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, automated measurements. However, the instrument requires the stem to have a minimum diameter of about 10 mm for reliable measurements. Leaf tissue can be used as an alternative to measure plant water status when plant species do not meet the minimum stem diameter. In situ leaf psychrometers are smaller and identical (in principle) instruments that expand the application to smaller plants such as grasses and shrubs.
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.
Figure 2 Leaf water status of two wheat plants were measured in a controlled growth chamber maintained a mean temperature of 25°C. Leaf 1 and 2, and leaf 3 and 4 were adjacent leaves from the same wheat plant respectively. In situ leaf psychrometers attached to PSY1 data loggers measured each wheat leaf every 10 minutes. Installations were installed on the topside of the leaf.
Wheat plants (4-weeks old) were grown in soil media in a temperature controlled growth chamber. The lighting system of the was set to give 16 hours of light and 8 hours of darkness regulated at a mean temperature of 25°C. Four leaf psychrometers were installed on the topside of the wheat leaf where:
Figure 3 -Diurnal leaf water status measurements collected every 10 minutes from four wheat leaves using in situ leaf psychrometers. Day and night was represented by alternating grey vertical columns. Irrigation was represented by the vertical blue lines, occurred daily to maintain adequate plant water status.
In Figure 3, leaf psychrometers provided reliable measurements of leaf water status during the first four days of data collection based on the immediate response to irrigation and night time recovery. The driest (most negative) leaf water status reached by leaf 1 and 3 was -3.9 MPa. However, leaf 1 showed evidence of unreliable measurements after the two-day drying period. The leaf water status did not show immediate response to irrigation. Leaf 4 showed the similar trends during the drying period, however the leaf water status recovered from the drought condition after irrigation. Leaves 2 and 4 were represented by the red and green lines, respectively. Leaf psychrometers demonstrated reliable measurements throughout the entire trial. During the time course, the leaf water status was never below -3 MPa.
In Situ leaf psychrometers installed on wheat plants provided up to 1 week 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.