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A new expression for ion leakage from plant tissue, the tissue ionic conductance (gT), is compared with electrical conductivity (EC) and a commonly used damage index (Id) to test the ability of each expression to correctly describe leakiness in two model systems representing examples of physiological processes with well-known effects on membrane permeability. In experiments in which drought-acclimated leaves were compared with nonacclimated leaves and senescing leaves were compared with nonsenescing leaves, Id contradicted our expectation that acclimated tissue would be less leaky than nonacclimated tissue, and gT1and EC confirmed this expectation. In a comparison of senescing and nonsenescing tissue, Id again contradicted our expectation that senescing tissue would be more leaky than nonsenescing, and EC and gT, were confirming. Using a diffusion analysis approach, we show that Id fails to account for variation in the concentration gradient between the tissue and the bathing solution and variation in the surface area through which efflux occurs. Furthermore, because Id is a parameter that relates treatment performance to control performance as a percentage value, it distorts the actual differences among treatments. The resulting artifacts lead to a presentation of membrane integrity which is probably incorrect.EC is a more direct measurement of net ion efflux and appears to be less vulnerable to artifact. However, because gTi is the only expression that explicitly includes chemical driving force and tissue surface area, it is the most reliable of the three expressions.Measuring solute leakage from plant tissue is a long-standing method for estimating membrane permeability in relation to environmental stresses, growth and development, and genotypic variation. Early published accounts used total electrolyte leakage, expressed as specific conductance (EC2) of the aqueous bathing solution in which the tissue was immersed, to indicate degree of damage resulting from chilling injury (3,4)

Tissue osmotic potential(Ψπ) and solute constituents were evaluated in leaves and roots of well-watered and water-stressed Prunus avium L. × pseudocerasus Lindl. `Colt' and Prunus cerasus L. `Meteor'. Osmotic potential at full turgorΨπ,sat decreased in response to water stress for leaves and roots of both cultivars. For `Colt', a cultivar with an indeterminate growth habit,Ψπ,sat decreased by 0.56 MPa and 0.38 MPa for terminal expanding leaves and older expanded leaves, respectively. For `Meteor', a cultivar with a determinate growth habit,Ψπ,sat decreased by ≈0.47 MPa in both terminal and older leaves. RootΨπ,sat was alike for both cultivars and showed a similar decrease of 0.20 MPa in response to water stress. Roots had considerably higherΨπ,sat than did leaves in both cultivars, irrespective of irrigation treatment. Soluble carbohydrates and potassium (K+) were the major solute constituents in both cultivars. Of the soluble carbohydrates, sorbitol was found in the greatest concentration and accounted for the bulk of water stress-induced solute accumulation in both cultivars. Regardless of the irrigation treatment, mature leaves of `Meteor' consistently had lowerΨπ,sat (typically 0.4 MPa) than `Colt'. This variation in Ψπ,sat between Prunus cultivars suggests the potential for selection of cultivars with low Ψπ,sat and possibly superior drought resistance.

A Minolta SPAD-502 leaf chlorophyll meter was used for nondestructive data collection on the chlorophyll and nitrogen (N) status of benjamin fig (Ficus benjamina) and cottonwood (Populus deltoides) to quantitatively evaluate foliage quality. The goal was to provide a specific calibration for interpreting SPAD data within a media study for each species. Triplicate SPAD readings were collected from each of six leaves, sampled from forty plants per species, then processed for foliar analysis. Leaf tissue disks were also collected directly over SPAD testing sites for chlorophyll concentration measurement. Significant linear correlations were found between SPAD data and chlorophyll concentrations (r2 = 0.90 in benjamin fig and r2 = 0.91 for cottonwood). A significant, but lower correlation was found between SPAD data and N concentration. The SPAD-N correlations improved from the fifth month to the ninth month harvest (r2 = 0.32 to 0.53 for benjamin fig and 0.26 to 0.42 for cottonwood). The SPAD-502 could be useful for in landscape plant management, and in time for production situations, but baseline data is needed. Consistent protocol in sample collection and seasonal timing is needed prior to use as a predictor for tissue N levels. Development of species, and perhaps cultivar, specific baseline data and sampling procedures will need development, but could yield a rapid, quantitative, in expensive field diagnostic for foliage quality for making cultural management decisions.

High species diversity is argued to be the most important requisite for a resilient urban forest. In spite of this, there are many cities in the northern hemisphere that have very limited species diversity within their tree population. Consequently, there is an immense risk to urban canopy cover, if these over-used species succumb to serious pests or pathogens. Recognition of this should motivate the use of less commonly used species. Analysis of plant traits, such as the leaf water potential at turgor loss (Ψ P0), can provide useful insights into a species' capacity to grow in warm and dry urban environments. Therefore, the aim of this study was to evaluate Ψ P0 of 45 tree species, the majority of which are rare in urban environments. To help evaluate the potential for using Ψ P0 data to support future decision-making, a survey of professionals engaged with establishing trees in urban environments was also used to assess the relationship between the measured Ψ P0 and the perceived drought tolerance of selected species. This study demonstrates that Ψ P0 gives strong evidence for a species' capacity to tolerate dry growing conditions and is a trait that varies substantially across species. Furthermore, Ψ P0 was shown to closely relate to the experience of professionals involved in establishing trees in urban environments, thus providing evidence of its practical significance. Use of plant traits, such as Ψ P0 , should, therefore, give those specifying trees confidence to recommend non-traditional species for challenging urban environments.

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SummaryGas exchange, tissue water relations, and leaf/root dry weight ratios were compared among young, container-grown plants of five temperate-zone, deciduous tree species (Acer negundo L., Bet&a papyrtfera Marsh, Malus baccata Borkh, Robinia pseudoacacia L., and Ulmus parvifolia Jacq.) under well-watered and water-stressed conditions. There was a small decrease (mean reduction of 0.22 MPa across species) in the water potential at which turgor was lost ('P'trp) in response to water stress. The 'Per for water-stressed plants was -1.18, -1.34, -1.61, -1.70, and -2.12 MPa for B. papyrifera, A. negundo, U. parvifolia, R. pseudoacacia, and M. baccata, respectively. Variation in 'f'rlr resulted primarily from differences in tissue osmotic potential and not tissue elasticity. Rates of net photosynthesis declined in response to water stress. However, despite differences in Y'rtr, there were no differences in net photosynthesis among water-stressed plants under the conditions of water stress imposed. In A. negundo and M. baccata, water use efficiency (net photosynthesis/transpiration) increased significantly in response to water stress. Comparisons among water-stressed plants showed that water use efficiency for M. baccata was greater than for B. papyrifera or U. parvtfolia. There were no significant differences in water use efficiency among B. papyrifera, U. parvtfolia, A. negundo, and R. pseudoacacia. Under water-stressed conditions, leaf /root dry weight ratios (an index of transpiration to absorptive capacity) ranged from 0.77 in R. pseudoacacia to 1.05 in B. papyrifera.

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