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Enabling better global research outcomes in soil, plant & environmental monitoring.

Psychrometer use In Medicinal Cannabis

Jonathan Stemeroff and Mike Dixon
University of Guelph

Standardisation of crop production and horticultural and environment control management strategies are particularly relevant goals for phyto-pharmaceutical crop producers. Standardisation of a crop production cycle, like any commercial or industrial process, is multifaceted and should be subject to continual evaluation and improvement.

In situ stem psychrometers (PSY1) (ICT International Pty. Ltd., Armidale, NSW) were used to quantify drought stress in response to modified irrigation scheduling for the ‘Wappa’ cultivar of Cannabis sativa (C. sativa).
Measurements were made every 15 minutes to provide a high degree of temporal resolution of plant water status.

The integrated or cumulative water potential (cWP) was correlated with cumulative vapour pressure deficit (cVPD) in order to establish a predictable relationship between this key environment variable and plant response. This will provide the grower with an irrigation management strategy based on a parameter (VPD) that is routinely monitored by their environment control system.

Three different irrigation regimes were tested and the results in terms of plant water status were monitored using the stem psychrometers (PSY1). The first treatment was the control which was the standard irrigation regime used (irrigation every 1-2 days). Additional treatments were (i) mild-water stress (irrigation every 2 days), and (ii) moderate-water stress (irrigation every 3 days). All plants were grown in 1.5 gallon pots and were hand watered with 2L of fertigated water. Nutrient rates and all other environment variables were the same for all treatments.

Instrument Installation:
A section of the water conducting tissue (xylem) was exposed by removing the bark with a razor blade as shown in Figure 1.


Figure 1: Exposed xylem tissue (water conducting tissue) to which a stem psychrometer will be attached (Photo credit: J. Stemeroff)

The installation site was rinsed and dried before attaching the stem psychrometer using a clamp. Clear silicone grease was applied around the psychrometer mounting point to seal surrounding tissue and provide a vapour seal between the edge of the psychrometer and the xylem as shown in Figure 2.


Figure 2: Installed stem psychrometer. Psychrometer face is placed against exposed xylem and clamped in place. (Photo credit: J. Stemeroff)

Insulation was added around the psychrometer using polyester batting (Figure 3) and heavy-duty aluminium foil (Figure 4). This insulation was used to maintain a stable temperature for each psychrometer installation.


Figure 3- Polyester insulation batting on psychrometer (Photo credit: J. Stemeroff)


Figure 4: Aluminium foil on psychrometer (Photo credit: J. Stemeroff)


Figure 5: Blocks of plants along bench perimeter (Photo credit: J. Stemeroff)

The plants were arranged into four equal sized blocks with plants randomly distributed within each block (Figure 5).
Blocks were spread over two benches using the perimeters for easy access to the psychrometers for maintenance.
When the benches were pushed together there was a large canopy creating similar environmental conditions for all plants (Figure 6).

At the end of the flower cycle all plants were harvested and had the major fan leaves removed. They were then dried according to standard procedure.


Figure 6: Plant Canopy showing consistent environment (Photo credit: J. Stemeroff)

The stem psychrometer provided detailed and accurate measurements of plant water status throughout the production cycle of this medicinal crop. The relationship between cWP (cumulative water potential) and cVPD (cumulative vapour pressure deficit) was established for each treatment and will provide fine tuning data for irrigation management strategies and environment control.
When using the data from the second half of the flower cycle, the correlation was stronger. This is likely due to the shift from vegetative growth to flower development and maturation, resulting in a more stable or consistent water demand.

“This study was co-funded by ABcann Medicinals Inc. and the Ontario Centres of Excellence with technical support and consultation from ICT International Pty. Ltd.”