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The hydraulic properties of xylem and phloem differ but the magnitude and functional consequences of the differences are not well understood., Phloem and xylem functional areas, hydraulic conduit diameters and conduit frequency along the stems of Picea abies trees were measured and expressed as allometric functions of stem diameter and distance from stem apex. Conductivities of phloem and xylem were estimated from these scaling relations., Compared with xylem, phloem conduits were smaller and occupied a slightly larger fraction of conducting tissue area. Ten times more xylem than phloem was annually produced along the stem. Scaling of the conduit diameters and cross-sectional areas with stem diameter were very similar in phloem and xylem. Phloem and xylem conduits scaled also similarly with distance from stem apex; widening downwards from the tree top, and reaching a plateau near the base of the living crown., Phloem conductivity was estimated to scale similarly to the conductivity of the outermost xylem ring, with the ratio of phloem to xylem conductivity being c. 2%. However, xylem conductivity was estimated to increase more than phloem conductivity with increasing tree dimensions as a result of accumulation of xylem sapwood. Phloem partly compensated for its smaller conducting area and narrower conduits by having a slightly higher conduit frequency. [ABSTRACT FROM AUTHOR]

Palms are an important component of tropical ecosystems, living alongside dicotyledonous trees, even though they have a very different growth pattern and vascular system. As monocots, vessels in palms are located within vascular bundles and, without a vascular cambium that many dicotyledonous trees possess, palms cannot add additional vessels to their vascular system as they get older and taller. This means that hydraulic architecture in palms is more predetermined, which may require a highly efficient hydraulic system. This preset nature, along with the decoupling of hydraulic and mechanical functioning to different cell types, may allow palms to have a more efficient hydraulic system than dicotyledonous trees. Therefore, this study seeks to determine the efficiency of the hydraulic system in the palm Iriartea deltoidea (Ruiz & Pav.) and compare this efficiency with other tree forms. We measured cross-sectional areas of roots, stems and fronds as well as leaf areas of I. deltoidea saplings. Likewise, cross-sections were made and vessel diameters and frequencies measured. This allowed for the calculation of theoretical specific conductivity (KS,calc), theoretical leaf-specific conductivity (KL,calc), and vessel diameter and vessel number ratios between distal and proximal locations in the palms. Iriartea deltoidea palms were found to have the largest, least frequent vessels that diverged most from the square packing limit (maximum number of vessels that fit into a given area) compared with other major tree forms, and they therefore invested the least space and carbon into water transport structures. Likewise, conduits tapered by ∼1/3 between ranks (root, bole and petiole), which represents an efficient ratio with regard to the trade-offs between safety and efficiency of the conducting system. Conduits also exhibited a high conservation of the sum of the conduit radii cubed (Σr3) across ranks, thereby approximating Murray's law patterning. Therefore, our results indicate that the palm I. deltoidea has a very efficient hydraulic system in terms of maintaining a large conducting capacity with a minimal vascular investment. This efficiency may allow palms to compete well with dicotyledonous trees in tropical and subtropical climates but other developmental factors largely restrict palms from regions that experience prolonged freezing temperatures. [ABSTRACT FROM AUTHOR]

In plants, water flows from roots to leaves through a complex network of xylem conduits. The xylem architecture is characterized by the conduit enlargement towards the stem base and the multiplication of conduits near the apices of lateral branches. The xylem architecture of a small ash tree was analysed by measuring the vessel hydraulic diameter (Dh) and number (N) at different heights along the stem and branches. Along the stem, Dh and N increased from the apex to the point of crown insertion. Below, Dh and N decreased and remained constant, respectively. In branches, the Dh and N of apices increased with distance from the ground (PL) (P < 0.001 and P < 0.0001, respectively), indicating that apical resistance (RAPEX) becomes lower in the most peripheral branches (P < 0.0001). At the level of branch nodes along the stem, the total conductive area (AC) of the stem and branches just above the node was 11% higher than that of the stem just below the node (P = 0.024), whereas the conductivity (Kh) remained invariant above and below (P = 0.76). The difference in AC (ΔAC) between the branches and stem above each node increased with the distance of the node position from the stem apex (L). The xylem architecture of the analysed tree was characterized by anatomical modifications likely aimed at equilibrating the different path length effects on the hydraulic resistance of the different branches. Conduit tapering and multiplication seem to play a crucial role for the achievement of equal hydraulic resistance of all the leaves in the crown. [ABSTRACT FROM PUBLISHER]

The diameter of vascular conduits increases towards the stem base. It has been suggested that this profile is an efficient anatomical feature for reducing the hydraulic resistance when trees grow taller. However, the mechanism that controls the cell diameter along the plant is not fully understood. The timing of cell differentiation along the stem was investigated. Cambial activity and cell differentiation were investigated in a Picea abies tree (11.5 m in height) collecting microsamples at nine different heights (from 1 to 9 m) along the stem with a 4 d time interval. Wood sections (8–12 μm thick) were stained and observed under a light microscope with polarized light to differentiate the developing xylem cells. Cell wall lignification was detected using cresyl violet acetate. The first enlarging cells appeared almost simultaneously along the tree axis indicating that cambium activation is not height-dependent. A significant increase in the duration of the cell expansion phase was observed towards the tree base: at 9 m from the ground, xylem cells expanded for 7 d, at 6 m for 14 d, and at 3 m for 19 d. The duration of the expansion phase is positively correlated with the lumen area of the tracheids (r2=0.68, P < 0.01) at the same height. By contrast, thickness of the cell wall of the earlywood did not show any trend with height. The lumen area of the conduits down the stem appeared linearly dependent on time during which differentiating cells remained in the expansion phase. However, the inductive signal of such long-distance patterned differentiation remains to be identified. [ABSTRACT FROM PUBLISHER]

recent paper by Meinzer et al. ( Oecologia, , 164:287-296) has reviewed the topic of conflicts between hydraulic efficiency and safety in vascular plants. The authors raised the question that reliable interpretations of results on the mechanisms behind this trade-off requires an integrated approach that considers the variations in hydraulic traits throughout the entire plant and between different individuals, species and environments. Nonetheless, the literature review was only partial. Here, we discuss that an important feature of the whole tree xylem architecture, conduit tapering, is one of traits that must be considered when planning analyses of hydraulic properties of plants and interpreting the results. [ABSTRACT FROM AUTHOR]

Low temperatures limit the fixation of photosynthates and xylogenesis. Here, we hypothesized that reduced longitudinal growth in trees at high altitude is related to the lower hydraulic efficiency of the transport system. Apical buds of Norway spruce (Picea abies) trees at high and low elevation were heated during 2006 and 2007. At the end of the experiment, trees were felled. Longitudinal increments and tracheid lumen areas were measured along the stem. Apical hydraulic conductivity (k) was estimated from anatomical data. Before heating, high-altitude trees showed fewer (P = 0.002) and smaller (P = 0.008) apical conduits, and therefore reduced k (P = 0.016) and stem elongation (P < 0.0001), in comparison with trees at low elevation. After 2 yr of heating, k increased at both high (P = 0.014) and low (P = 0.047) elevation. Only highaltitude trees showed increased stem elongation, which reached the same magnitude as that of controls at low elevation (P = 0.735). Heating around the apical shoots did not appear to induce significant changes in conduit dimension along the rest of the stem. The total number and size of xylem elements at the stem apex are strongly constrained by low temperatures. Trees at high altitude are therefore prevented from building up an efficient transport system, and their reduced longitudinal growth reflects strong hydraulic limitations. [ABSTRACT FROM AUTHOR]

We studied the effect of tree architecture on xylem anatomy in three Betula pendula Roth., three Picea abies (L.) H. Karst. and three Pinus sylvestris (L.) trees (mean age 35 years). First, the analysis of conduit anatomy in different tree parts showed that conduits tapered and their frequency increased from roots (≥2 mm) to stem, from stem to branches and further to leaf petioles in B. pendula. Conduit anatomy in lateral and main roots, as well as lateral and main branches, significantly differed from each other in all the studied species. The increase in conduit diameter and decrease in frequency from the pith to the bark were clear aboveground, but variable patterns were observed belowground. In the leaf petioles of B. pendula, conduit diameter increased and conduit frequency decreased with increasing individual leaf area. Second, the results concerning the scaling of conduit diameter were compared with the predictions of the general vascular scaling model (WBE model) and Murray's law. The scaling parameter values at the tree level corresponded with the predictions of the WBE model in all the studied trees except for one tree of both conifer species. However, the scaling parameter values changed from one tree compartment to another rather than remaining uniform inside a tree, as assumed by the WBE model. The assumptions of the WBE model of a constant conductivity ratio, constant tapering and an unchanged total number of conduits were not fulfilled. When the conductivity ratio and relative tapering were plotted together, the results aboveground corresponded quite well with Murray's law: the conductivity ratio increased when relative tapering decreased. Our results support the theory that trees adjust both their macro- and microstructure to maximize their water transport efficiency, but also to prevent embolism and ensure mechanical safety. [ABSTRACT FROM AUTHOR]

•Recent research suggests that increasing conduit tapering progressively reduces hydraulic constraints caused by tree height. Here, we tested this hypothesis using the tallest hardwood species, Eucalyptus regnans. •Vertical profiles of conduit dimensions and vessel density were measured for three mature trees of height 47, 51 and 63 m. •Mean hydraulic diameter ( Dh) increased rapidly from the tree apex to the point of crown insertion, with the greatest degree of tapering yet reported ( b > 0.33). Conduit tapering was such that most of the total resistance was found close to the apex (82–93% within the first 1 m of stem) and the path length effect was reduced by a factor of 2000. Vessel density ( VD) declined from the apex to the base of each tree, with scaling parameters being similar for all trees ( a = 4.6; b = −0.5). • Eucalyptus regnans has evolved a novel xylem design that ensures a high hydraulic efficiency. This feature enables the species to grow quickly to heights of 50–60 m, beyond the maximum height of most other hardwood trees. [ABSTRACT FROM AUTHOR]

Xylem conduits increase in size from the apex downwards. While conduit tapering in the stem has often been reported to converge towards a common pattern among all plants, information on conduit tapering in small plants and roots is extremely scarce. We selected 10 small trees (height < 3 m) of Pinus cembra L. and Larix decidua Miller along an altitudinal gradient and measured diameter and conduit dimensions along stems and roots in the last annual ring. Sections of 10 µm were cut from wooden disks taken at different heights in the stem and in the roots and then stained with safranine. Slides were observed under a microscope, the lumen areas of conduits were measured and mean hydraulic diameters (Dh) calculated. Dh increased from stem tip (Dh at 1 cm from the apex averaged 10.75 µm; s = 2.33) to base (Dh from 20.70 to 30.54 µm), following a power trajectory (i.e., Dh = a·Lb, with L being the distance from the tip). Such degrees of conduit tapering may have a considerable effect in minimizing the hydraulic constraints. Despite trees at the treeline being older and smaller than in the subalpine forest (age: 28–70 years at the treeline; 18–39 years in the subalpine forest), conduit tapering did not differ significantly between sites, suggesting that tree height is the main factor controlling the basipetal increase in conduit lumens. In the roots, the increase in conduit dimensions continued towards their tips, even more steeply than in the stem. The widest conduits were measured around the root tips (around 40 µm). Conduit tapering resulted as a stable structural feature in small plants as well as in tall trees. [ABSTRACT FROM AUTHOR]

• Vertical conduit tapering is proposed as an effective mechanism to almost eliminate the increase in hydraulic resistance with increased height. Despite this potential role, very little is known about its changes during ontogeny. • Here, conduit tapering and stem morphology of young/small and old/tall individuals of Acer pseudoplatanus in the field, as well as 3-yr-old grafted trees from both age classes, were analysed. The distribution of hydraulic resistance along stems was also determined in a subsample of trees. • Substantial conduit tapering was found in small trees (field-grown and grafted from both age classes), whereas values were lower in tall trees, indicating that tapering was a size-related, not an age-related process. Apical conduit diameters were larger in tall trees and were inversely correlated with the degree of tapering. Hydraulic resistance increased less than linearly with distance from the apex. Its scaling against distance was indistinguishable from that predicted from anatomical measurements. • Conduit tapering was an effective but partial mechanism of compensation for the increase in hydraulic resistance with tree height. Size-related changes in tapering and in apical conduit diameters may be explained by the combined need to reduce the build-up of hydraulic resistance while minimizing the carbon costs of building vessel walls. [ABSTRACT FROM AUTHOR]