Minimum Water Potential Research Articles

Virgin olive oil and olive fruit minor constituents as affected by irrigation management based on SWP and TDF as compared to ETc in medium-density young olive orchards (Olea europaea L. cv. Cornicabra and Morisca)

Abstract This study investigated the effect of different irrigation managements, based on the measurement of the stem water potential (SWP) and the trunk diameter fluctuations (TDF) of the olive tree, as compared to the ET-FAO methodology, with regard to the minor constituents composition of virgin olive oil (VOO) and olive fruit in two different experimental olive orchards (Olea europaea L. Cornicabra cv. and Morisca cv.). No clear relationships between the water stress integral, both seasonal and from DOY 229–277, and the oleuropein content in the drupes were found. Nevertheless, a good agreement between the content of this biophenol and the minimum SWP of the olive trees, measured from the beginning of August to the end of the irrigation season, were found (r2 = 0.88–0.95 and r2 = 0.90–0.95 in Cornicabra and Morisca cultivars, respectively). A lower minimum water potential corresponded therefore with a higher biophenol content in the drupe and consequently with a superior phenolic content in VOO. In both cultivars, the volatile compounds most affected by the water status of olive trees were hexanal, E-2-hexenal and hexan-1-ol, showing an inverse relationship with the water stress integral observed. Furthermore, it was observed that the irrigation scheduling based on the SWP measurement, with a threshold of −1.2 MPa provided VOO in both cultivars with a similar phenolic and volatile composition with regard to those obtained with the FAO treatment.

Performance of Siberian elm ( Ulmus pumila) on steppe slopes of the northern Mongolian mountain taiga: Drought stress and herbivory in mature trees

Vegetation, soil conditions, age structure, water relations, light response of the chlorophyll fluorescence yield, and herbivore damage were studied in native savanna-like stands of Siberian elm ( Ulmus pumila) on south-facing slopes in the mountain taiga of the western Khentey Mountains in northern Mongolia. These stands limited to the middle and lower parts of the slopes consist of single trees or small groups of trees of up to 5 m height. Parts of the tree crowns are characteristically dead. The stands have a marked shrub layer of, e.g., small individuals of U. pumila as well as Spiraea aquilegifolia, Padus asiatica, and Ribes diacantha, and a diverse, but patchy ground vegetation. U. pumila is limited to loose, deep, stony soils on the slopes, which preferably occur near rock outcrops, mostly in contact to mountain steppe. Fine-grained, non-skeletal soils are avoided by U. pumila, but inhabited by meadow steppe. The preference for stony soils is attributable to the high water requirements of U. pumila, as rock cracks are known to form a reservoir for rain and dew water. The high demand for water is a partly a consequence of a low VPD sensitivity of the stomata regulation known for U. pumila. High transpiration rates may help U. pumila to avoid elevated leaf temperatures under high solar irradiation on the exposed steppe slopes. Increasing non-photochemical quenching at a PPFD > 2000 μmol m −2 s −1 gives additional evidence of the good adaptation of U. pumila to sunlit environments. Minimum shoot water potentials far above the point of zero turgor throughout the growing season suggest sufficient water supply in summer in years with average rainfall. Literature data indicate that U. pumila can also suffer lethal damage on the rocky slopes during exceptionally dry summers. Insect herbivory was of subordinate significance for the elm trees. Therefore, desiccation is thought to be the main cause of the frequent occurrence of dead crown parts and the early death of the elm trees on the sun-exposed slopes. Mean age of the sample trees was as low as 17 years. At this age, the trees are not fertile, but proliferate vegetatively. The low lifespan of U. pumila on the sun-exposed slopes suggests that these slopes are a suboptimal habitat for this tree species. The dependence on loose, stony soils alone explains the irregular distribution of U. pumila on the sun-exposed slopes and rules out the formation of closed elm forest at these sites.

Water relations and photosynthetic performance in Larix sibirica growing in the forest-steppe ecotone of northern Mongolia

Shoot water relations were studied in Siberian larch (Larix sibirica Ledeb.) trees growing at the borderline between taiga and steppe in northern Mongolia. Larix sibirica is the main tree species in these forests covering 80% of Mongolia's forested area. Minimum shoot water potentials (Psi(m)) close to the point of zero turgor (Psi(0)) repeatedly recorded throughout the growing season suggest that the water relations in L. sibirica were often critical. The Psi(m) varied in close relation to the atmospheric vapor pressure deficit, whereas Psi(0) was correlated with monthly precipitation. Young larch trees growing at the forest line to the steppe were more susceptible to drought than mature trees at the same sites. Furthermore, isolated trees growing on the steppe exhibited lower Psi(m) and recovered to a lower degree from drought overnight than the trees at the forest line. Indications of drought stress in L. sibirica were obtained in two study areas in Mongolia's forest-steppe ecotone: one in the mountain taiga of the western Khentey in northernmost Mongolia, the other in the forest-steppe at the southern distribution limit of L. sibirica on Mt. Bogd Uul, southern Khentey. Larix sibirica growing in riverine taiga with contact to the groundwater table was better water-supplied than the larch trees growing at the forest line to the steppe. Larch trees from the interior of light taiga forests on north-facing slopes, however, exhibited more critical water relations than the trees at the forest line. Frequent drought stress in mature trees and even more in young larch trees at the forest-steppe borderline suggests that L. sibirica does not have the potential to encroach on the steppe under the present climate, except in a sequence of exceptionally moist and cool years. A regression of the present borderline between forest and steppe is likely to occur, as average temperatures are increasing everywhere and precipitation is decreasing regionally in Mongolia's taiga forest region. Higher stomatal conductance concomitant to lower Psi(m) in trees of northern-slope forests compared to trees from the forest line to the steppe may be the result of a recent increase in drought intensity that affects better drought-adapted trees at the forest edge less than the trees in the forest interior. We conclude that drought is a key factor explaining the forest-steppe borderline in northern Mongolia. The proportion of forests within the present vegetation pattern of forests on north-facing slopes and the grasslands on south-facing slopes in Mongolia's forest-steppe ecotone is not likely to increase under the present climate, but may decrease with increasing aridity due to global warming.

Growth, physiological characteristics and ion distribution of NaCl stressed Alhagi sparsifolia seedlings

Alhagi sparsifolia is a leguminous perennial desert species that is plays an important role in dune stabilization and revegetation of degraded desert ecosystems. We investigated the effects of three different levels of salinity (50, 150, 250 mmol/L NaCl) on the growth, shoot photosynthetic parameters and salt distribution amongst different plant organs in one-year-old A. sparsifolia seedlings in a pot experiment over a 50 d period. The minimum (predawn) and maximum (midday) water potentials of A. sparsifolia seedlings decreased with the increase of external NaCl concentrations as a consequence of the osmotic or water deficit effect of saline solutions outside the roots. Salinity also reduced gas exchange parameters in A. sparsifolia, with seedlings subjected to salinity having lower photosynthesis rates and reduced stomatal conductances compared to the control. The reductions in photosynthetic rates in high salinity treatments of the A. sparsifolia seedlings were mainly caused by stomatal limitation. Consequently plants growing at greater external NaCl concentrations had significantly lower biomass accumulation compared to the control grown at 50 mmol/L. However, plants exposed to higher salinity were able to maintain growth throughout the experiment but allocated a greater proportion of biomass belowground. Plants exposed to higher external salinity levels had increased concentrations of Na+ and Cl− ions in shoots and roots, suggesting that A. sparsifolia seedlings were utilizing Na+ and Cl− as osmolytes to increase the cellular osmolality and decrease their water potential. We observed the greatest NaCl concentrations in the plants treated with 150 mmol/L NaCl indicating that there may be a threshold level of NaCl that can be tolerated by the plants. In conclusion our results indicate that A. sparsifolia seedlings are moderately salt tolerant. Photosynthetic gas exchange parameters were reduced by greater external salinity but the seedlings maintained substantial photosynthetic rates even under high salinity stress, were able to maintain growth over the 50 d experimental period and showed no signs of salinity toxicity or damage.

Differential summer water use by Pinus edulis and Juniperus osteosperma reflects contrasting hydraulic characteristics

Previous studies of pinyon-juniper woodlands show that Pinus edulis Engelm. makes better use of soil water from summer precipitation pulses than does co-occurring Juniperus osteosperma (Torr.) Little. To investigate the basis of this difference, we examined seasonal variation in cavitation and hydraulic conductance. Pinus edulis remained isohydric over the growing season. Minimum water potentials never fell below -2.3 MPa, and the extent of xylem cavitation remained near constant during the dry season. In contrast, J. osteosperma was anisohydric, reaching water potentials as low as -6.9 MPa, and experiencing progressively greater xylem cavitation as the dry season progressed despite having more cavitation-resistant xylem than P. edulis. We conducted an irrigation experiment to observe the responses of the study species to a summer pulse of water. Although sap flow increased in both species in response to the 25-mm irrigation pulse, only J. osteosperma responded to the 10-mm pulse. This was inconsistent with the response of P. edulis to light rain events and may have been due to a difference in the distribution of irrigation water and rain water between the under- and between-canopy areas. Whole-plant conductance increased following the 25-mm irrigation in P. edulis but remained constant in J. osteosperma. We hypothesized that this difference was caused, in part, by differential refilling of embolized xylem. Area specific hydraulic conductivity was 66% higher in roots of irrigated P. edulis trees relative to roots of control trees 3 days after the 25-mm irrigation (t = 2.14, P = 0.02, df = 16). There was no change in hydraulic conductivity of the roots of J. osteosperma or in the stems of either species. Our results indicate that the response to an irrigation pulse in P. edulis depended on cavitation avoidance in stems and the reversal of cavitation in roots, resulting in increased whole-plant conductance and water uptake. In contrast, J. osteosperma failed to exploit light summer rain events but was able to extract deep soil water at low water potentials.

Seed dormancy in red rice. XII: Population-based analysis of dry-afterripening with a hydrotime model

AbstractRed rice (Oryza sativaL.) dispersal units (florets) were dry-afterripened for 0–8 weeks and subsequently incubated at 30°C in polyethylene glycol (PEG) solutions with water potentials from 0 to − 1.6 MPa. Germination percentages and rates increased with dry-afterripening and water potential of the incubation medium. The seed population exhibited a normal distribution of base water potentials (Ψb, i.e. minimum water potential allowing germination) among individual seeds, characterized by three parameters: the hydrotime constant (θH), the mean base water potential (Ψb) and the standard deviation of the base water potential distribution (σΨb). Changes in germination during afterripening could be described by modifications of such parameters, particularlyΨb, which was employed to derive an index, DH(ARX = Ψb(ARX) − Ψb(ARN), where DH(ARX) represents a measure of dormancy of the seed population (in MPa) based on the hydrotime model,Ψb(ARX) is the mean base water potential of the seed population at any afterripening timeX, andΨb(ARN) is the mean water potential of the non-dormant (fully afterripened) population. The introduction of this index permitted interpretation of afterripening as a measurable reduction in the dormancy status of the seed, with progressive acquisition of both full germinative capacity and maximum germination rate, as anticipated by the hydrotime model. Moreover, secondary dormancy was induced proportionally to the reduction in water potential in the dark. Susceptibility to secondary dormancy induction was defined through DI(ARX), an index analogous to DH(ARX). These indices revealed that, in red rice, both breaking of primary dormancy and the inducibility of secondary dormancy followed decay kinetics with different sensitivities to the duration of dry storage.

Evolution of hydraulic traits in closely related species pairs from mediterranean and nonmediterranean environments of North America

Chaparral shrubs in California experience cool, wet winters and hot, dry summers characteristic of mediterranean-type climates; by contrast, morphologically similar close relatives in central Mexico experience summer rainfall. A comparison of closely related species pairs was conducted to examine whether evolutionary divergences in plant hydraulic conductivity were associated with contrasting seasonality of precipitation. Six species pairs in Santa Barbara, California and Tehuacan, Mexico were chosen to test for repeated directional divergences across the habitat contrast. Additionally, evolutionary correlations were examined using phylogenetically independent contrasts (PICs) among a suite of hydraulic traits, including stem- and leaf-specific conductivity, resistance to embolism, wood density, inverse Huber value, and minimum seasonal water potential. Leaf-specific conductivity was generally higher in California, but for most hydraulic traits the species pairs exhibited varied evolutionary trajectories across the climate contrast. A significant correlation was found between divergences in xylem resistance to embolism and minimum seasonal water potential, but no evolutionary trade-off was found between resistance and stem conductivity. Higher leaf-specific conductivity may be adaptive in California, where soil and atmospheric droughts coincide during summer months. This response is consistent with a hydraulic strategy of high leaf water supply under high evaporative demand to prevent excessive drops in water potential.

Responses to flooding and drought stress by two citrus rootstock seedlings with different water‐use efficiency

Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6‐month‐old seedlings of Carrizo citrange [Citrus sinensis (L.) Osb. ×Poncirus trifoliata L.; Carr] and Cleopatra mandarin (Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor () than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and were higher in Cleo than in Carr. Net CO2 assimilation rate (ACO2) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water‐use efficiency (ACO2/transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO2 concentration (Ci) in stressed plants than in control plants indicated that non‐stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F0, minimum fluorescence in dark‐adapted leaves)] were more important limitations on ACO2 than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short‐term DS and flooding stress than Carr.

Response of Buttonwood (Conocarpus erectus) Trees to Drought Conditions

Six-month-old buttonwood (Conocarpus erectus L.) seedlings were grown in containers under different soil water potentials (Ψsoil). The objective of the work was: 1) to determine the minimum soil water potential at which Conocarpus trees can survive and grow fairly well, 2) to study the soil-plant water relationship at different irrigation regimes, and 3) to examine the capacity of Conocarpus seedlings for osmotic adjustment via solute accumulation. Seedling growth was not affected significantly at soil water potential above –0.1 MPa (between 40 and 30% Field Capacity (FC). At lowerΨsoil, plant height, leaf area and shoot and root dry weights became disrupted by water deficit. Water stress decreased the osmotic potential (Ψπ) of leaves and roots. Leaves tended to osmoregulate their cell sap through osmotic adjustment processes as their content of soluble sugars increased. The positive survival under low Ψsoil could be related to increased osmotic adjustment. Ψsoil values were found to be more useful than FC values to estimate water requirements and use over an extended period of time, for plants grown under different soil types and different environmental conditions. Conocarpus seedlings can withstand reasonable water stress and can survive at moderately low water potential but, in contrast to other studies, this can not be classified as a high drought tolerant or resistant species.

Interannual consistency in canopy stomatal conductance control of leaf water potential across seven tree species

We investigated interannual variability of canopy transpiration per unit ground area (E (C)) and per unit leaf area (E (L)) across seven tree species in northern Wisconsin over two years. These species have previously been shown to be sufficient to upscale stand-level transpiration to the landscape level during one growing season. Our objective was to test whether a simple plant hydraulic model could capture interannual variation in transpiration. Three species, wetland balsam fir (Abies balsamea (L.) Mill), basswood (Tilia Americana L.) and speckled alder (Alnus rugosa (DuRoi) Spreng), had no change in E (C) or E (L) between 2000 and 2001. Red pine (Pinus resinosa Ait) had a 57 and 19% increase in E (C) and E (L), respectively, and sugar maple (Acer saccharum Marsh) had an 83 and 41% increase in E (C) and E (L), respectively, from 2000 to 2001. Quaking aspen (Populus tremuloides Michx) had a 50 and 21% decrease in E (C) and E (L), respectively, from 2000 to 2001 in response to complete defoliation by forest tent caterpillar (Malascoma distria Hüber) and subsequent lower total leaf area index of the reflushed foliage. White cedar (Thuja occidentalis L.) had a 20% decrease in both E (C) and E (L) caused by lowered surface water in wetlands in 2001 because of lower precipitation and wetland flow management. Upland A. balsamea increased E (L) and E (C) by 55 and 53%, respectively, as a result of release from light competition of the defoliated, overstory P. tremuloides. We hypothesized that regardless of different drivers of interannual variability in E (C) and E (L), minimum leaf water potential would be regulated at the same value. Minimum midday water potentials were consistent over the two years within each of the seven species despite large changes in transpiration between years. This regulation was independently verified by the exponential saturation between daily E (C) and vapor pressure deficit (D) and the tradeoff between a reference canopy stomatal conductance (G (S)) and the sensitivity of G (S) to D, indicating that trees with high G (S) must decrease G (S) in response to atmospheric drought faster than trees with low G (S). Our results show that models of forest canopy transpiration can be simplified by incorporating G (S) regulation of minimum leaf water potential for isohydric species.

Ecological relevance of minimum seasonal water potentials

The minimum seasonal water potential a plant experiences, ψmin, provides an important measure of plant water status, as it reflects the maximum water deficit that leaves and xylem must tolerate to maintain physiological activity. ψmin also acts as a selective force on xylem structure which, in turn, generates correlations between ψmin and numerous hydraulic traits. This review focuses on the ecological relevance of ψmin as a reflection of overall plant hydraulic strategy. The focus is on plant functional strategies with respect to soil drought, but we conclude with preliminary findings on the role of atmospheric drought.

Pathogenicity of Armillaria Isolates Inoculated on Five Quercus Species at Different Watering Regimes

One of three fungal isolates of Armillaria mellea (Vahl: Fr.) P. Kummer, A. gallica Marxm. et Romagn. and A. tabescens (Scop.: Fr.) Emel. was inoculated on 1,440 three-year-old potted seedlings of five Quercus species (Q. cerris L., Q. ilex L., Q. pubescens Willd., Q. robur L. and Q. trojana Webb.) grown at different watering regimes in a greenhouse. Inoculum was represented by a piece of an oak branch colonised with the fungus (or sterile, as a control), which was attached to the unwounded main root of each oak seedling. During the growing season, differences in water availability among seedlings were measured monthly using minimum water potential assessments on noninoculated seedlings receiving an equal amount of water. Although all three Armillaria isolates induced infection, the A. mellea isolate was most pathogenic in all cases, while the A. gallica isolate showed a statistically equal degree of pathogenicity only on the least watered seedlings. Of the Quercus species, Q. ilex showed the greatest number of infected seedlings, Q. robur the smallest. Reducing the water supply to potted oak seedlings could be a useful indicator for detecting differences in pathogenicity between Armillaria species.

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.

Water relations of baobab trees (Adansonia spp. L.) during the rainy season: does stem water buffer daily water deficits?

Baobab trees are often cited in the literature as water-storing trees, yet few studies have examined this assumption. We assessed the role of stored water in buffering daily water deficits in two species of baobabs (Adansonia rubrostipa Jum. and H. Perrier and Adansonia za Baill.) in a tropical dry forest in Madagascar. We found no lag in the daily onset of sap flow between the base and the crown of the tree. Some night-time sap flow occurred, but this was more consistent with a pattern of seasonal stem water replenishment than with diurnal usage. Intrinsic capacitance of both leaf and stem tissue (0.07-0.08 and 1.1-1.43 MPa(-1), respectively) was high, yet the amount of water that could be withdrawn before turgor loss was small because midday leaf and stem water potentials (WPs) were near the turgor-loss points. Stomatal conductance was high in the daytime but then declined rapidly, suggesting an embolism-avoidance strategy. Although the xylem of distal branches was relatively vulnerable to cavitation (P50: 1.1-1.7 MPa), tight stomatal control and minimum WPs near--1.0 MPa maintained native embolism levels at 30-65%. Stem morphology and anatomy restrict water movement between storage tissues and the conductive pathway, making stored-water usage more appropriate to longer-term water deficits than as a buffer against daily water deficits.

Effetti di carenze idriche prolungate su pinete mediterranee: insegnamenti da due esperimenti in Italia meridionale

A synthesis is presented of two studies carried out on the effects of imposed water shortage in two Mediterranean pine stands growing in Southern Italy, highlighting how physiological responses and structural acclimation can combine in the overall response of mature pine trees to long-term drought. The impact of a severe reduction in water availability on the water relations, transpiration and growth of 50-year-old Pinus halepensis trees (growing along the Ionian coast, Puglia, experiment 1) and 35-year-old Pinus laricio trees (growing in Sila, Calabria, experiment 2) was studied in two replicated experiments, in which the water available to trees was restricted by suspending a `roof’ made up by transparent polyethylene sheet. The experimental effects were followed for 12 months in experiment 1 and for 36 months in experiment 2, comparing “covered” plots and “uncovered” control plots. In experiment 1 trees experienced a severe water stress during the experimental period; a direct and rapid impact of drought was evident on the water relations and gas exchanges; there was an evident decrease of predawn water potential and transpiration was strongly reduced by stomatal closure (short-term physiological adjustment). A few months after the removal of the covering, no difference in whole plant hydraulic resistance was observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees, but no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. In experiment 2, albeit rather extreme at the first sight, the treatment resulted in a realistic simulations of water availability as predicted by climate change scenarios; a 35% reduction in terms of timber yield and a 30% reduction of leaf area was estimated over the experiment in covered plots; however, the reduction of radial growth over the three years translated into a mere 2% reduction in sapwood area, so that a shift of resources from leaves to sapwood was apparent after three years (structural adjustment). As for the effect of treatment on transpiration, a 43% reduction of transpiration was observed in covered trees over the summer, but recovering to the same control values was measured over the autumn when, due to differences in leaf area, leaf specific transpiration was greater in covered trees; no significant differences was found for leaf hydraulic resistance and minimum daily values of water potential between covered and control plots; this suggests that stomatal closure was effective in counterbalancing the effects of soil dryness, maintaining minimum water potential values rather constant (homeostasis of water potential), largely decoupled from predawn water potential (isohydric behaviour).

IRRIGATION OF WALNUT TREES MANAGING THE WATER POTENTIAL

The Creysse Research Centre has tested the use of a portable, manually-operated pressure chamber to manage irrigation based on measurements of the minimum water potential of the xylem. This method was compared to management methods based on tensiometric monitoring or soil water balance. The findings from two years of the study showed the merits and the feasibility of the method: easy installation and readouts, relevance of measurement data and possibility of actuating irrigation at given threshold values.

Food webs

Shrubs of the Great Basin desert in Utah are subjected to a prolonged summer drought. One potential consequence of drought is a reduced water transport capability of the xylem. This is due to drought-induced cavitation. We used the centrifuge method to measure the vulnerability of root and stem xylem to cavitation in six native shrub species. The shrubs fall into three categories with regards to rooting depth, vegetative phenology and plant water status during drought. The “summer green” group (Chrysothamnus viscidiflorus, Atriplex canescens, Atriplex confertifolia) sustains summer drought with a relatively shallow root system (<2.5 m), but maintains leaf area. A “drought deciduous” group (Grayia spinosa, Tetradymia glabrata) has shallow roots but responds to drought by dropping leaves. A “phreatophytic” group (e.g. Chrysothamnus nauseosus) avoids low water potentials by developing a deep root system that gives them access to deeper soil moisture. We hypothesized that cavitation resistance across these groups was adjusted to the amount of drought stress a species experienced. Drought experience was quantified by measuring plant water potentials over two growing seasons. As expected, we found a significant relationship between the cavitation resistance of roots and stems of a species and the minimum seasonal water potential. Shallow rooted, drought deciduous species showed the lowest water potentials prior to leaf shedding and they had the most resistant xylem. The phreatophytic shrub Chrysothamnus nauseosus always maintained favourable water potentials and had the most susceptible xylem, and summer green species were intermediate. This correlation between cavitation resistance and water potential suggests that a safe xylem is associated with some “cost” for the plant. We could identify two costs or trade-offs. The first trade-off was between safety and transport efficiency as reflected by vessel diameter. In 5 out of 7 species, there was an intra specific relationship between vessel diameter and cavitation resistance. Roots, having larger vessels than branches, were also more vulnerable to xylem dysfunction. There was also an inter specific correlation between vessel diameter and cavitation resistance, but only for combined root and stem data. A second trade-off was found between safety and construction cost. Resistant xylem was strongly correlated with high wood densities, suggesting that thicker cell walls and/or narrower conduits translate into lower air permeability. Air entry into the conduit is known to be the cause of cavitation.

FUNCTIONAL STRATEGIES OF CHAPARRAL SHRUBS IN RELATION TO SEASONAL WATER DEFICIT AND DISTURBANCE

The study of interspecific variation in plant ecological strategies has revealed suites of traits associated with leaf life span and with maximum levels of water deficit (measured as leaf water potentials). Here, the relationship between these sets of traits was examined in a study of 20 co-occurring chaparral shrubs that vary in leaf habit, rooting depth, and regeneration strategies. Leaf life span (LLS) and minimum seasonal water potentials (ψmin) were not significantly correlated, suggesting that they are associated with independent aspects of functional variation. Multiple regression analyses of a large suite of physiological, functional, and phenological attributes in relation to these two “anchor traits” supported this view. Short LLS was significantly associated with high specific leaf area, high carbon assimilation and leaf nitrogen (per mass), early onset of growth, and a multistemmed, short stature growth form. This suite of traits was also associated with opportunistic regeneration following physical disturbance. Area-based gas exchange was not tightly linked to LLS. Low ψmin (i.e., greater water deficit) was associated with high wood density, small vessel diameters, thin twigs, low leaf area : sapwood area ratios, and early onset of leaf abscission. Among the evergreen species, this suite of traits was most characteristic of post-fire seeders, which depend on high drought tolerance for post-fire regeneration of seedlings. Plant stature was the only trait associated with both the LLS axis and the ψmin axis of functional variation. A two-dimensional strategy space, approximately defined by LLS and ψmin, can be used to distinguish contrasting strategies of drought tolerance vs. avoidance, and alternative modes of regeneration following fire and other disturbance. This conceptual scheme illustrates the strength of a trait-based approach to defining plant strategies in relation to resource availability and disturbance.

The hydraulic architecture of Pinaceae – a review

We reviewed the literature to examine the vulnerability to water stress-induced embolism of Pinaceae relative to other conifers and to study the inter-relationships among the main traits involved in the hydraulic function within the Pinaceae. Results showed that Pinaceae (particularly the genus Pinus) are more vulnerable to xylem embolism, and show less variability in this character, than other conifers. Detailed data from 12 populations of Pinaceae (11 species) from three different areas (Pinol and Sala 2000; Martinez-Vilalta and Pinol 2002; Oliveras et al. 2003) was used to study the relationships among hydraulic properties of stems. These included: leaf-to-wood area ratio (AL:AW), wood- and leaf-specific hydraulic conductivity (KW and KL, respectively), vulnerability to xylem embolism (� 50 PL C), carbon isotope composition of needles (δ 13 C) and minimum needle water potential (minimum � L). Results showed that hydraulic properties tended to be more correlated among each other than with indicators of environmental (precipitation to potential evapotranspiration ratio, P/E) or physiological water stress (minimum

Eco-physiological behaviour of some mediterranean plants as suitable candidates for reclamation of degraded areas

Diurnal time course of the water relations of Ceratonia siliqua, Quercus coccifera, Pistacia terebinthus and Olea oleaster was studied on healthy and degraded sites for getting a suitable parameter describing the water stress impact on vegetation, and selecting the species suitable for reforestation. Out of the species used C. siliqua, showed high maximum stomatal conductance ( g L), with high relative water content (RWC) under field conditions, and relatively constant minimum (midday) water potential ( Ψ min) and pre-dawn water potential ( Ψ pd's). O. oleaster plants appeared to be unable to prevent dehydration inspite of consistent decrease of stomatal conductance ( g L), when subjected to increasing water stress. Stomatal closure in fact, was not sufficient to prevent water loss and relative water content (RWC) dropped to about 70%. P. terebinthus species is water spender. Under water shortage condition inspite of complete closure of stomata (stomatal conductance dropped to 0.08 s cm −1), relative water content showed variable values. Q. coccifera typically showed high stomatal conductance activity and also high relative water content (average 83%) on healthy site, but on the degraded site it reduced stomatal conductance, thus maintaining high RWCs (over 80%) and preventing leaf water potential ( Ψ L) to drop to critical values.