• Background and Aims Sapwood traits like vessel diameter and intervessel pit characteristics play key rolesin maintaining hydraulic integrity of trees. Surprisingly little is known about how... Show more• Background and Aims Sapwood traits like vessel diameter and intervessel pit characteristics play key rolesin maintaining hydraulic integrity of trees. Surprisingly little is known about how sapwood traits covary with treeheight and how such trait-based variation could affect the efficiency of water transport in tall trees. This studypresents a detailed analysis of structural and functional traits along the vertical axes of tall Eucalyptus grandis trees. • Methods To assess a wide range of anatomical and physiological traits, light and electron microscopy wasused, as well as eld measurements of tree architecture, water use, stem water potential and leaf area distribution. • Key Results Strong apical dominance of water transport resulted in increased volumetric water supply per unitleaf area with tree height. This was realized by continued narrowing (from 250 to 20 μm) and an exponentialincrease in frequency (from 600 to 13 000 cm−2) of vessels towards the apex. The widest vessels were detected atleast 4 m above the stem base, where they were associated with the thickest intervessel pit membranes. In addition,this study established the lower limit of pit membrane thickness in tall E. grandis at ~375 nm. This minimumthickness was maintained over a large distance in the upper stem, where vessel diameters continued to narrow.• Conclusions The analyses of xylem ultrastructure revealed complex, synchronized trait covariation and trade-offs with increasing height in E. grandis. Anatomical traits related to xylem vessels and those related to architectureof pit membranes were found to increase efficiency and apical dominance of water transport. This study underlinesthe importance of studying tree hydraulic functioning at organismal scale. Results presented here will improveunderstanding height-dependent structure–function patterns in tall trees. Show less
Li, S.; Lens, F.P.; Espino, S.; Karimi, Z.; Klepsch, M.; Schenk, H.J.; ... ; Jansen, S. 2016
Pit membranes in bordered pits between neighbouring vessels play a major role inthe entry of air-water menisci from an embolised vessel into a water-filled vessel(i.e., air-seeding). Here, we... Show morePit membranes in bordered pits between neighbouring vessels play a major role inthe entry of air-water menisci from an embolised vessel into a water-filled vessel(i.e., air-seeding). Here, we investigate intervessel pit membrane thickness (TPM)and embolism resistance (P50, i.e., the water potential corresponding to 50% lossof hydraulic conductivity) across a broad range of woody angiosperm species.Data on TPM and double intervessel wall thickness (TVW) were compiled based onelectron and light microscopy. Fresh material that was directly fixated for transmission electron microscopy (TEM) was investigated for 71 species, while non-freshsamples were frozen, stored in alcohol, or air dried prior to TEM preparation for anadditional 60 species. TPM and P50 were based on novel observations and literature.A strong correlation between TPM and P50 was found for measurements based onfreshly fixated material (r = 0.78, P <0.01, n = 37), and between TPM and TVW(r = 0.79, P < 0.01, n = 59), while a slightly weaker relationship occurred betweenTVW and P50 (r = 0.40, P <0.01, n = 34). However, non-fresh samples showed nocorrelation between TPM and P50, and between TPM and TVW. Intervessel pit membranes in non-fresh samples were c.28% thinner and more electron dense thanfresh samples. Our findings demonstrate that TPM measured on freshly fixatedmaterial provides one of the strongest wood anatomical correlates of drought-induced embolism resistance in angiosperms. Assuming that cellulose micro fibrilsshow an equal spatial density, TPM is suggested to affect the length and the shapeof intervessel pit membrane pores, but not the actual pore size. Moreover, theshrinking effect observed for TPM after dehydration and frost is associated withan increase in micro bril density and porosity, which may provide a functionalexplanation for embolism fatigue. Show less
The evolution of lignified xylem allowed for the efficient transport of water under tension,but also exposed the vascular network to the risk of gas emboli and the spread of gas betweenxylem... Show moreThe evolution of lignified xylem allowed for the efficient transport of water under tension,but also exposed the vascular network to the risk of gas emboli and the spread of gas betweenxylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposesthat the safety of xylem (its ability to resist embolism formation and spread) should trade offagainst xylem efficiency (its capacity to transport water). We tested this safety–efficiency hypothesis in branch xylem across 335 angiosperm and 89gymnosperm species. Safety was considered at three levels: the xylem water potentials where12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r2 < 0.086), no species hadhigh efficiency and high safety, supporting the idea for a safety–efficiency tradeoff. However,many species had low efficiency and low safety. Species with low efficiency and low safetywere weakly associated (r2 < 0.02 in most cases) with higher wood density, lower leaf- to sap-wood-area and shorter stature. There appears to be no persuasive explanation for the considerable number of species withboth low efficiency and low safety. These species represent a real challenge for understandingthe evolution of xylem. Show less
