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Tree water relations and their dependence on microclimate and soil moisture were studied over several months in young oaks (Quercus robur L.) subjected in large lysimeter-based open top chambers to environments with a controlled soil water supply. Automated single point dendrometers and the recently developed leaf patch clamp pressure (LPCP) probe were used for monitoring water-related stem radius variations (ΔW) and turgor-dependent leaf patch pressures (Pp). Both parameters showed distinct diurnal patterns with sharp negative and positive peaking of ΔW and Pp, respectively, after solar noon and recovery to initial levels in the evening. During the day, varying solar radiation was responsible for short time fluctuations of Pp in the range of minutes to hours reflecting feedback regulation of leaf turgor by sunlight driven stomatal movements. At longer timescales, i.e. days to months, atmospheric vapour pressure deficit (VPD) and soil water content (SWC) were the main determinants of ΔW and Pp. Daily minimum and maximum values of ΔW and Pp decreased and increased, respectively, with increasing VPD or decreasing SWC and recovery of ΔW and Pp in the evening was impeded by low SWC. In well-watered oaks, daily positive peaking of Pp preceded daily negative peaking of ΔW; these time lags gradually increased with increasing soil drought, suggesting hydraulic uncoupling of stem and leaves.

The non-invasive leaf patch clamp pressure (LPCP) probe measures the attenuated pressure of a leaf patch, P(p) , in response to an externally applied magnetic force. P(p) is inversely coupled with leaf turgor pressure, P(c) , i.e. at high P(c) values the P(p) values are small and at low P(c) values the P(p) values are high. This relationship between P(c) and P(p) could also be verified for 2-m tall olive trees under laboratory conditions using the cell turgor pressure probe. When the laboratory plants were subjected to severe water stress (P(c) dropped below ca. 50 kPa), P(p) curves show reverse diurnal changes, i.e. during the light regime (high transpiration) a minimum P(p) value, and during darkness a peak P(p) value is recorded. This reversal of the P(p) curves was completely reversible. Upon watering, the original diurnal P(p) changes were re-established within 2-3 days. Olive trees in the field showed a similar turnover of the shape of the P(p) curves upon drought, despite pronounced fluctuations in microclimate. The reversal of the P(p) curves is most likely due to accumulation of air in the leaves. This assumption was supported with cross-sections through leaves subjected to prolonged drought. In contrast to well-watered leaves, microscopic inspection of leaves exhibiting inverse diurnal P(p) curves revealed large air-filled areas in parenchyma tissue. Significantly larger amounts of air could also be extracted from water-stressed leaves than from well-watered leaves using the cell turgor pressure probe. Furthermore, theoretical analysis of the experimental P(p) curves shows that the propagation of pressure through the nearly turgorless leaf must be exclusively dictated by air. Equations are derived that provide valuable information about the water status of olive leaves close to zero P(c) .

ABSTRACT:The non-invasive, magnetic leaf patch clamp pressure probe (also termed ZIM-probe) allows for the first time to measure continuously turgor pressure changes of plant leaves over long periods of time with high precision and in real time. The probe has become an important tool in plant physiology, molecular biology and ecology, but also in agriculture because the probe is very robust and user-friendly. Growers receive the information about the water status of their plants by wireless telemetry, mobile network and internet on an as-needed basis and can thus adjust very precisely both the timing of irrigation and the quantity of water to apply. Effects of air and leaf temperature, relative humidity, illumination and wind on turgor pressure can be monitored very sensitively both under indoor and outdoor conditions. Even the effects of blue and red light as well as of oscillations of stomata aperture on turgor pressure can be monitored by the probe with high sensitivity. Similarly, water deficit due to increase of the osmotic pressure in the nutrition The applications of the magnetic probe are numerous and one can expect highly interesting developments in plant water relations in the nearest future.

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Southwest Australian Banksia woodlands are highly diverse plant communities that are threatened by drought- or temperature-induced mortality due to the region's changing climate. We examined water relations in dominant Banksia menziesii R. Br. trees using magnetic leaf patch clamp pressure (ZIM-) probes that allow continuous, real-time monitoring of leaf water status. Multiple ZIM-probes across the crown were complemented by traditional ecophysiological measurements. During summer, early stomatal downregulation of transpiration prevented midday balancing pressures from exceeding 2.5 MPa. Diurnal patterns of ZIM-probe and pressure chamber readings agreed reasonably well, however, ZIM-probes recorded short-term dynamics, which are impossible to capture using a pressure chamber. Simultaneous recordings of three ZIM-probes evenly spaced along leaf laminas revealed intrafoliar turgor gradients, which, however, did not develop in a strictly basi- or acropetal fashion and varied with cardinal direction. Drought stress manifested as increasing daily signal amplitude (low leaf water status) and occasionally as rising baseline at night (delayed rehydration). These symptoms occurred more often locally than across the entire crown. Microclimate effects on leaf water status were strongest in crown regions experiencing peak morning radiation (East and North). Extreme spring temperatures preceded the sudden death of B. menziesii trees, suggesting a temperature- or humidity-related tipping point causing rapid hydraulic failure as evidenced by collapsing ZIM-probe readings from an affected tree. In a warmer and drier future, increased frequency of B. menziesii mortality will result in significantly altered community structure and ecosystem function.