During my studies at Wageningen University & Research, we primarily investigated how light, in terms of intensity, spectrum, and its interaction with other environmental parameters, like managing internal water flow, affects plant growth, morphology, and the synthesis of pharmacologically active compounds, such as cannabinoids and terpenoids. However, we always had to ensure that all other variables (except the one under investigation) were held constant and, most importantly, up to the standards of commercial production.
One morphological parameter to consider was root growth and development to ensure sufficient water and nutrient uptake under extreme conditions. Allowing for healthy root growth and development in early plant cultivation ensures that high light intensities and, thus, photosynthesis can be maintained.
Before we discuss the physiology and practices used to ensure vigorous root growth, we should consider why we grow plants and how plants determine which organs they should invest energy in.
Plant cultivation can be optimized in many ways, either in terms of the weight of harvestable product per unit of ground area or in the concentration of valuable compounds per unit of ground area. For various crops, these compounds can vary; for instance, they may include total soluble sugars in fruits and vegetables, or cannabinoids and terpenoids in cannabis.
Total soluble sugars are classified as primary metabolites, such as glucose, sucrose, and fructose, which are necessary for plant growth but also determine the sweetness of the product. Cannabinoids and terpenoids are classified as plant specialized metabolites. These compounds are not directly involved in plant growth but rather in plant defense against environmental stresses such as drought, flooding, high light intensity, and herbivory damage.
Managing Internal Water Flow for Plant Growth Optimization
For plants grown for their specialized metabolites, we must consider how irrigation strategies can optimize both plant growth and, consequently, the harvestable flowers. Also, how can we increase the concentration of cannabinoids and terpenoids by introducing stressful conditions, without limiting plant growth?
Here, the growth-defense trade-off comes into play. Carbohydrates that are synthesized through photosynthesis can be either directed towards primary metabolites to increase plant growth, or towards plant specialized metabolites to improve plant defense (but also the quality of the harvestable product!). Under environmental conditions in which stressors are absent, plant growth is prioritized over plant defense, which, from an evolutionary perspective, makes sense. Young plants should allocate most of their energy to stem elongation and leaf area expansion. This strategy enables plants to outgrow their neighbors and compete more effectively for light, thereby increasing their chances of survival through greater growth.
Internal Water Flow to Ensure Root Initiation
Now, when we consider irrigation, we want to ensure that root initiation (the formation of adventitious roots from cut tissue) is increased, which can be done by applying external rooting powders or gels. One of them is auxin, the primary plant hormone that accumulates at the base of the cutting. Under these conditions, we must ensure that the relative humidity is maintained at around 95%, as there is limited water uptake to compensate for water lost through transpiration. This would otherwise be comparable to being left in the desert without any water.
However, as soon as calluses and root tips form, we want to gradually lower the relative humidity to 75% to stimulate these passive roots to actively grow. The amount of water and nutrients taken up directly depends on the surface area the roots cover within a given volume of substrate. For this reason, we also don’t want the taproot to encircle the substrate and become rootbound. Instead, we want to stimulate the taproot to actively grow side-roots and root hairs, which exponentially increase the root surface area and thus the capacity to take up water and nutrients.
This can be achieved during vegetative growth by maintaining a substrate water content of no more than 80% and allowing it to dry back overnight to 60%, thereby stimulating the roots to actively search for water.
How-To Calculate Substrate Water Content
To calculate the substrate water content of the substrate, there are quite a few nifty sensors out there, but an old-school method could suffice, although the weight of the plant must be considered:
Water content %= Wwet- WdryVsubstrate x 100
Where:
Wwet= weight of substrate when moist (g)
Wdry= weight of substrate when dry (g)
Vsubstrate= known substrate volume (L)
Furthermore, the activity of the roots is also determined by the vapor in the air, which governs internal water flow from the roots through the xylem, into the leaf, and through the stomata out into the environment. Relative humidity does not describe how strong this ‘pull’ from the environment is; it only tells us the amount of water vapor in the air as a percentage of the maximum amount of water vapor that the air could hold at the same temperature.
VPD
The vapor pressure deficit (VPD) indicates the difference between the actual water vapor in the air and the vapor pressure at saturation. For example, a relative humidity of 65% corresponds to a VPD of 1.11 kPa at 25 °C and 1.48 kPa at 30°C. Indicating that at 25 °C and 65% relative humidity, the air can hold 1.11 kPa more water vapor before reaching saturation, and 1.48 kPa at 30 °C before reaching saturation. This unit provides insight into the amount of water being extracted from the plant by the environment, thereby determining the internal water flow from the roots to the stomata.
As mentioned earlier, during the early phases of plant cultivation, we aim to promote vigorous vegetative growth. This can be maintained at high temperatures and moderate relative humidity. For cannabis, this would be around 27 °C during the day and 65% relative humidity.
To understand the influence of temperature and relative humidity on transpiration, feel free to calculate the VPD yourself through the following calculation:
VPD=es-ea=es×(1-RH100)
Where:
es=0.6108×exp?[(17.27T)/(T+237.3)]
es=saturation vapor pressure in kPa
ea=Actual vapor pressure in kPa
RH=the ratio of es and ea*100 in %
T=Air temperature in °C
Up to this point, we have discussed the physiology of plants, focusing on how environmental factors such as light, temperature, humidity, and substrate water content interact to influence plant growth and internal water flow.
In the next article, we will delve more deeply into how you can use VPD, substrate water content, irrigation frequency, and shot sizes to actively steer vegetative growth during the early cultivation phases, and later adjust these environmental setpoints to induce stress, thereby increasing the production of your pharmaceutical active compounds.
