Trunk-embedded microtensiometers for tree water stress monitoring

The impact of climate uncertainty on water availability and subsequent effects on tree health and yield is prompting industry to find tools that can monitor water stress levels accurately and timelessly within an orchard.  

Key points 

• FloraPulse remote sensor offers real alternative to traditional water stress monitoring methods. 
• Remote sensors could be used as triggers for irrigation timing to increase water use efficiency. 

As part of the PIPS 4 Profit Pear production systems for future climates (AP22002) project, Agriculture Victoria researchers are monitoring irrigation water use across orchards in the Goulburn Valley and evaluating the suitability of different technologies to monitor tree water stress efficiently and accurately. 

Although several plant and soil parameters can be used to assess water stress, the most reliable indicator is stem water potential. Stem water potential is the amount of water tension within the xylem which changes based on the level of tree water stress. For many years, portable pressure chambers have been used for accurate measurements of stem water potential, with a proven track record. However, they can be time consuming, leaves undergo destructive measurements and data is not recorded continuously. These drawbacks have led to research into other sensing tools that may help reduce labour input through more autonomous data capture to monitor real-time plant water stress. 

A trunk-embedded microtensiometer was recently developed by FloraPulse, following extensive research at Cornell University, United States. The sensor has the market edge of continuously monitoring stem water and sending the captured data directly into a cloud-based platform, using FloraPulse’s dataloggers. This enables plant water stress to be tracked in real-time with no labour input. An annual subscription also provides irrigation decision support. As alternatives to FloraPulse dataloggers, traditional dataloggers such as the Campbell Scientific CR1000 can be interfaced with the microtensiometers. These sensors offer a practical alternative to traditional tools and provide real-time water stress of the plant. 

Testing microtensiometer technology

Although FloraPulse and scientists overseas have validated these sensors in several crops, including pear, the Agriculture Victoria team assessed the technology’s capabilities under the Goulburn Valley conditions within the AP22002 project. To assess the accuracy and practicality of the sensors in estimating stem water potential, a small experiment was set up at the Tatura SmartFarm. One FloraPulse sensor was implanted next to a trunk dendrometer in a mature pear tree (post-harvest) – a trunk dendrometer measures the shrinkage and swelling of the trunk, which has previously shown to be a good indicator of plant water stress. The FloraPulse and trunk dendrometer readings followed a similar trend over a period of six days (Figure 1). 

Fig 1: FloraPulse and dendrometer readings plotted over several days.

The FloraPulse sensor was also tested against manual measures of stem water potential using a pressure chamber (PMS 600, PMS Instrument Company, Albany, OR, US). Pressure chamber readings commenced at predawn and were taken at approximately one-hour intervals. A good relationship was found between the stem water potential readings obtained with FloraPulse and the portable pressure chamber (Figure 2), although the FloraPulse sensor underestimated the stem water potential at low values and overestimated it at higher values. This indicates that data obtained with FloraPulse sensors requires initial calibration to obtain absolute values of stem water potential. 

Fig 2: Stem water potential measured with a portable pressure chamber plotted against the FloraPulse sensor readings.

An additional benefit of microtensiometers over dendrometers is that they don’t require adjustment for changes in trunk diameter as a tree grows. The sensor also responded quickly after irrigation, indicating that it is sensitive to changes in water stress. In practice, these sensors could be used as triggers for irrigation timing to avoid over or under irrigating. However, use over multiple seasons needs to be evaluated to determine whether performance is maintained over time and whether readjustments or recalibrations are necessary.  

FloraPulse sensor installed in a pear tree.

Acknowledgement  

This study is a component of the PIPS 4 Profit program’s Pear production systems for future climates (AP22002) project, funded by Hort Innovation, using the Hort Innovation Apple and Pear research and development levies, contributions from the Australian Government and co-investment from Agriculture Victoria. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture. 

This article was first published in the Winter 2024 edition of AFG.