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Lasers & Sources

Lasers for noninvasive monitoring of sap flow in trees

A laser-based technique can monitor sap flow in trees, solving a long-standing problem in plant biology with additional implications for climate change modeling.
20 March 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0653
Background

Two distinct types of sap flow occur simultaneously in plants. First, water travels upwards from roots to leaves through xylem cells that form a network of microcapillaries spanning the inner region of the stem. Second, sugars produced in the leaves through photosynthesis travel downwards through phloem cells located beneath the bark. Measuring plant water use is important in ecology-related fields from irrigation science to physiology. In addition, phloem flow monitoring coupled with levels of soil and atmospheric CO2 can make a significant contribution to detailed understanding of the terrestrial carbon cycle and, hence, to the dynamics of climate change.

Magnetic resonance imaging (MRI) is at present the only genuinely noninvasive technique for measuring both xylem and phloem sap flow,1 but equipment is bulky, expensive, and unsuited to field measurements. Instead, a variety of intrusive techniques use heat to trace sap flow in the field. Due to probe implantation, these methods disrupt the flow being measured and are unsuitable for phloem monitoring due to the fragility of the tissue.

We have developed a noncontact, laser-based system capable of automated xylem flow monitoring in small trees. We have also demonstrated that the system is capable of resolving phloem sap flow.2


Figure 1. Comparison of xylem flow velocities in golden alder determined by our heat pulse technique and by magnetic resonance imaging of flow.
Application of laser pulse

The underlying principle of measurement consists in applying heat to the surface of the stem and studying the evolution of the temperature over time, above and below the point of heating. When a moving fluid is heated locally, heat is transported downstream by convection and conduction, but upstream only via conduction. Flow velocity can be deduced from the downstream convective temperature profile. Heating is achieved by delivering a laser pulse (?=812nm, pulse length 1-6s, optical power at the sample ~0.5W) to a small area of the stem (1mm high × 5mm wide). An IR camera is used as a noncontact thermometric probe.

Results

To date, all our experiments have been carried out on potted broadleaf saplings where trunk diameter does not exceed 2cm at the point of heating. In the future we will extend our work to much more mature trees. With relatively small plants, reference xylem flow rate measurements are readily available by monitoring the plant's weight loss through evapotranspiration (ET), i.e., water loss through the leaves. As a first approximation, we used a 1D solution of the heat equation for combined convection and conduction in a wood-and-water matrix to calculate flow velocities from thermometric data.

Although this model implies that radial heat transfer occurs instantaneously, we were able to demonstrate a linear relationship between ET data and calculated xylem flow velocities. Furthermore, assuming that the total cross-sectional area of the stem was conductive, a near 1:1 agreement was found between ET rates and calculated xylem flow rates (see Figure 1). Our series of simultaneous heat pulse and MRI xylem flow measurements, carried out at the NMR Centre at Wageningen University in the Netherlands, revealed that only ~25% of the total stem area of the sapling studied was conductive. We consequently revised the mathematical model to account for radial conduction and thermal anisotropy along the radial and longitudinal grains. This led to a near 1:1 agreement with MRI flow data (see Figure 1).

Unequivocal detection of phloem flow was achieved in an oak sapling (see Figure 2). Monitoring of slow-flowing phloem sap in small plants is complicated, however, due to the underlying fast-flowing xylem.


Figure 2. (a) Temperature profiles above and below the point of heating for an intact stem and (b) after removal of phloem tissue. The absence of temperature rise below the point of heating demonstrates that phloem sap flow was observed in (a).
Conclusions

We have demonstrated a noninvasive technique for monitoring water flow in small trees. Furthermore, we observed phloem flow that may hold key information about the terrestrial carbon cycle.2 Future work will focus on improving the mathematical model used for data analysis. In addition, we hope to increase sensitivity to systematically resolve phloem flow.


Carole Helfter
EPS Physics, Heriot-Watt University
Edinburgh, United Kingdom
Carel W. Windt and Henk Van As
Wageningen University
Wageningen, The Netherlands
Maurizio Mencuccini
The University of Edinburgh
Edinburgh, United Kingdom
Duncan P. Hand
Heriot-Watt University
Edinburgh, United Kingdom