Differences In Water Relations Research Articles

Differences in photosynthetic and water use characteristics of four co-existing tree species at the treeline of the Japanese Alps

ABSTRACT Physiological characteristics of trees in the alpine treeline are critical for assessing the vulnerability of alpine forest ecosystems, but differences in water relations between co-existing deciduous and evergreen trees remain poorly understood. In this study, we analyzed the photosynthetic and water use characteristics of two deciduous broad-leaved (Sorbus matsumurana and Betula ermanii) and two evergreen coniferous (Abies mariesii and Pinus pumila) tree species at the treeline – i.e. the line connecting the highest patches of forest composed of trees at least three meters high – of Mt. Norikura in central Japan, to explore the physiological adaptations of trees to alpine environments. We monitored diurnal changes in leaf gas exchange rates and leaf and soil water potentials in each tree species during the growing season, as well as bulk leaf water relations based on pressure – volume curves and whole-tree hydraulic structures. The four species under study relied on different physiological characteristics at both leaf and plant levels for adaptation to their natural habitat. For maintaining water in leaves, S. matsumurana showed higher water use efficiency (WUE; the ratio of photosynthesis to transpiration) and osmotic adjustment. In contrast, B. ermanii showed lower WUE, while maintaining higher soil-to-leaf hydraulic conductivity with less stomatal control. In turn, A. mariesii showed higher WUE and leaf succulence. Meanwhile, P. pumila showed lower WUE but compensated for water loss through higher leaf hydraulic capacitance and tissue elasticity. Our observations demonstrate that tree species rely on specific physiological characteristics to adapt to the alpine treeline environment.

Yerba mate (Ilex paraguariensis) agroforestry systems: intraspecific differences in water relations and hydraulic architecture.

Intensive farming systems benefit from the additional ecosystem services provided by tree integration, which generate different growing conditions for the main crop. We studied yerba mate (Ilex paraguariensis ) responses to growing conditions in monoculture (the conventional cropping system of yerba mate) and in three agroforestry systems: (1) yerba mate+Balfourodendron riedelianum ; (2) yerba mate+Peltophorum dubium ; and (3) yerba mate+Toona ciliata . Mainly, we focused on water relations and the hydraulic architecture of yerba mate. Agroforestry cropping systems provided a shade cover of around 34-45% and yielded as high as the conventional system. The shade cover influenced the allocation pattern to enhance leaf light capture, incrementing the leaf area to the sapwood area at the branch level. We also found a higher specific hydraulic conductivity in stems of yerba mate plants in consortium with T. ciliata than in the conventional cropping system, as well as higher resistance to water deficits due to lower vulnerability to embolism in the stems. During a severe drought, yerba mate plants had a similar stem and leaf water potential in both agricultural systems. Still, plants in monoculture had lower hydraulic safety margins and higher signs of leaf damage and mortality. This indicates that integrating trees into the yerba mate cultivation increases water stress resistance which would be beneficial to avoid restrictions on crop productivity under severe droughts induced by climate change.

Stem girth changes in response to soil water potential in lowland dipterocarp forest in Borneo: An individualistic time-series analysis.

Time-series data offer a way of investigating the causes driving ecological processes as phenomena. To test for possible differences in water relations between species of different forest structural guilds at Danum (Sabah, NE Borneo), daily stem girth increments (gthi), of 18 trees across six species were regressed individually on soil moisture potential (SMP) and temperature (TEMP), accounting for temporal autocorrelation (in GLS-arima models), and compared between a wet and a dry period. The best-fitting significant variables were SMP the day before and TEMP the same day. The first resulted in a mix of positive and negative coefficients, the second largely positive ones. An adjustment for dry-period showers was applied. Interactions were stronger in dry than wet period. Negative relationships for overstorey trees can be interpreted in a reversed causal sense: fast transporting stems depleted soil water and lowered SMP. Positive relationships for understorey trees meant they took up most water at high SMP. The unexpected negative relationships for these small trees may have been due to their roots accessing deeper water supplies (if SMP was inversely related to that of the surface layer), and this was influenced by competition with larger neighbour trees. A tree-soil flux dynamics manifold may have been operating. Patterns of mean diurnal girth variation were more consistent among species, and time-series coefficients were negatively related to their maxima. Expected differences in response to SMP in the wet and dry periods did not clearly support a previous hypothesis differentiating drought and non-drought tolerant understorey guilds. Trees within species showed highly individual responses when tree size was standardized. Data on individual root systems and SMP at several depths are needed to get closer to the mechanisms that underlie the tree-soil water phenomena in these tropical forests. Neighborhood stochasticity importantly creates varying local environments experienced by individual trees.

Trunk radial growth, water and carbon relations of mature apple trees on two size-controlling rootstocks during severe summer drought.

The use of size-controlling rootstocks is central to modern high-density fruit production systems. While biological mechanisms responsible for vigor control are not fully understood, differences in water relations and carbohydrate storage ability have been suggested as two potential factors. To better understand the processes that control growth vigor, we analyzed the trunk radial variation at seasonal and diurnal timescales and measured the midday leaf water potential (ΨMD), leaf gas exchange and concentrations of non-structural carbohydrates (NSC) in apple trees of variety 'Jonagold' grafted on two rootstocks of contrasting growth vigor (dwarfing J-TE-G vs invigorating J-TE-H). The measurements were conducted during an exceptionally hot and dry summer. We found that smaller annual trunk radial increments in dwarfed trees were primarily due to an earlier cessation of trunk secondary growth. The interdiurnal trunk circumference changes (ΔC) were slightly lower in dwarfed trees, and these trees also had fewer days with positive ΔC values, particularly during the driest summer months. The trunks of dwarfed trees shrank gradually during the drought, showed less pronounced diurnal variation of trunk circumference and the maximum trunk daily shrinkage was only weakly responsive to the vapor pressure deficit. These results indicated that lower turgidity in the cambial region may have limited the trunk radial expansion in dwarfed trees during the hot and dry days. Dwarfed trees also maintained lower ΨMD and leaf gas exchange rates during the summer drought. These parameters decreased in parallel for both rootstock combinations, suggesting their similar drought sensitivity. Similar concentrations and seasonal dynamics of NSC in both rootstock combinations, together with their similar spring growth rates, suggest that NSC reserves were not directly limiting for growth. Our results support the prominent role of water relations in rootstock-induced size-controlling mechanisms and highlight the complexity of this topic.

Differences in water relations and total carbohydrate content between precommercially thinned and unthinned Pinus nigra subsp. pallasiana stands

Variations in seasonal responses in water relations and total carbohydrate content (TCC) in one-yearold shoots from precommercially thinned (PCT) and unthinned Anatolian black pine stands were assessed during three seasons (sampling in May, July and September) in 2015–2017. Three different treatments were established: unthinned control with 4 941 stems·ha–1 and two thinned spacing levels (2–2.5 and 3–3.5 m) where 2 133 stems·ha<sup>–1</sup> and 1 093 stems·ha<sup>–1</sup> were left, respectively. Differences in osmotic potential at turgor loss point (Ψπ<sub>TLP</sub>) between the thinned and unthinned plots appeared only during a water shortage (September) in the second season, with the thinned stands showing lower Ψπ<sub>TLP</sub> than the unthinned stands. Seasonal variation in terms of Ψπ<sub>TLP</sub> was detected in the 3–3.5 m spacing trees. PCT were effective on osmotic potential at full turgor (Ψπ<sub>100</sub>), relative water content (RWC), symplastic water at saturated point per dry weight of the shoot and dry weight fraction. In both the thinned and unthinned plots, a gradual decline was observed in RWC from May to September in all three years. Generally, although TCC was found to be higher in the 3–3.5 m spacing, control plots were also high in carbohydrates in some periods. Results reflect the ability of this species to survive in changing environments by PCT during dry periods in the three studied seasons. Although PCT has an effect on water potential components and TCC, it is not clear exactly how effective the PCT density is in osmotic adjustment.

Drought responses and their effects on radial stem growth of two co-occurring conifer species in the Mediterranean mountain range

Key messagePatterns of stem radial variations showed thatCedrus libaniA. Rich. was less limited by summer drought than co-occurringJuniperus excelsaM. Bieb.Cedrus libanirecovered faster from tree water deficit and showed significantly higher radial growth rates and annual stem increments thanJ. excelsa. However, the ability ofJ. excelsato grow more hours per year may indicate a potential benefit in more extreme conditions.ContextKnowledge about species-specific drought responses is needed to manage productive forests in drought prone areas. Under water shortage, trees commonly show stem shrinkage, which is assumed to inhibit growth.AimsWe investigated whether the two co-existing conifers Juniperus excelsa M. Bieb. and Cedrus libani A. Rich. (growing at the Taurus Mountains, SW-Turkey) show differences in water relations and stem growth in order to evaluate their respective drought tolerance.MethodsStem radius changes were hourly monitored over 2 years using high-resolution point dendrometers. Radial stem growth, tree water deficit-induced stem shrinkage, and maximum daily shrinkage were extracted from stem radius change measurements, investigated for their patterns, and related to environmental conditions.ResultsCedrus libani recovered from tree water deficit under higher temperature and vapor pressure deficit than J. excelsa. The number of hours during which stem growth occurred was higher for J. excelsa; however, growth rates and annual increments were significantly lower than in C. libani. Both species showed highest maximum daily shrinkage during the driest months indicating the ability to maintain gas exchange all year round.ConclusionJuniperus excelsa showed a more conservative growth strategy while C. libani was less limited by summer drought and showed more annual stem increment under the conditions investigated.

Coffee performs better than amomum as a candidate in the rubber agroforestry system: Insights from water relations

Rubber (Hevea brasiliensis) plantations have been facing a double challenge of land degradation and seasonal drought in Southeast Asia. Various cash crops are recently interplanted with rubber trees to face these issues. However, the water relations between rubber trees and the intercrops remain poorly understood. This study aims to evaluate the influences of three cash intercrops, namely two herbaceous plants (Amomum villosum and Alpinia oxyphylla) and a woody beverage (Coffea arabica), on rubber water utilization through both spatial and temporal scales. We investigated the plant water-absorption dynamics, root biomass, and intrinsic water use efficiency (WUEi) throughout a whole year (2017–2018). The results showed that rubber trees (43.5 ± 2.6%) and intercrops (69.1 ± 3.2%) highly depended on soil water from the 0–20 cm depths. An interspecific water competition occurred in all the rubber-based agroforestry practices, because of their similar water source and root distribution in the vertical soil profiles. Overall, the WUEi of rubber trees was relatively higher during the dry season (δ13C: −30.79 ± 1.12‰) compared to the rainy season (δ13C: −31.65 ± 0.99‰). Coffee (C. arabica) better facilitated the soil water availability than the other intercrops, suggesting its suitability as an intercrop for rubber trees. Alpinia-oxyphylla (A. oxyphylla) played a moderate role on soil water retention. Amomum (A. villosum), however, aggravated the soil water deficit in the agroforestry practice. Given the differences in water relations to rubber trees, the introduction of woody crops rather than herbaceous crops can improve the resistance of rubber plantation to the frequent drought stress in this region.

Environmental determinants of biocrust carbon fluxes across Europe: possibilities for a functional type approach

Due to the well-known importance of biocrusts for several ecosystem properties linked to soil functionality, we aim to go deeper into the physiological performance of biocrusts components. Possible functional convergences in the physiology of biocrust constituents would facilitate the understanding of both species and genus distributional patterns and improve the possibility of modelling their response to climate change. We measured gas exchange in the laboratory under controlled conditions of lichen- and moss-dominated biocrusts from four environmentally different locations in Europe. Field data were used to determine the natural hydration sources that drive metabolic activity of biocrusts. Our results show different activity drivers at the four sites. Within site analyses showed similar C fixation for the different crust types in the three sites without hydric stress whilst light use related parameters and respiration at 15 °C were similar in the between sites analyses. There were significant differences in water relations between the biocrusts types, with moss-dominated crusts showing higher maximum and optimum water contents. The functional type approach for biocrusts can be justified from a physiological perspective when similar values are found in the within and between site analyses, the latter indicating habitat independent adaptation patterns. Our multi-site analyses for biocrusts functional performance provide comparisons of C fluxes and water relations in the plant-soil interface that will help to understand the adaptation ability of these communities to possible environmental changes.

Irrigation of ‘Green walls’ is necessary to avoid drought stress of grass vegetation (Phleum pratense L.)

Climate change will lead to increased frequencies of weather extremes in Central Europe, such as severe heat waves and droughts, causing high stress intensities for plants. Drought stress also represents a critical factor on many re-cultivated areas with poor edaphic conditions and small soil depths. This includes living wall systems which are applied to cover shotcrete walls and re-integrate them in landscape surroundings. Due to the vertical arrangement of these systems, which limits absorption of rain water, artificial irrigation was considered mandatory, but experimental evidence is lacking. Therefore, we compared soil and plant water relations of test plots on a ‘Green wall’ system with active and deactivated irrigation. During the vegetation period 2017, soil water potential and plant hydraulic parameters (stomatal conductance, relative water content, midday leaf water potential and osmotic potential) of Phleum pratense, the dominating grass on study plots, were measured.In summer 2017, many weeks with humid weather conditions, but also a pronounced heat period were registered. During this period, considerable physiological responses were observed and respective recovery after subsequent days of heavy precipitation. Phleum pratense showed reductions in stomatal conductance (−148.6 ± 118.8 mmolm−2 s−1), relative water content (−9.3 ± 10.1%) and midday leaf water potential (−0.3 ± 0.5 MPa) on the test plot during heat days. Upon precipitation, differences in water relations between plots ceased within few days. Results indicate that integrated irrigation systems are a necessary component of living wall systems.

Dynamic niche partitioning in root water uptake facilitates efficient water use in more diverse grassland plant communities

Abstract Efficient extraction of soil water is essential for the productivity of plant communities. However, research on the complementary use of resources in mixed plant communities, and especially the impact of plant species richness on root water uptake, is limited. So far, these investigations have been hindered by a lack of methods allowing for the estimation of root water uptake profiles. The overarching aim of our study was to determine whether diverse grassland plant communities in general exploit soil water more deeply and whether this shift occurs all the time or only during times of enhanced water demand. Root water uptake was derived by analysing the diurnal decrease in soil water content separately at each measurement depth, thus yielding root water uptake profiles for 12 experimental grasslands communities with two different levels of species richness (4 and 16 sown species). Additional measurements of leaf water potential, stomatal conductance, and root traits were used to identify differences in water relations between plant functional groups. Although the vertical root distribution did not differ between diversity levels, root water uptake shifted towards deeper layers (30 and 60 cm) in more diverse plots during periods of high vapour pressure deficit. Our results indicate that the more diverse communities were able to adjust their root water uptake, resulting in increased water uptake per root area compared to less diverse communities (52% at 20 cm, 118% at 30 cm, and 570% at 60 cm depth) and a more even distribution of water uptake over depth. Tall herbs, which had lower leaf water potential and higher stomatal conductance in more diverse mixtures, contributed disproportionately to dynamic niche partitioning in root water uptake. This study underpins the role of diversity in stabilizing ecosystem function and mitigating drought stress effects during future climate change scenarios. Furthermore, the results provide evidence that root water uptake is not solely controlled by root length density distribution in communities with high plant diversity but also by spatial shifts in water acquisition. A plain language summary is available for this article.

Response of three broccoli cultivars to salt stress, in relation to water status and expression of two leaf aquaporins

The aim of this study was to compare differences in water relations in the leaves of three broccoli cultivars and differential induction of the expression of PIP2 aquaporin isoforms under salt stress. Although broccoli is known to be moderately tolerant to salinity, scarce information exists about the involvement of leaf aquaporins in its adaptation to salinity. Thus, leaf water relations, leaf cell hydraulic conductivity (Lpc), gas exchange parameters and the PIP2 expression pattern were determined for short- (15h) and long- (15days) term NaCl treatments. In the long term, the lower half-time of water exchange in the cells of cv. Naxos, compared with Parthenon and Chronos, and its increased PIP2 abundance may have contributed to its Lpc maintenance. This unmodified Lpc in cv. Naxos under prolonged salinity may have diluted NaCl in the leaves, as suggested by lower Na(+) concentrations in the leaf sap. By contrast, the increase in the half-time of water exchange and the lower PIP2 abundance in cvs. Chronos and Parthenon would have contributed to the reduced Lpc values. In cv. Parthenon, there were no differences between the ε values of control and salt-stressed plants; in consequence, cell turgor was enhanced. Also, the increases in BoPIP2;2 and BoPIP2;3 expression in cv. Chronos for the short-term NaCl treatment suggest that these isoforms are involved in osmotic regulation as downstream factors in this cultivar, in fact, in the short-term, Chronos had a significantly reduced osmotic potential and higher PIP2 isoforms expression.

Water relations of selected wallum species in dry sclerophyll woodland on the lower north coast of New South Wales, Australia

The present study examined the water relations of wallum dry sclerophyll woodland on the lower north coast of New South Wales (NSW). Wallum is the regionally distinct vegetation of Quaternary dunefields and beach ridge plains along the eastern coast of Australia. Wallum sand masses contain large aquifers, and previous studies have suggested that many of the plant species may be groundwater dependent. However, the extent of this dependency is largely unknown, despite an increasing reliance on the aquifers for groundwater extraction. Fifteen species from five growth-form categories and seven plant families were investigated. The pre-dawn and midday xylem water potential (ψx) of all species was monitored over a 20-month period from December 2007 to July 2009. Pressure–volume curve traits were determined for each species in late autumn 2008, including the osmotic potential at full (π100) and zero (π0) turgor, and bulk modulus of elasticity (ε). Carbon isotope ratios (δ13C) were determined in mid-autumn 2008 to measure water use efficiency (WUE). Comparative differences in water relations could be loosely related to growth forms. A tree (Eucalyptus racemosa subsp. racemosa) and most large shrubs had low midday ψx, π100 and π0, and high ε and WUE; whereas the majority of small and medium shrubs had high midday ψx, π100 and π0, and low ε and WUE. However, some species of similar growth form displayed contrasting behaviour in their water relations (e.g. the herbs Caustis recurvata var. recurvata and Hypolaena fastigiata), and such differences require further investigation. The results suggest that E. racemosa subsp. racemosa is likely to be groundwater dependent, and large shrubs such as Banksia aemula may also utilise groundwater. Both species are widespread in wallum, and therefore have the potential to play a key role in monitoring ecosystem health where aquifers are subject to groundwater extraction.

Species distribution and crown decline are associated with contrasting water relations in four common sympatric eucalypt species in southwestern Australia

Drought-associated vegetation declines are increasingly observed worldwide. We investigated whether differences in water relations can potentially explain the distribution and vulnerability to drought-induced decline of four common tree species in Mediterranean southwestern Australia. We compared seasonal and daily water relations of four eucalypt species (i.e. C. calophylla, E. accedens, E. marginata, E. wandoo) when co-occurring as well as on nearby typical sites for each species. When co-occurring, species generally inhabiting drier regions (i.e. E. accedens, E. wandoo) had lower summer leaf water potentials, osmotic potential, and vulnerability to cavitation and higher stomatal conductance and relative sapflow velocity. Both wetter zone species (e.g. C. calophylla and E. marginata) had remarkably high vulnerabilities to cavitation for Mediterranean woody species but showed greatly improved leaf water status on nearby sites where they dominate. Using local soil moisture retention curves of saprolitic clay layers underlying southwestern Australia we show the large disadvantage that the wetter zone species have in terms of accessing tightly bound water in these layers. Our work shows that species distribution and local dominance of four dominant overstorey species in southwestern Australia is largely a function of plant water relations interacting with local soil profiles. The observed differences in water relations amongst species are consistent with some of the declines that have been observed in recent decades.

Transpiration sensitivity of urban trees in a semi-arid climate is constrained by xylem vulnerability to cavitation

Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m(T); slope of the relationship between tree transpiration and ln D) and transpiration at D=1 kPa (E(Tref)) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m(T) was significantly related to vulnerability to cavitation of branches (P(50)). While m(T) did not appear to differ between ring- and diffuse-porous species, the relationship between m(T) and P(50) was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring- versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.

Allocation to leaf area and sapwood area affects water relations of co-occurring savanna and forest trees

Water availability is a principal factor limiting the distribution of closed-canopy forest in the seasonal tropics, suggesting that forest tree species may not be well adapted to cope with seasonal drought. We studied 11 congeneric species pairs, each containing one forest and one savanna species, to test the hypothesis that forest trees have a lower capacity to maintain seasonal homeostasis in water relations relative to savanna species. To quantify this, we measured sap flow, leaf water potential (Psi(L)), stomatal conductance (g (s)), wood density, and Huber value (sapwood area:leaf area) of the 22 study species. We found significant differences in the water relations of these two species types. Leaf area specific hydraulic conductance of the soil/root/leaf pathway (G (t)) was greater for savanna species than forest species. The lower G (t) of forest trees resulted in significantly lower Psi(L) and g (s) in the late dry season relative to savanna trees. The differences in G (t) can be explained by differences in biomass allocation of savanna and forest trees. Savanna species had higher Huber values relative to forest species, conferring greater transport capacity on a leaf area basis. Forest trees have a lower capacity to maintain homeostasis in Psi(L) due to greater allocation to leaf area relative to savanna species. Despite significant differences in water relations, relationships between traits such as wood density and minimum Psi(L) were indistinguishable for the two species groups, indicating that forest and savanna share a common axis of water-use strategies involving multiple traits.

Shade tolerance, photoinhibition sensitivity and phenotypic plasticity of Ilex aquifolium in continental Mediterranean sites

Shade tolerance, plastic phenotypic response to light and sensitivity to photoinhibition were studied in holly (Ilex aquifolium L.) seedlings transported from the field to a greenhouse and in adult trees in the field. All plants were growing in, or originated from, continental Mediterranean sites in central Spain. Seedlings tolerated moderate but not deep shade. Mortality was high and growth reduced in 1% sunlight. Survival was maximal in 12% sunlight and minimal in full sunlight, although the relative growth rate of the seedlings surviving in high light was similar to that of plants in moderate shade. Maximum photochemical efficiency at predawn was significantly lower in sun plants than in shade plants in the field, revealing chronic photoinhibition that was most pronounced in winter. Plasticity in response to available light varied according to the variable studied, being low for photosynthetic capacity and stomatal conductance, and high for specific leaf area, root:shoot ratio and leaf area ratio, particularly in seedlings. Differences in water relations and hydraulic features between sun and shade plants in the field were marginal. High water potential at the turgor loss point of field-grown plants suggested that holly is sensitive to drought during both the seedling and the adult stage. Low relative growth rates in both high and low light with low physiological plasticity in response to light indicate the existence of a stress-tolerance mechanism. We conclude that holly is a facultative understory plant in areas of oceanic and relatively mild climate, but an obligate understory plant in dry continental areas such as the study site. The impact of abandonment of traditional management practices and climate change on these Mediterranean populations is discussed.

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.

Variation in water relations characteristics of terminal shoots of Port-Orford-cedar (Chamaecyparis lawsoniana) seedlings.

We measured water relations attributes of the terminal shoots of 3-year-old Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murr.) Parl.) seedlings that represented its geographic range. Pressure-volume curves were developed and osmotic potentials at full (psi(sf)) and zero turgor (psi(sz)), relative water content at zero turgor, and an index of tissue elasticity (IE) were calculated for 38 families during early, mid- and late summer at an inland nursery, and for 12 of these families during mid- and late summer at a coastal nursery. Compared with other conifer species, psi(sz) was high (-1.4 to -1.5 MPa) and declined in seedlings at both nurseries as the season progressed. Both IE and osmotic amplitude (psi(sf)-psi(sz)) increased during the season. Osmotic potential at zero turgor was lower and osmotic amplitude greater in seedlings at the inland nursery than at the coastal nursery. Correlations of water relations attributes with geographic location of the seed sources were weak and usually not significant. High elevation southern sources exhibited smaller differences in psi(sz) between nurseries than low elevation northern sources. The small differences in water relations attributes among sources and between nurseries suggest that some may be of marginal physiological importance; however, sources that produced larger seedlings appeared to be less desiccation tolerant. We conclude that, when moving genotypes during reforestation, decisions based on patterns in tree size and timing of growth will account for these small differences in water relations.

Differences in water relations, leaf ion accumulation and excretion rates between cultivated and wild species of Limonium sp. grown in conditions of saline stress

Summary Plants of two genotypes, Limonium pectinatum (wild species) and the hybrid Limonium latifolium x Limonium caspium cv. Beltlaard grown in greenhouse conditions were irrigated with a 200 mM NaCl solution for 4 months (saline treatment). The dry weight of cv. Beltlaard plants was significantly reduced by salinity but remained unchanged in Limonium pectinatum . At the end of the experiment none of the parameters related to floral stems (number, height and dry weight) in Limonium pectinatum was affected to any significant degree by salts, while the treated plants of the Beltlaard genotype showed significant reductions in these parameters compared with the control plants. Leaf water relations under saline conditions showed a similar behaviour in both genotypes, achieving the same degree of leaf osmotic adjustment through the accumulation of Na + and Cl − . Although the highest levels of these ions were found in treated plants of Limonium pectinatum , the lower accumulation of salts in the tissues of cv. Beltlaard plants generated major toxicity and nutritional disturbances, which negatively affected growth. The Na + and Cl − excretion rate through salt glands was higher in Limonium pectinatum than in cv. Beltlaard , which indicated that the wild species was more efficient decreasing the toxic salt content of its tissues. The ultrastructure of these salt glands is depicted and described.

Acclimation of photosynthesis and respiration to simulated climatic warming in northern and southern populations of Acer saccharum: laboratory and field evidence.

Physiological acclimation and genotypic adaptation to prevailing temperatures may influence forest responses to future climatic warming. We examined photosynthetic and respiratory responses of sugar maple (Acer saccharum Marsh.) from two portions of the species' range for evidence of both phenomena in a laboratory study with seedlings. A field study was also conducted to assess the impacts of temperature acclimation on saplings subjected to an imposed temperature manipulation (4 degrees C above ambient temperature). The two seedling populations exhibited more evidence of physiological acclimation to warming than of ecotypic adaptation, although respiration was less sensitive to short-term warming in the southern population than in the northern population. In both seedling populations, thermal compensation increased photosynthesis by 14% and decreased respiration by 10% in the warm-acclimated groups. Saplings growing in open-top field chambers at ambient temperature and 4 degrees C above ambient temperature showed evidence of temperature acclimation, but photosynthesis did not increase in response to the 4 degrees C warming. On the contrary, photosynthetic rates measured at the prevailing chamber temperature throughout three growing seasons were similar, or lower (12% lower on average) in saplings maintained at 4 degrees C above ambient temperature compared with saplings maintained at ambient temperature. However, the long-term photosynthetic temperature optimum for saplings in the field experiment was higher than it was for seedlings in either the 27 or the 31 degrees C growth chamber. Respiratory acclimation was also evident in the saplings in the field chambers. Saplings had similar rates of respiration in both temperature treatments, and respiration showed little dependence on prevailing temperature during the growing season. We conclude that photosynthesis and respiration in sugar maple have the potential for physiological acclimation to temperature, but exhibit a low degree of genetic adaptation. Some of the potential for acclimation to a 4 degrees C increase above a background of naturally fluctuating temperatures may be offset by differences in water relations, and, in the long term, may be obscured by the inherent variability in rates under field conditions. Nevertheless, physiologically based models should incorporate seasonal acclimation to temperature and permit ecotypic differences to influence model outcomes for those species with high genetic differentiation between regions.