It was hypothesised that as water was withheld from the juvenile tree, soil moisture and sap flow would decline, and that night-time plant water potential will become more negative.
In January 2013, during the summer in Australia, a juvenile lime tree was instrumented with a SMM Soil Moisture Meter, with MP306 Soil Moisture Sensors, and a SFM1 Sap Flow Meter, and a PSY1 Stem Psychrometer. These instruments measure, respectively, volumetric water content, sap flow (plant water use), and plant water potential (Figure 1). It was hypothesised that as water was withheld from the juvenile tree, soil moisture and sap flow would decline, and that night-time plant water potential will become more negative.
The tree was 60cm tall and stem diameter was 2cm diameter growing in a large pot. A single MP306 Soil Moisture Sensor was installed at 6cm depth. The SFM1 Sap Flow Meter can measure woody stems with a minimum diameter of 1cm so this juvenile tree was of a sufficient size. Insulation was placed around the SFM1 to ensure external heat did not affect measurements (Figure 2). Similarly, the PSY1 Stem Psychrometer can be installed on small stems and it needs insulation (Figure 3). Data were collected over a one week period in mid-January 2013.
Figure 4 shows the results of the experiment. On the second day of measurement the temperature reached over 40°C however the sap velocity pattern suggests there was no indication of water stress. As the week progressed, and soil moisture content decreased to approximately 13%, peak sap velocity declined. Night-time stem water potential also became more negative. This decline in sap flow, and the fact that midday water potential remained at -2MPa as the soil dried, suggests stomata closed in order to maintain plant hydration.