Newton Tran & Alec Downey
University of Guelph
Measuring plant water status has become the most accurate method for developing efficient and effective horticultural management practices. In-situ stem psychrometers are the most responsive field applicable instruments available to monitor plant water status. When paired with the PSY1 data logging system (ICT International Pty., Armidale, NSW, Australia) it allows for high temporal, wireless, and automated measurements. In-situ leaf psychrometers are smaller and identical (in principle) instruments that expand the application to smaller plants such as grasses and shrubs. Corn plants were selected as a trial species to expand applications for the leaf psychrometer.
A flat portion (top or underside) of the leaf was inserted into the leaf psychrometer clamp to outline the installation area. The clamp was secured into position to a stand. The cuticle layer of the leaf was abraded by rotating 400-600 grit sand paper to increase water vapour exchange between the leaf tissue and the leaf psychrometer. Arrangement of the clamp and tools used for abrasion are shown in Figure 1.
Figure 1 – Leaf placed into psychrometer clamp to outline installation site before abrasion (Left). A leaf abrasion tool can be constructed using hole punched (1/4” diameter) sand paper glued to wooden dowels.
After the leaf surface was abraded, the surface was rinsed and dried to remove potential solute contaminants that may offset the leaf water status. The leaf psychrometer was prepared by applying a ring of silicon grease on the leaf psychrometer perimeter and spread across the surface of the psychrometer. A slight downward pressure and rotation was applied to evenly spread the grease surrounding the installation site. The leaf psychrometer was secured by using the tightening screw.
Three corn plants were grown in a temperature controlled growth chamber with 16 hours of light and 8 hours of darkness. Two leaf psychrometers were installed per plant at different heights. One leaf psychrometer was installed on the top leaf while the other was installed at the bottom leaf of the plant.
Figure 2 – In situ leaf psychrometers installed on two different leaf locations, located on the top and bottom leaf of
the same plant.
Figure 3 – Diurnal leaf water status measurements collected every 10 minutes from one corn plant. In situ leaf psychrometers were installed on the top and bottom leaf. Irrigation was represented by the vertical blue lines, occurred daily to maintain adequate plant water status. The green and black line represents the leaf water status of the top and bottom leaves, respectively.
In Figure 3, the leaf water status measurements from the top and bottom corn leaves over a 16-day period demonstrated similar diurnal responses. Corn plants were subjected to two drying phases and was immediately irrigated when the leaf water status showed evidence of excessive water stress conditions. During the first irrigation event, the water status remained above -0.50 MPa which indicated the plants were not under significant stress, however the soils of the containers visually looked dry. After irrigation, the plant was subjected to another drying period of 9 days. The water status after these 9 days decreased to -1.2 MPa before irrigation was applied. The installation provided reliable measurements based on the response produced by irrigation in the leaf water status. Overall the corn plants were able to provide reliable measurements for a 1 to 2 week duration. Reliable measurements were dependent on a systematic abrasion technique and consistent monitoring of the leaf water status.
To expand on the response seen in Figure 3, the w-shaped response in the corn leaves provides an opportunity to understand how the leaf manages water resources diurnally. One interpretation of this phenomena is, immediately after the lights were switched on, the stomata began gas exchange with the environment. The water status decreased to develop a potential gradient for water movement into the leaves and into the environment. Immediately after the plants obtained sufficient resources from the environment, the stomata close to reduce water loss and recover leaf water status to prevent mid-day stress. Although a midday stress did occur, this management system allowed the plant to maintain a higher leaf water status than if the water status was to decrease immediately
In Situ leaf psychrometers installed on corn plants provided up to 2 weeks of reliable measurements. The installation duration period is dependent on the systematic abrasion technique that carefully removes the cuticle layer of the leaf. Removing too much of the cuticle will reduce the installation time by increasing oxidation on the abraded surface. Immediate response to irrigation and fluctuations of day and night is a strong determinate of the leaf psychrometer providing reliable measurements. Measuring the plant water status opens opportunities to further understanding physiological response of corn.