Comparative monitoring of temporal and spatial changes in tree water status using the non-invasive leaf patch clamp pressure probe and the pressure bomb

Abstract

Real-time monitoring of plant water status under field conditions remains difficult to quantify. Here we give evidence that the magnetic-based leaf patch clamp pressure (LPCP) probe is a non-invasive and online-measuring method that can elucidate short- and long-term temporal and spatial dynamics of leaf water status of trees with high precision in real time. Measurements were controlled remotely by telemetry and data transfer to the Internet. Concomitant measurements using the pressure chamber technique (frequently applied for leaf water status monitoring) showed that both techniques yield in principle the same results despite of the high sampling variability of the pressure chamber data. There was a very good correlation between the output pressure signals of the LPCP probe and the balancing pressure values (on average r² = 0.90 ± 0.05; n = 8), i.e. the external pressure at which water appears at the cut end of a leaf under pressure chamber conditions. Simultaneously performed direct measurements of leaf cell turgor pressure using the well-established cell turgor pressure probe technique evidenced that both techniques measure relative changes in leaf turgor pressure. The output pressure signals of the LPCP probe and the balancing pressure values were inversely correlated to turgor pressure. Consistent with this, the balancing pressure values and the cell turgor pressure values could be fitted quite well by the same firm theoretical backing derived recently for the LPCP probe (Zimmermann et al., 2008). This finding suggests that use of the LPCP probe technique in agricultural water management can be built up on the knowledge accumulated on spot leaf or stem water potential measurements.

Introduction

Irrigation scheduling involves determining precisely both the timing of irrigation and the quantity of water to apply. Irrigation scheduling decisions are frequently based on the determination of soil moisture content or soil moisture tension. Local measurements of soil water status have, however, the drawback that they do not give direct information about the water needs of a plant (Naor et al., 2008 and literature quoted there). By contrast, determination of plant-based water stress indicators allows optimum irrigation and thus high efficiency use of the worldwide scarce water resources (Steduto et al., 2007).

Irrigation scheduling is usually based on soil water measurements. An inherent problem determine the water content is that changes in the bulk soil water content do not necessarily reflect plant water demands (Jones, 2004). Higher precision in the application of irrigation is expected by use of plant-based methods, i.e. by methods which directly measure the water supply of plants. There are several plant-based approaches that are used for monitoring water stress in trees such as psychrometers (McBurney, 1988), sap flow techniques (Fernández et al., 2001, Fernández et al., 2006, Green et al., 2003, Smith and Allen, 1996), water content variations in stems by time domain reflectometry (Nadler et al., 2003, Nadler et al., 2006) and stem dendrometers (Goldhamer and Fereres, 2001, Zweifel et al., 2000, Zweifel et al., 2001). These methods have found broad applications in basic research, but are too sophisticated for routine applications by growers (at least at the present). Therefore, leaf or stem water potential, measured with the pressure chamber (Scholander et al., 1965) at midday, has been proposed as standard parameter to determine the plant water status for irrigation scheduling of fruit trees (Fereres and Goldhamer, 2003, Naor, 2001). Advantage of the pressure chamber is that no sophisticated instrumentation is required and that measurements are easy to be performed. Balancing pressure, P_b, values are measured by this technique, i.e. the external pressure is recorded at which water appears at the cut end of a leaf or a leafy twig kept at atmosphere. P_b values are usually in the megapascal range. Drawback of the pressure chamber technique is that it is destructive, slow, labour intensive and unsuitable for automation. Thus, spot measurements are usually only possible which may prevent precise water management of crops and fruit trees. Moreover, P_b values determined on various leaves or twigs taken at the same time show very often a large scatter. Among other things, values can also depend on the microclimate at the measuring site (Zimmermann et al., 2004).

The disadvantages of the pressure chamber are not shared by the recently developed online-measuring leaf patch clamp pressure (LPCP) probe (Zimmermann et al., 2008). The online-measuring LPCP probe measures plant water status in real time and is characterised by high precision, operating convenience, automation suitability and minimum costs. Multiple probes can, in principle, be clamped on leaves over the entire height of a plant for unravelling the dynamics of leaf water status at different sites of a plant. For real-time evaluation data can be sent via wireless telemetric units to a data logger or more conveniently to a GPRS (General packet radio service) modem linked to an Internet server via a mobile phone network. The entire setup allows simultaneous data processing of multiple probes.

The probe technique measures the pressure transfer function of an intact leaf, i.e. the attenuation of an externally applied clamp pressure by the leaf tissue. The clamp pressure is generated by springs or – more user-friendly – by magnets. Its magnitude depends on leaf-specific structural features. Theory and experiments on grapevine, banana and lianas have shown (Westhoff et al., 2009, Zimmermann et al., 2008, Zimmermann et al., 2010) that the pressure transfer through a leaf patch is dictated predominantly by turgor pressure. High turgor pressure prevents pressure transfer through the leaf and, in turn, the output pressure P_p measured by the probe is small. At very low turgor pressure the transfer function assumes values close to unity, i.e. the applied pressure is transferred to the pressure sensor at most and P_p assumes a maximum value (Zimmermann et al., 2008). By contrast to the balancing pressure values P_p values are much smaller ranging between 5 kPa (corresponding to maximum turgor pressure) to 200 kPa (corresponding to minimum turgor pressure).

To assess the potential of the LPCP probe as a novel and universally usable water stress monitoring method for agricultural water management and for the elucidation of the mechanisms of water ascent in tall trees, we have applied the LPCP probe to leaves of various tree species. Among other things, an important aim was to figure out whether the LPCP probe can replace the pressure chamber for measuring water relations of trees. Despite large differences in morphology, compressibility and turgescence of the leaves of the various species we will demonstrate that the LPCP probe technique measures similar diurnal changes in leaf water status as the pressure chamber technique, but continuously and at a much higher resolution and accuracy. We will further show by direct cell turgor pressure measurements using the cell turgor pressure probe (Zimmermann et al., 2004) that the agreement between the pressure chamber and probe data are due to the fact that both techniques ultimately monitor relative changes in leaf turgor pressure.