This article is modified from Perez-Harguindeguy et al (2013). The “New handbook for standardised measurement of plant functional traits worldwide” is a product of and is hosted by Nucleo Diversus (with additional Spanish translation). For more on this trait and on its context as part of the entire trait handbook visit its primary site Nucleo DiverSus at http://www.nucleodiversus.org/?lang=en
Species facing soil water shortage can avoid water stress to a degree by dropping leaves, or delay the development of water stress in their tissues by rooting deeply, or by shutting stomata and losing stored water only slowly through their cuticle. Alternatively, tissues may tolerate physiological desiccation. The bulk leaf water potential ( L; units MPa) is a simple indicator of leaf water status; the more negative the value, the more dehydrated the leaf.
When measured pre-dawn, the plant may have become equilibrated with the soil during the night, and the L may thus represent the soil water potential in the -average’ root zone. However, recent work has shown instances of substantial disequilibrium between pre-dawn L and soil water potential as a result of several mechanisms, including nocturnal transpiration, cavitation in the xylem and osmolyte accumulation in the cell walls. Thus, pre-dawn L may be more negative than the soil water potential, and should be used only as a tentative index of soil water availability.
During the day, L will decline below the soil water potential as a result of transpiration into the atmosphere. When measured in the dry season, the midday L can provide a useful index of the degree of physiological drought experienced. Thus, the minimum value for L that a plant reaches, usually at midday at the driest, hottest time of year, can be used as an index of the tolerance to water shortage that the species (or individuals and populations) demonstrate (assuming that the plants are still healthy and not drought-injured).
What and how to collect
Measurement of minimum values of L is typically carried out at the end of the hot, dry season for evergreen species and in Mediterranean winter-rain ecosystems. However, in summer-rain ecosystems, the time of year at which drought stress is maximal may not be obvious. Repeated-measurements in different seasons can help find the real minimum L for each species.
Depending on the type of pressure chamber used (see below), either leaves or short, terminal, leafy twigs should be collected. Samples should be collected at midday and, as previously indicated (see SLA), from shoots or individuals located in the sun. Leaves should have been exposed to direct sun for at least 30 min before collection (avoid cloudy days). We recommend measuring samples as soon as possible, or at least within half an hour of collecting all samples (with the number of samples depending on the number of pressure chambers available) over a period of no more than half an hour between the first and last measurement. Samples should be collected into sealable plastic bags, into which one has just exhaled to increase moisture and CO2 to try to minimise shoot transpiration within the bag. Samples sealed in plastic bags should be kept refrigerated and in darkness (e.g. in a refrigerated picnic fridge, or an insulated cooler box containing pre-frozen cooling bars or ice).
The simplest way to measure leaf water potential is with a pressure chamber, or Scholander bomb (see diagram in Fig. 1 -attached). This consists of a pressure container into which the sample (leaf or terminal twig) is placed, a manometer or pressure gauge to measure the pressure inside the chamber, and as a pressure source, a pressure tank of liquid N, connected to the chamber through a needle valve and pressure-safe (normally copper) tubing. Many models with different characteristics are commercially available.
A leaf or shoot is placed inside the chamber, with its cut end projecting to the exterior through the sealing port. Pressure, from the N tank, is then gradually increased in the chamber. When a drop of water appears at the cut end of the specimen, the -balance pressure’ indicated by the gauge or manometer is recorded. Assuming that the xylem osmotic potential is very low, the balance pressure represents the equilibrium water potential of the plant material in the chamber, multiplied by -1 Leaf water potential is conventionally expressed in MPa. Minimum leaf water potentials usually vary from near 0 to -5 MPa, but can be lower in (semi-)arid ecosystems. Extreme care should be taken when pressure chambers are under high pressures.
Fig. 1. Measuring water potential with a pressure chamber. A cut branch (or leaf or compound leaf) is placed inside the chamber, with the cut end protruding from the seal. Once the chamber has been sealed (hermetically closed), pressure is gradually applied from the gas cylinder. When the pressure in the chamber equals the xylem pressure, a drop of water appears at the cut surface. Assuming that the xylem osmotic potential is very low, the balance pressure represents the equilibrium water potential of the plant material in the chamber.
References on theory,significance and large datasets:
Ackerly DD (2004) Functional strategies of chaparral shrubs in relation to seasonal water deficit. Ecological Monographs 74, 25-44. doi:10.1890/03-4022
Bartlett M, Scoffoni C, Sack L (2012) The determinants of leaf turgor loss point and prediction of drought tolerance of species and biomes: a global meta-analysis. Ecology Letters 15, 393-405. doi:10.1111/j.1461-0248.2012.01751.x
Bhaskar R, Ackerly DD (2006) Ecological relevance of minimum seasonal water potentials. Physiologia Plantarum 127, 353-359. doi:10.1111/j.1399-3054.2006.00718.x
Bucci SJ, Goldstein G, Meinzer FC, Scholz FG, Franco AC, Bustamante M (2004) Functional convergence in hydraulic architecture and water relations of tropical savanna trees: from leaf to whole plant. Tree Physiology 24, 891-899. doi:10.1093/treephys/24.8.891
Hinckley TM, Lassoie JP, Running SW (1978) Temporal and spatial variations in the water status of forest trees. Forest Science 20, a0001-z0001.
Jacobsen AL, Pratt RB, Davis SD, Ewers FW (2008) Comparative community physiology: nonconvergence in water relations among three semi-arid shrub communities. New Phytologist 180, 100-113. doi:10.1111/j.1469-8137.2008.02554.x
Lenz TI, Wright IJ, Westoby M (2006) Interrelations among pressure-volume curve traits across species and water availability gradients. Physiologia Plantarum 127, 423-433. doi:10.1111/j.1399-3054.2006.00680.x
References on methods:
Scholander PF (1966) The role of solvent pressure in osmotic systems. Proceedings of the National Academy of Sciences, USA 55, 1407-1414. doi:10.1073/pnas.55.6.1407
Turner NC (1988) Measurement of plant water status by the pressure chamber technique. Irrigation Science 9, 289-308. doi:10.1007/BF00296704