage. On the other hand, high RH can lead to slower growth rates and more disease susceptibility for plants in the flower stage.
Evapotranspiration describes the two processes (evaporation and transpiration) in which there is transfer of water from the soil and leaf surfaces into the air. The process is controlled by temperature, RH, and air flow. As temperature increases, the amount of water vapor that will saturate the air increases, resulting in decreased RH. This results in an increased vapor pressure deficit (VPD), pulling more water from the roots and driving increased evapotranspiration. The result of this process, which was triggered by increased temperature, is a cooling of the leaf surface. The opposite occurs when the temperature decreases. In this way, plants adapt to changing temperature with altered water/nutrient flow. Increased air flow will increase evapotranspiration as well as improve CO2 availability to the plants.
Plants are governed by complicated biochemical reactions which are controlled by many different signals from the environment. Assuming that correct light levels have been determined, it is important to maintain the other environmental parameters such as CO2, water, nutrient supply, temperature, and RH so that maximum productivity can be obtained.