Photosynthetic pathway alters hydraulic structure and function in woody plants

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (K_s ) was 16%–98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher K_s values compared with three C₄ species. Leaf specific conductivity (K_L ; hydraulic conductivity per leaf area) was 60%–98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO₂ variation, may also affect the evolution of wood structure and function.     

Photosynthesis, water use efficiency and stable carbon isotope composition are associated with anatomical properties of leaf and xylem in six poplar species

Although fast‐growing Populus species consume a large amount of water for biomass production, there are considerable variations in water use efficiency (WUE) across different poplar species. To compare differences in growth, WUE and anatomical properties of leaf and xylem and to examine the relationship between photosynthesis/WUE and anatomical properties of leaf and xylem, cuttings of six poplar species were grown in a botanical garden. The growth performance, photosynthesis, intrinsic WUE (WUE_i), stable carbon isotope composition (δ¹³C) and anatomical properties of leaf and xylem were analysed in these poplar plants. Significant differences were found in growth, photosynthesis, WUE_i and anatomical properties among the examined species. Populus cathayana was the clone with the fastest growth and the lowest WUE_i/δ¹³C, whereas P. × euramericana had a considerable growth increment and the highest WUE_i/δ¹³C. Among the analysed poplar species, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance (g_s) and lowest WUE_i/δ¹³C. Moreover, significant correlations were observed between WUE_i and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUE_i and δ¹³C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUE_i. It is anticipated that some poplar species, e.g. P. × euramericana, are better candidates for water‐limited regions and others, e.g. P. cathayana, may be better for water‐abundant areas.     

Photosynthetic pathway alters xylem structure and hydraulic function in herbaceous plants

Plants using the C₄ photosynthetic pathway have greater water use efficiency (WUE) than C₃ plants of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C₄ plants do not need to acquire and transport as much water as C₃ species. Because the structure of xylem tissue reflects hydraulic demand by the leaf canopy, a reduction in water transport requirements due to C₄ photosynthesis should affect the evolution of xylem characteristics in C₄ plants. In a comparison of stem hydraulic conductivity and vascular anatomy between eight C₃ and eight C₄ herbaceous species, C₄ plants had lower hydraulic conductivity per unit leaf area (K_L) than C₃ species of similar life form. When averages from all the species were pooled together, the mean K_L for the C₄ species was 1.60 × 10^?4 kg m^?1 s^?1 MPa^?1, which was only one‐third of the mean K_L of 4.65 × 10^?4 kg m^?1 s^?1 MPa^?1 determined for the C₃ species. The differences in K_L between C₃ and C₄ species corresponded to the two‐ to three‐fold differences in WUE observed between C₃ and C₄ plants. In the C₄ species from arid regions, the difference in K_L was associated with a lower hydraulic conductivity per xylem area, smaller and shorter vessels, and less vulnerable xylem to cavitation, indicating the C₄ species had evolved safer xylem than the C₃ species. In the plants from resource‐rich areas, such as the C₄ weed Amaranthus retroflexus, hydraulic conductivity per xylem area and xylem anatomy were similar to that of the C₃ species, but the C₄ plants had greater leaf area per xylem area. The results indicate the WUE advantage of C₄ photosynthesis allows for greater flexibility in hydraulic design and potential fitness. In resource‐rich environments in which competition is high, an existing hydraulic design can support greater leaf area, allowing for higher carbon gain, growth and competitive potential. In arid regions, C₄ plants evolved safer xylem, which can increase survival and performance during drought events.     

Inter-species variation of photosynthetic and xylem hydraulic traits in the deciduous and evergreen Euphorbiaceae tree species from a seasonally tropical forest in south-western China

The objective of the present study was to examine the functional coordination among hydraulic traits, xylem characteristics and gas exchange rates across three deciduous Euphorbiaceae tree species (Hevea brasiliensis, Macaranga denticulata and Bischofia javanica) and three evergreen Euphorbiaceae tree species (Drypetes indica, Aleurites moluccana and Codiaeum variegatum) from a seasonally tropical forest in south-western China. The deciduous tree species were more vulnerable to water stress-induced embolism than the evergreen tree species. However, the deciduous tree species generally had higher maximal rates of sapwood and leaf-specific hydraulic conductivity (K _S and K _L), respectively. Compared with the evergreen tree species, the deciduous tree species, however, possessed a lower density of sapwood and a wider diameter of xylem vessels. Regardless of leaf phenology, the hydraulic vulnerability and conductivity were significantly correlated with sapwood density and mean vessel diameter. Furthermore, the hydraulic vulnerability was positively correlated with water transport efficiency. In addition, the deciduous tree species exhibited higher maximal photosynthetic rates (A _max) and stomatal conductance (g _max), but lower water use efficiency (WUE). Interestingly, the A _max, g _max and WUE were strongly correlated with K _S and K _L across the deciduous and evergreen tree species. These results suggest that xylem structure, rather than leaf phenology, accounts for the difference in hydraulic traits between the deciduous tree species and the evergreen tree species. Meanwhile, our results show that there is a significant trade-off between hydraulic efficiency and safety, and a strong functional correlation between the hydraulic capacity and gas exchange rates across the deciduous and evergreen tree species.     

Influence of nutrient versus water supply on hydraulic architecture and water balance in Pinus taeda

We investigated the hydraulic consequences of a major decrease in root‐to‐leaf area ratio (A_R:A_L) caused by nutrient amendments to 15‐year‐old Pinus taeda L. stands on sandy soil. In theory, such a reduction in A_R:A_L should compromise the trees’ ability to extract water from drying sand. Under equally high soil moisture, canopy stomatal conductance (G_S) of fertilized trees (F) was 50% that of irrigated/fertilized trees (IF), irrigated trees (I), and untreated control trees (C). As predicted from theory, F trees also decreased their stomatal sensitivity to vapour pressure deficit by 50%. The lower G_S in F was associated with 50% reduction in leaf‐specific hydraulic conductance (K_L) compared with other treatments. The lower K_L in F was in turn a result of a higher leaf area per sapwood area and a lower specific conductivity (conducting efficiency) of the plant and its root xylem. The root xylem of F trees was also 50% more resistant to cavitation than the other treatments. A transport model predicted that the lower A_R:A_L in IF trees resulted in a considerably restricted ability to extract water during drought. However, this deficiency was not exposed because irrigation minimized drought. In contrast, the lower A_R:A_L in F trees caused only a limited restriction in water extraction during drought owing to the more cavitation resistant root xylem in this treatment. In both fertilized treatments, approximate safety margins from predicted hydraulic failure were minimal suggesting increased vulnerability to drought‐induced dieback compared with non‐fertilized trees. However, IF trees are likely to be so affected even under a mild drought if irrigation is withheld.     

Photosynthetic,hydraulic and biomass properties in closely related C3 and C4 species

In plants, most water is absorbed by roots and transported through vascular conduits of xylem which evaporate from leaves during photosynthesis. As photosynthesis and transport processes are interconnected, it was hypothesized that any variation in water transport demand influencing water use efficiency (WUE), such as the evolution of C₄ photosynthesis, should affect xylem structure and function. Several studies have provided evidence for this hypothesis, but none has comprehensively compared photosynthetic, hydraulic and biomass allocation properties between C₃ and C₄ species. In this study, photosynthetic, hydraulic and biomass properties in a closely related C₃ Tarenaya hassleriana and a C₄ Cleome gynandra are compared. Light response curves, measured at 30°C, showed that the C₄ C. gynandra had almost twice greater net assimilation rates than the C₃ T. hassleriana under each increasing irradiation level. On the contrary, transpiration rates and stomatal conductance were around twice as high in the C₃, leading to approximately 3.5 times higher WUE in the C₄ compared with the C₃ species. The C₃ showed about 3.3 times higher hydraulic conductivity, 4.3 times greater specific conductivity and 2.6 times higher leaf‐specific conductivity than the C₄ species. The C₃ produced more vessels per xylem area and larger vessels. All of these differences resulted in different biomass properties, where the C₄ produced more biomass in general and had less root to shoot ratio than the C₃ species. These results are in support of our previous findings that WUE, and any changes that affect WUE, contribute to xylem evolution in plants.     

Divergent scaling of respiration rates to nitrogen and phosphorus across four woody seedlings between different growing seasons

Empirical studies indicate that the exponents governing the scaling of plant respiration rates (R) with respect to biomass (M) numerically vary between three‐fourth for adult plants and 1.0 for seedlings and saplings and are affected by nitrogen (N) and phosphorus (P) content. However, whether the scaling of R with respect to M (or N and P) varies among different phylogenetic groups (e.g., gymnosperms vs. angiosperms) or during the growing and dormant seasons remains unclear. We measured the whole‐plant R and M, and N and P content of the seedlings of four woody species during the growing season (early October) and the dormant season (January). The data show that (i) the scaling exponents of R versus M, R versus N, and R versus P differed significantly among the four species, but (ii), not between the growing and dormant seasons for each of the four species, although (iii) the normalization constants governing the scaling relationships were numerically greater for the growing season compared to the dormant season. In addition, (iv) the scaling exponents of R versus M, R versus N, and R versus P were numerically larger for the two angiosperm species compared to those of the two gymnosperm species, (v) the interspecific scaling exponents for the four species were greater during the growing season than in the dormant season, and (vi), interspecifically, P scaled nearly isometric with N content. Those findings indicate that the metabolic scaling relationships among R, M, N, and P manifest seasonal variation and differ between angiosperm and gymnosperm species, that is, there is no single, canonical scaling exponent for the seedlings of woody species.     

Dwarf shrub hydraulics: two Vaccinium species (Vaccinium myrtillus,Vaccinium vitis‐idaea) of the European Alps compared

Vaccinium myrtillus and Vaccinium vitis‐idaea are two dwarf shrubs widespread in the European Alps. We studied the hydraulics of these species hypothesizing that (1) the hydraulic architecture of dwarf shrubs differs from trees, (2) hydraulic properties reflect the species' ecological amplitude and (3) hydraulic properties vary spatially and seasonally. Key hydraulic parameters (osmotic potential, turgor loss point, xylem hydraulic conductivity, vulnerability to drought‐induced embolism, stomata closure, drought‐induced cell damage and embolism repair) and related wood anatomical traits (conduit diameter and conduit wall reinforcement) were analyzed at four sites in Tyrol, Austria. Both species exhibited low hydraulic safety as well as low hydraulic efficiency. Fifty percentage embolism accumulated at ?2.08 (V. myrtillus) and ?1.97 MPa (V. vitis‐idaea), 88% stomata closure was at ?2.19 and ?2.35 MPa, respectively. After drought, both species showed embolism repair on re‐watering. Site‐specific variation within species was low, while seasonal changes in embolism resistance and turgor loss point were observed. Results indicate that studied Vaccinium species have a high risk for embolism formation. This is balanced by refilling capacities, which are probably based on the small growth height of dwarf shrubs. V. vitis‐idaea, which occurs on drier sites, showed more efficient repair and a lower turgor loss point than V. myrtillus.     

Growing‐season temperature and precipitation are independent drivers of global variation in xylem hydraulic conductivity

Stem xylem‐specific hydraulic conductivity (K_S) represents the potential for plant water transport normalized by xylem cross section, length, and driving force. Variation in K_S has implications for plant transpiration and photosynthesis, growth and survival, and also the geographic distribution of species. Clarifying the global‐scale patterns of K_S and its major drivers is needed to achieve a better understanding of how plants adapt to different environmental conditions, particularly under climate change scenarios. Here, we compiled a xylem hydraulics dataset with 1,186 species‐at‐site combinations (975 woody species representing 146 families, from 199 sites worldwide), and investigated how K_S varied with climatic variables, plant functional types, and biomes. Growing‐season temperature and growing‐season precipitation drove global variation in K_S independently. Both the mean and the variation in K_S were highest in the warm and wet tropical regions, and lower in cold and dry regions, such as tundra and desert biomes. Our results suggest that future warming and redistribution of seasonal precipitation may have a significant impact on species functional diversity, and is likely to be particularly important in regions becoming warmer or drier, such as high latitudes. This highlights an important role for K_S in predicting shifts in community composition in the face of climate change.     

Hydraulic architecture of deciduous and evergreen dry rainforest tree species from north-eastern Australia

Hydraulic conductivity and xylem anatomy were examined in stems of two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., and two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., from a seasonally dry rainforest in north Queensland, Australia. The deciduous species possessed hydraulic architecture typical of drought-sensitive plants, i.e. low wood density, wider xylem vessels, higher maximal rates of sapwood specific hydraulic conductivity (K_s) and high vulnerability to drought-induced embolism. In contrast, the evergreen species had lower rates of K_h and leaf specific conductivity (K_L) but were less susceptible to embolism. The evergreen species experienced leaf water potentials <–4.0 MPa during the dry season, while the deciduous species shed their leaves before leaf water potentials declined below –2.0 MPa. Thus, the hydraulic architecture of the evergreens allows them to withstand the greater xylem pressure gradients required to maintain water transport to the canopy during the dry season. Our results are consistent with observations made in neotropical dry forests and demonstrate that drought-deciduous species with low wood density and high water storage capacity are likely to be more hydraulically efficient, but more vulnerable to embolism, than coexisting evergreens.     

Water use efficiency of twenty-five co-existing Patagonian species growing under different soil water availability

The variation of plant water use efficiency (WUE) with water availability has two interacting components: a plastic response, evident when individuals of the same genotype are compared (e.g. wet versus dry years), and an interspecific response, evident when different species living in habitats with different water availability are compared. We analysed the WUE of 25 Patagonian species that belong to four life forms (grasses, shrubs, annual herbs and perennial herbs) in relation to the climatic conditions of 2 years and the mean historic water availability experienced by each species. To estimate water availability, we calculated the effective soil water potential (EWP) of each species, based on available information about soil water dynamics, phenology and root system structure. To estimate WUE, we used isotopic discrimination of leaf C (Δ¹³C) and mean annual water vapour difference between leaves and atmosphere (Δe) measured in situ. For the plastic response, for every species and life form, WUE increased from the dry to the wet year. We hypothesize that photosynthesis was less nutrient limited in the wet than in the dry year, facilitating higher net photosynthesis rates per unit of stomatal conductance in the wet year. For the interspecific response, WUE was lower in species native to drier habitats than in species native to wetter habitats. This response was mostly accounted for by a decrease in Δe with EWP. Annual herbs, which avoid drought in time (they have the earliest growth cycle), and shrubs, which avoid drought in space (they have the deepest roots), showed the highest EWP and WUE. We conclude that the conventional wisdom which states that the highest WUE occurs within a species during the driest years, and among species in the driest habitats, does not always hold true, and that co-existing life forms drastically differ in water availability and water economy.     

Bundle sheath lignification mediates the linkage of leaf hydraulics and venation

The lignification of the leaf vein bundle sheath (BS) has been observed in many species and would reduce conductance from xylem to mesophyll. We hypothesized that lignification of the BS in lower‐order veins would provide benefits for water delivery through the vein hierarchy but that the lignification of higher‐order veins would limit transport capacity from xylem to mesophyll and leaf hydraulic conductance (K_leaf). We further hypothesized that BS lignification would mediate the relationship of K_leaf to vein length per area. We analysed the dependence of K_leaf, and its light response, on the lignification of the BS across vein orders for 11 angiosperm tree species. Eight of 11 species had lignin deposits in the BS of the midrib, and two species additionally only in their secondary veins, and for six species up to their minor veins. Species with lignification of minor veins had a lower hydraulic conductance of xylem and outside‐xylem pathways and lower K_leaf. K_leaf could be strongly predicted by vein length per area and highest lignified vein order (R² = .69). The light‐response of K_leaf was statistically independent of BS lignification. The lignification of the BS is an important determinant of species variation in leaf and thus whole plant water transport.     

Interspecific patterns underlying variations in allometric relationship of sympatric Sterculiaceae species in a Bornean rainforest

We compared aboveground tree forms among closely related species in two genera of the Sterculiaceae (Scaphium and Heritiera) in a Bornean mixed dipterocarp forest. Two significant allometric patterns were detected: a negative correlation between the height at the onset of branching and the slope of the species-specific Cr (crown width)-D (stem diameter) allometric relationship for juveniles (D<10 cm), and a negative correlation between H _max (observed maximum height) and the Cr-D slope. The slope of the Cr-D allometric relationship of branched trees was significantly steeper than that of monoaxial (unbranched) trees in most species. These results suggest that the branching growth habit is better adapted than the monoaxial growth habit to crown expansion, and that the morphology of short species is better adapted to crown expansion than that of tall species. We did not detected significant correlations between the height at the onset of branching and the slope of the H (height)-D allometric relationship for juvenile trees, and between H _max and the H-D slope. In addition, the monoaxial and branched juvenile of most species did not differ significantly in the allometric slopes of the H-D relationship. Therefore, the study does not support the hypotheses that a monoaxial growth habit favors rapid height growth and that tall species have allometries better adapted to height growth.     

Trade-off between water transport efficiency and leaf life-span in a tropical dry forest

Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determined for both groups. They differed in seasonal changes of both, wood water content (W _c) and wood specific gravity (G). During the dry season, the W _c in drought-deciduous species declined and the minimum value was recorded when leaf fall was complete. At this time, the volumetric fraction of gas (V _g) increased indicating air entry into xylem vessels. In contrast, W _c, G and V _g changed only slightly throughout the year for evergreen species. Maximum hydraulic conductivity of drought-deciduous species was 2–6 times that of the evergreen species. but was severely reduced at leaf fall. In the evergreen species, similar water conductivities were measured during wet and dry seasons. The trade-off between xylem water transport capacity and leaf lifespan found in species coexisting in this forest reveals the existence of contrasting but successful adaptations to this environment. Drought-deciduous species maximize production in the short term with higher water transport efficiency which leads to the seasonal occurrence of embolisms. Conversely, the behaviour of evergreen species with reduced maximum efficiency is conservative but safe in relation to xylem embolism.     

Allometric scaling relationship between above‐ and below‐ground biomass within and across five woody seedlings

Allometric biomass allocation theory predicts that leaf biomass (M_L) scaled isometrically with stem (M_S) and root (M_R) biomass, and thus above‐ground biomass (leaf and stem) (M_A) and root (M_R) scaled nearly isometrically with below‐ground biomass (root) for tree seedlings across a wide diversity of taxa. Furthermore, prior studies also imply that scaling constant should vary with species. However, litter is known about whether such invariant isometric scaling exponents hold for intraspecific biomass allocation, and how variation in scaling constants influences the interspecific scaling relationship between above‐ and below‐ground biomass. Biomass data of seedlings from five evergreen species were examined to test scaling relationships among biomass components across and within species. Model Type II regression was used to compare the numerical values of scaling exponents and constants among leaf, stem, root, and above‐ to below‐ground biomass. The results indicated that M_L and M_S scaled in an isometric or a nearly isometric manner with M_R, as well as M_A to M_R for five woody species. Significant variation was observed in the Y‐intercepts of the biomass scaling curves, resulting in the divergence for intraspecific scaling and interspecific scaling relationships for M_L versus M_S and M_L versus M_R, but not for M_S versus M_R and M_A versus M_R. We conclude, therefore, that a nearly isometric scaling relationship of M_A versus M_R holds true within each of the studied woody species and across them irrespective the negative scaling relationship between leaf and stem.     

Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina (K_leaf) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance (g_s) and CO₂ uptake (A_leaf) were associated with short‐term changes in K_leaf. On days of high evaporative demand mid‐day depression of K_leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than ?1 MPa implying a refilling mechanism, or that loss of K_leaf was not linked to cavitation. Laboratory measurement of the response of K_leaf to Ψ_leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K_leaf and Ψ_leaf predicted from measured transpiration, xylem water potential and the K_leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K_leaf, g_s and A_leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.     

An empirical assessment of tree branching networks and implications for plant allometric scaling models

Several theories predict whole‐tree function on the basis of allometric scaling relationships assumed to emerge from traits of branching networks. To test this key assumption, and more generally, to explore patterns of external architecture within and across trees, we measure branch traits (radii/lengths) and calculate scaling exponents from five functionally divergent species. Consistent with leading theories, including metabolic scaling theory, branching is area preserving and statistically self‐similar within trees. However, differences among scaling exponents calculated at node‐ and whole‐tree levels challenge the assumption of an optimised, symmetrically branching tree. Furthermore, scaling exponents estimated for branch length change across branching orders, and exponents for scaling metabolic rate with plant size (or number of terminal tips) significantly differ from theoretical predictions. These findings, along with variability in the scaling of branch radii being less than for branch lengths, suggest extending current scaling theories to include asymmetrical branching and differential selective pressures in plant architectures.     

Thermal acclimation of leaf and root respiration: an investigation comparing inherently fast- and slow-growing plant species

We investigated the extent to which leaf and root respiration (R) differ in their response to short‐ and long‐term changes in temperature in several contrasting plant species (herbs, grasses, shrubs and trees) that differ in inherent relative growth rate (RGR, increase in mass per unit starting mass and time). Two experiments were conducted using hydroponically grown plants. In the long‐term (LT) acclimation experiment, 16 species were grown at constant 18, 23 and 28 °C. In the short‐term (ST) acclimation experiment, 9 of those species were grown at 25/20 °C (day/night) and then shifted to a 15/10 °C for 7 days. Short‐term Q₁₀ values (proportional change in R per 10 °C) and the degree of acclimation to longer‐term changes in temperature were compared. The effect of growth temperature on root and leaf soluble sugar and nitrogen concentrations was examined. Light‐saturated photosynthesis (A_sat) was also measured in the LT acclimation experiment. Our results show that Q₁₀ values and the degree of acclimation are highly variable amongst species and that roots exhibit lower Q₁₀ values than leaves over the 15–25 °C measurement temperature range. Differences in RGR or concentrations of soluble sugars/nitrogen could not account for the inter‐specific differences in the Q₁₀ or degree of acclimation. There were no systematic differences in the ability of roots and leaves to acclimate when plants developed under contrasting temperatures (LT acclimation). However, acclimation was greater in both leaves and roots that developed at the growth temperature (LT acclimation) than in pre‐existing leaves and roots shifted from one temperature to another (ST acclimation). The balance between leaf R and A_sat was maintained in plants grown at different temperatures, regardless of their inherent relative growth rate. We conclude that there is tight coupling between the respiratory acclimation and the temperature under which leaves and roots developed and that acclimation plays an important role in determining the relationship between respiration and photosynthesis.     

Why vines have narrow stems: Histological trends in Bauhinia (Fabaceae)

Summary Xylem (wood) tissue in plants functions both for mechanical support and water transport. Since vines are mechanical parasites, they allocate less biomass for xylem tissue than do free-standing trees or shrubs. With-in the genus Bauhinia, stems of vine species were found to have not only less xylem per distal leaf area, but also less phloem and cortical tissue when compared to tree and shrub species. The phloem and cortical reductions are interpreted as an indirect effect of the developmental/geometric constraints imposed by the evolution of a reduced mechanical system. Apparently vines overcame these constraints with the evolution of wider vessels and wider sieve tubes and with many types of variant (anomalous) secondary growth. The long and wide vessels of vines, which compensate hydraulically for the reduced xylem areas, may help limit the distribution of vine species in nature.     

Tree water dynamics in a drying and warming world

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (F_D), stomatal conductance (G_s), hydraulic conductivity (K_L) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower G_s and F_D were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of G_s and F_D to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced F_D, G_s and K_L. Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in G_s, F_D and K_L took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi‐arid regions.