UNDERSTANDING LEAF TRANSPIRATION MECHANISMS WITH LGR WATER VAPOUR ISOTOPIC ANALYZER

Leaf transpiration is the process in which plant roots absorb water and then release the water in the form of vapour through the leaves. It is an important factor in the water cycle as it is one of the major sources of water into the atmosphere (10%). The leaf transpiration process is nearly identical to perspiration or sweating in animals.

Leaf transpiration is the process in which plant roots absorb water and then release the water in the form of vapour through the leaves. It is an important factor in the water cycle as it is one of the major sources of water into the atmosphere (10%). The leaf transpiration process is nearly identical to perspiration or sweating in animals.

The stable isotopic composition of plant transpired water (d_T) is the result of complex interactions between liquid water at the evaporation site, water vapor in ambient air, and the environmental conditions inside and outside the leaf. It is a powerful tracer used to characterize plant processes in the fields of ecology, plant physiology and hydrology.

In practice, the d_T value is rarely directly measured due to the intensive labor and time involved in cryogenic water vapor collection and measurement using isotopic ratio mass spectrometry methods (IRMS). Even with recent advances for faster sampling, traditional IRMS based cold-trap methods are still not capable of resolving values of d_T at minute to hourly resolutions. Because of the lack of fast-response sensors capable of resolving the isotopic composition of water vapor in ambient air, indirect methods are often used to estimate d_T . The plant stem water is typically useed as a proxy for the value of. However the underlying assumptions on isotopic steady state of the leaves are generally valid only for timescales much greater than the turnover time of water in the leaves and in the absence of rapidly changing environmental. On small timescales of minutes to hours, non-steady state isotopic enrichment is also common in many natural systems from leaf to canopy scales, especially during early morning and late afternoon, when transpiration rates are lower.

In the attached paper ”Direct quantification of leaf transpiration isotopic composition”, a group of biologists and environmental scientists from Princeton, New South Wales and Australia National universities describe a new direct method for continuous measurement of d_T based on isotopic mass balance of both bulk water vapor and the stable isotopes of water vapor (¹⁸O or ²H) within a leaf chamber.

Schematic of the leaf transpiration chamber setup (Wang et al.)

Their experiments used an LGR Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) Water Vapour Isotopic Analyzer setup for 1 Hz rate measurements and coupled to a transparent leaf chamber containing a transpiring leaf in a flow-through system arrangement.

ABB LGR Water Vapour Isotopic Analyzer WVIA-911

The method was applied to obtain direct measurements of the isotopic composition of leaf transpiration under field conditions in central Kenya. It was also tested in laboratory conditions using a Dew Point Generator as a continuous water vapor source with specific isotopic compositions using different combinations of dew point temperature setting and liquid water source, in order to simulate transpiring leaves.

Schematic of the experimental setup for directly quantifying leaf transpiration isotopic composition in field (a) and laboratory (b) settings. The OA-ICOS instrument refers for LGR analyzer. For laboratory measurements a dew point generator is placed into the transpiration chamber, mimicking a transpiring leaf. (Wang et al.)

The field trials in Kenya demonstrated the method’s ability to capture rapid d_T changes in response to environmental fluctuations that occur on the time scale of seconds to minutes.

The real-time isotopic measurements in laboratory conditions with artificial leaves were compared to modelling results based on traditional gas exchange principle equations in steady and non-steady states, and they showed excellent agreement.