Competitive strategies for water availability in two Mediterranean Quercus species

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour ( g _L), water potential ( Ψ _L) and relative water content ( RWC ) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area ( K _RL). Quercus cerris behaved as a drought-tolerant species, with strong reductions of K _RL, Ψ _L, and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing g _L, maintaining Ψ _L and RWC high and K _RL at the same level as that measured in the spring. A 'conductance ratio' ( CR ) was calculated in terms of the ratio of g _L to K _RL. Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought-tolerant with drought-avoiding species can be generalized is also discussed.     

Relax and refill: xylem rehydration prior to hydraulic measurements favours embolism repair in stems and generates artificially low PLC values

Diurnal changes in percentage loss of hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that diurnal PLC changes might arise as a consequence of an experimental artefact, that is, air entry into xylem conduits upon cutting stems, even if under water, while under substantial tension generated by transpiration. Rehydration procedures prior to hydraulic measurements have been recommended to avoid this artefact. In the present study, we show that xylem rehydration prior to hydraulic measurements might favour xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. When xylem tension relaxation procedures were performed on stems where refilling mechanisms had been previously inhibited by mechanical (girdling) or chemical (orthovanadate) treatment, PLC values measured in stems cut under native tension were the same as those measured after sample rehydration/relaxation. Our data call for renewed attention to the procedures of sample collection in the field and transport to the laboratory, and suggest that girdling might be a recommendable treatment prior to sample collection for PLC measurements.     

Axial‐to‐radial water permeability of leaf major veins: a possible determinant of the impact of vein embolism on leaf hydraulics?

The leaf hydraulic conductance (K_L) was measured in Prunus laurocerasus L. and Juglans regia L. in which previous measurements had revealed different impacts of dehydration on K_L. Leaves of P. laurocerasus lost 8% of their K_L at water potentials (Ψ_L) of ?2.0 MPa. Leaflets of J. regia showed K_L losses of 40% at Ψ_L = ?1.0 MPa. When major veins were blocked using cyanoacrylate to simulate their embolism, the K_L of P. laurocerasus was reduced by 57% but that of J. regia leaflets was reduced by 80%. Such differences were hypothesized to be due to different axial‐to‐radial permeabilites of major veins. Infiltration of leaves with Phoxine B revealed that P. laurocerasus major veins were largely leaky in the radial direction whereas those of J. regia leaflets showed prevailing axial water transport. Differences between species in terms of axial‐to‐radial water permeability were confirmed by measurements of changes of hydraulic resistance along the midrib. The two hydraulic models are discussed in terms of leaf vulnerability to embolism and plant adaptation to dry habitats.     

Distribution of Vessel Ends in Stems of some Diffuse- and Ring-porous Trees: The Nodal Regions as 'Safety Zones' of the Water Conducting System

We report measurements of the number of vessels ending withinthe nodes in comparison with that in the internodes Such measurementshave been carried out during autumn 1982 and 1983 in 1-year-oldtwigs of three species with diffuse-porous wood (Vitis vintferaL. Populus deitoides Bartr and Olea europaea L) and three ring-poroustrees (Juglans nigra L, Castanea sativa Mill, Sophora JaponicaL) In all three species with diffuse-porous wood, a significantlyhigher percentage of vessels ending in the nodes (at the leafgap) was recorded when compared with that in the internodesNo significant differences were found between nodes and internodesin this respect in ring-porous trees. Since vessel ends arecapable of screening out gaseous emboli and/or particles biggerthan 1 µm in diameter, we interpret the nodes of diffuse-poroustrees as stem zones providing safety for the water conductingsystem .The reason why ring-porous trees are likely not to needsuch ‘safety zones’ is also discussed Vitis vinifera L, grapevine, Populus deltoides Bartr, poplar, Olea europaea L, olive, Juglans nigra L, Eastern black walnut, Castanea sativa Mill, European chestnut, Sophora japonica L, Japanese pagoda tree, vessel ends, nodes, internodes     

Three Different Methods for Measuring Xylem Cavitation and Embolism: A Comparison

Three different methods for measuring xylem embolism due towater cavitation were compared—the acoustic method, thehydraulic method and the anatomical method. Young plants ofCeratonia siliqua L. were water stressed for 9, 16 and 23 d. Xylem cavitation was detected by counting ultrasound (100–300kHz) acoustic emissions (AE) from 1-year-old twigs (acousticmethod). Xylem embolism was detected by measuring the loss ofhydraulic conductivity of twigs of the same age (hydraulic method).The blockage of single xylem conduits was detected by perfusingSafranin into the xylem of 1-year-old twigs of stressed plantsand measuring the number and the diameters of non-conductingxylem conduits, under the microscope (anatomical method). It was noted that: (a) the number of AE and the loss of conductivityincreased with the water stress applied; (b) a linear relationseemed to exist between the number of AE and the loss of conductivity,suggesting that the AE counted could be only (or mainly) producedin the xylem conduits; (c) the vulnerability of the xylem conduitsto embolism was a direct function of their diameter; and (d)the measured loss of conductivity was of the same order of magnitudeas the theoretical one. The three methods gave fairly similar results. Nonetheless,they are not alternative to one another in that: (a) the acousticmethod allows continuous recordings to be made but does notprovide information about the actual damage suffered by plants;(b) the hydraulic method is very informative but destructive;and (c) the anatomical method is very useful both in phytogcographicaland in genetic improvement studies. Ceratonia siliqua L., Carob tree, water stress, xylem embolism, acoustic method, hydraulic method, anatomical method     

Different Aspects of Cavitation Resistance in Ceratonia siliqua, a Drought-Avoiding Mediterranean Tree

Potted plants of Ceratonia siliqua L., growing in a greenhouse,were used to detect xylem cavitation (in terms of ultrasoundacoustic emissions AE) in internodes and node-to-petiole (N-P)junctions, after different periods of drought (9, 16 and 23d). Diurnal AE were only 100 in internodes of watered (W) plantsbut 320, 1250 and 2460 in 9-, 16- and 23-d stressed ones. InN-P junctions, AE were only 15 to 20% with respect to internodes. Stem perfusion with dye allowed measurement of the percentageof xylem conduit transverse area blocked by cavitation. Thiswas 2% in internodes of W-plants and 5.2, 13.8 and 40.4% inthose of 9-, 16- and 23-d stressed ones. In N-P junctions, 18.5%of the xylem conduit transverse area was blocked in the 23-dstressed plants only. The major resistance to cavitation exhibitedby the N-P junctions is interpreted in terms of their greaternumber of narrow xylem conduits. The percentage of blocked xylemconduits within a range of diameters showed that the narrowera xylem conduit, the less likely it was that cavitation wouldoccur. After rewatering, the release of the xylem blockage causedby cavitation occurred within 2 h. Our data suggest that C.siliqua can be considered to be a cavitation avoider, especiallyin its stem-to-leaf transition zones. Ceratonia siliqua L., Carob tree, cavitation avoidance, xylem architecture, ultrasonic acoustic emissions     

Taxonomy and evolution of European brown frogs

Phylogenetic relationships within the Euro-Mediterranean stock of the Rana temporaria species group (R. temporaria, R. iberica, R. cameranoi, R. macrocnemis, R. holtzi, R. dalmatina, R. graeca, R. latastei) were studied by (1) morphometric analysis, (2) osteological study and (3) electrophoresis. The three data sets were analysed independently and phenograms and cladograms examined for congruent patterns. The tree resulting from electrophoretic analysis is the most clearly resolved (and being also compatible with the other two) is assumed to represent the most likely phylogeny of the Mediterranean brown frogs. Patterns indicated by electrophoretic data matched the molecular clock-hypothesis. This suggests that the first split of the main stock of Mediterranean brown frogs separated ancestors of Rana temporaria and R. iberica from the others during the Late Pleistocene. A second split occurred separating the ancestors of what are currently the other big-bodied and the small-bodied brown frogs. The relationships between and within the three main stocks are discussed in the context of biogeographical and geological data.     

Organ-Specific Metastases in Melanoma: Experimental Animal Models

Metastases from certain primary tumors frequently exhibit specific organ preference. Animal models have been developed to induce in a reproducible fashion the formation of organ-specific metastases by malignant melanoma cells. Some of these models rely on the use of immunodeficient mice, which can support the growth of murine as well as human malignant melanomas. Moreover, immunodeficient mice, because of their diminished ability to mount an effective immune response, allow the expression of malignant properties (e.g., preferential colonization of certain organs), which are intrinsic to transplanted melanoma cells. This review discusses the relevant factors (and limitations) of some of the animal models used to study the in vivo properties of melanoma cells.     

Drought Avoidance Strategy in Ceratonia siliqua L., a Mesomorphic-leaved Tree in the Xeric Mediterranean Area

We report measurements of diurnal changes in leaf relative watercontent (RWC), water potential (