By: Due Diligence Horticulture

February 3, 2025

Plant growth is driven by CO2 assimilated through photosynthesis. The exchange of CO2 from the atmosphere into the leaf happens through small pores on the leaf surface called ‘stomata’. For CO2 to diffuse inside the leaf and made available to the site of photosynthesis, water must diffuse out of the stomata through a process called transpiration. Photosynthesis (growth) and transpiration (water use) are positively related (Figure 1). The ratio of growth to transpiration is called water use efficiency (WUE). Practically defined as the amount of biomass or yield produced per  unit of water transpired by the crop, WUE is  typically expressed in units of grams per liter (g per L). 

graph representing water use efficiency is the ratio of biomass to transpiration
Figure 1: Water use efficiency is the ratio of biomass to transpiration.

WUE can be affected by multiple environmental factors, but the most critical in controlled  environments are CO2 concentration[1,2] and vapor pressure deficit (VPD)[3]. Higher WUE is desirable  because plants produce more biomass with less water, making it a critical concept in sustainable agriculture. The concept of WUE is not only critical for managing water but also in optimizing nutrient solution concentrations[4,5].  

Here we will review how CO2 and VPD influence WUE, and how changes in WUE impact nutrition of cannabis. By understanding these principles, water and nutrient management can be optimized to maximize yield while minimizing waste and unnecessary costs.

Environmental Factors Influencing WUE:

In the absence of environmental stress (e.g. drought, nutrient deficiency), the two most important  environmental factors that impact WUE are CO2 concentration and VPD. At ambient CO2 (~420 ppm)  and a VPD of 1.2 kPa, WUE is about 3 grams per liter6. Elevated CO2 reduces transpiration and  increases growth, thus WUE increases with increasing CO2. Similarly, low VPD (high relative  humidity) reduces transpiration, so WUE increases with decreasing VPD. Estimates for WUE at  elevated CO2 (1200 ppm) and low VPD are as high as 6 g per L, but 4 to 5 g per L is more typical[6]

Application of WUE in Nutrient Solution Optimization:

The necessary concentration of nutrients in solution can be calculated by multiplying the desired concentration in plant tissue by the WUE. Numerous sources are available containing reference  ranges for optimal ranges of nutrients in cannabis leaves (Table 1, 2) [7].

Published reference macronutrients values for floral hemp leaf tissue concentrations
Table 1: Published reference macronutrients values for floral hemp leaf tissue concentrations.
Hemp Leaf Tissue Nutrient Ranges
Hemp Leaf Tissue Nutrient Ranges: Refinement of Reference Standards for Floral Hemp | NC State Extension Publications. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges

For example, if the plant tissue requires 3% (30 mg per g) nitrogen and the WUE is 3 g of biomass per  liter of water, the optimal nitrogen concentration in the solution is 90 mg per L (ppm). To achieve 100  ppm (1 mg per g) iron in the leaves, the solution concentration should be 0.3 mg per L (ppm).

Relating Environmental Effects on WUE to Changes in Nutrition:

As WUE changes with environmental conditions (e.g., elevated COor humidity), the concentration of the nutrient solution needs to be adjusted. For instance, WUE in high COenvironments can increase from 3 g per L to 6 g per L, meaning nutrient solution concentrations must be doubled to maintain optimal nutrient supply[6]. The values in Table 3 effectively provide upper and lower limits for optimal nutrient solution concentration. Recent research related to nutrition of cannabis generally agree with the ranges provided in Table 3 [8–12]. It’s important to remember that the optimal concentration depends on other factors, including the volume of water delivered. For example, 2 liters at 15 ppm P delivers the same amount of P as 1 L at 30 ppm P. The amount of excess nutrient solution (leachate) is therefore an important consideration. A leachate fraction of 10% is a common target in horticulture, but it is possible to supply low and steady concentration without leaching, resulting in no waste and no excess costs[13].

Calculated nutrient solution concentration at a WUE of 3 and 6 g per L
Table 3: Calculated nutrient solution concentration at a WUE of 3 and 6 g per L. Adapted from Langenfeld et al. (2022)

Conclusion:

While there will be differences in WUE and the optimum nutrient solution concentration among species, cultivars, and environmental conditions,  this theoretical framework is supported by  decades of research. Cannabis has proved to be  unique in several aspects of nutrition (e.g. hyper accumulation of P in flowers[8,11]), but mineral  nutrition of plants is well studied. 

Understanding WUE and how it changes with the environment enables growers to strategically adjust nutrient solution based on the environment their plants are growing in. A well-balanced fertilizer  solution delivered at the appropriate concentration improves plant health, promotes environmental  sustainability, and drives financial growth.

 

References

  1. Morison, J. I. L. Sensitivity of stomata and water use efficiency to high CO2. Plant Cell Environ 8, 467–474  (1985). 
  2. Hui, D. et al. Canopy radiation- and water-use efficiencies as affected by elevated [CO2]. Global Change  Biology 7, 75–91 (2001). 
  3. Rawson, H. M., Begg, J. E. & Woodward, R. G. The effect of atmospheric humidity on photosynthesis,  transpiration and water use efficiency of leaves of several plant species. Planta 134, 5–10 (1977). 
  4. Hoagland, D. R. The Water-Culture Method for Growing Plants without Soil. (College of Agriculture,  University of California, Berkeley, Calif, 1950). 
  5. Bugbee, B. NUTRIENT MANAGEMENT IN RECIRCULATING HYDROPONIC CULTURE. in Acta  Horticulturae 99–112 (2004). doi:10.17660/ActaHortic.2004.648.12. 
  6. Langenfeld, N. J., Pinto, D. F., Faust, J. E., Heins, R. & Bugbee, B. Principles of Nutrient and Water  Management for Indoor Agriculture. Sustainability 14, 10204 (2022). 
  7. Hemp Leaf Tissue Nutrient Ranges: Refinement of Reference Standards for Floral Hemp | NC State  Extension Publications. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges. 
  8. Shiponi, S. & Bernstein, N. The Highs and Lows of P Supply in Medical Cannabis: Effects on Cannabinoids,  the Ionome, and Morpho-Physiology. Frontiers in Plant Science 12, (2021). 
  9. Saloner, A. & Bernstein, N. Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis  (Cannabis sativa L.). Industrial Crops and Products 167, 113516 (2021).
  10. Saloner, A. & Bernstein, N. Effect of Potassium (K) Supply on Cannabinoids, Terpenoids and Plant Function  in Medical Cannabis. Agronomy 12, 1242 (2022).
  11. Westmoreland, F. M. & Bugbee, B. Sustainable Cannabis Nutrition: Elevated root-zone phosphorus  significantly increases leachate P and does not improve yield or quality. Frontiers in Plant Science 13, (2022).
  12. Bevan, L., Jones, M. & Zheng, Y. Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless  Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in Plant  Science 12, 2587 (2021).
  13. Langenfeld, N. J. & Bugbee, B. Sustainable Hydroponics Using Zero-discharge Nutrient Management and  Automated pH Control. (2024) doi:10.21273/HORTSCI17975-24.