North Carolina State University researchers Professor Ricardo Hernandez and Cristian Collado worked with Current’s Arize Element L1000 LED top lighting in a greenhouse setting to explore the impact of light levels on the production of cuttings, plant growth, flower production, quality, and water use of a cannabis sativa variety commonly cultivated for its high levels of CBD. The impact of different levels of light throughout the vegetative and reproductive phases of growth was isolated by controlling all other environmental factors, such as temperature, fertigation, CO2, and water usage.
Cannabis water use efficiency (WUE) refers to the amount of water a cannabis plant uses to produce a certain amount of biomass or yield. Supplemental light, such as artificial lighting in indoor cultivation, can have significant effects on a plant’s water use efficiency.
Increased Photosynthesis
Supplemental light, especially in indoor growing environments, can enhance photosynthesis in cannabis plants. When plants can capture more light energy, they can convert more carbon dioxide and water into sugars and other organic compounds. This increased photosynthetic activity can potentially lead to improved water use efficiency, as more water is used for productive processes.
Transpiration and Stomatal Regulation
Transpiration is the process by which water is released from a plant’s leaves through small openings called stomata. These openings also allow for the exchange of gasses, including carbon dioxide and oxygen. When more light is available, plants often open their stomata wider to take in more carbon dioxide, which can lead to increased water loss through transpiration. This could potentially decrease water use efficiency if not properly managed.
Optimal Lighting Management
To maximize water use efficiency under supplemental light, it’s important to manage light levels effectively. Providing the right amount of light for the growth stage of the cannabis plant can help maintain a balance between photosynthesis and transpiration. Using light intensity and duration strategies, growers can optimize the plant’s ability to produce energy while minimizing excessive water loss.
Growing Medium and Watering Techniques
The choice of growing medium (soil, coco coir, hydroponics, etc.) and the watering techniques employed can also influence cannabis water use efficiency. Proper substrate choice and irrigation practices can help regulate water availability to the plant roots, preventing both water stress and waterlogging — both of which can impact WUE.
Genetics and Environmental Factors
Cannabis cultivars vary in their response to light intensity and other environmental factors. Some strains may exhibit better water use efficiency under supplemental light compared to others. Additionally, environmental conditions such as temperature, humidity, and CO2 levels can also influence water use efficiency.
To push these limits, Callado and Hernandez regulated and analyzed the quantity and demand of resources and plant growth factors on an ongoing basis. They added light and water-control and measuring capabilities to every plot in the greenhouse, in addition to measuring temperature and evapotranspiration.
Cannabis crops were grown under four light levels using two Current dimmable fixtures per plot supplementing sunlight. The L1000 PPB lighting fixtures delivered uniform supplemental light intensities of 150, 300, 500, and 700 μmol m⁻² s⁻¹ for 18 hours, while the Daily Light Integral (DLI) from the sun and LEDs were, on average around 18, 30, 40, and 52 mol m⁻² d-1. However, they present preliminary results for the three highest light levels.
Moreover, the fertigation system was triggered independently at each plot when the pots’ water container capacities were 80%. This maintained consistent water and nutrient levels in pots regardless of the crop growth rates. Finally, the water use was quantified with load cells (scales) under the plants.
The Results and Conclusions
It’s easy to conclude from known knowledge that the impact of supplemental light on cannabis water use efficiency can be complex and depends on various factors, including light intensity, duration, genetics, and environmental conditions. Proper management of these factors, along with optimized growing practices, can help improve water use efficiency in cannabis cultivation.
As the cannabis industry continues to evolve, research and experimentation in this area will provide more insights into how to achieve the best water use efficiency outcomes.
The results from Callado and Hernandez suggest that increasing the light amount not only increases the number of branches or cuttings per plant but also could increase the water demand (Figure 2b) and water-use efficiency to produce cuttings (less water per cutting) (Figure 2b).
In other words, plants grown under an average DLI of 30 mol m-2 d-1 for 21 days produced close to 29 cuttings per plant, while plants grown at 52 mol m-2 d-1 produced 47 cuttings per plant from new secondary branches.
Furthermore, plants grown under 30 mol m-2 d-1 produced 2.5 cuttings per every liter of water, while plants grown under 52 mol m-2 d-1 produced 4.3 cuttings per the same liter of water. This means the crops were more efficient at transforming water into branches under higher light intensities.
So, how does this impact commercial growers?
The current research highlights the ability of a cannabis crop to use higher light levels to increase yield and water-use efficiency (higher yield per liter of water). The water-use efficiency for cutting production went from 2.5 to 4.3 cuttings per liter of evapotranspirated water when growing plants under 30 versus 52 moles of light per day, respectively. This would mean that to produce 100 cuttings using 52 moles of light, growers needed 23 liters of water instead of 40 liters under 30 moles of light.
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