High potential for foliar water uptake in early stages of leaf development of three woody angiosperms.

Foliar water uptake (FWU) is a widespread mechanism that may help plants cope with drought stress in a wide range of ecosystems. FWU can be affected by various leaf traits, which change during leaf development. We exposed cut and dehydrated leaves to rainwater and measured FWU, changes in leaf water potential after 19 h of FWU (ΔΨ), minimum leaf conductance (gmin ), and leaf wettability (abaxial and adaxial) of leaves of Acer platanoides, Fagus sylvatica, and Sambucus nigra at three developmental stages: unfolding (2-5-day-old), young (1.5-week-old) and mature leaves (8-week-old). FWU and gmin were higher in younger leaves. ΔΨ corresponded to FWU and gmin in all cases but mature leaves of F. sylvatica, where ΔΨ was highest. Most leaves were highly wettable, and at least one leaf surface (adaxial or abaxial) showed a decrease in wettability from unfolding to mature leaves. Young leaves of all studied species showed FWU (unfolding leaves: 14.8 ± 1.1 µmol m-2 s-1 ), which may improve plant water status and thus counterbalance spring transpirational losses due to high gmin . The high wettability of young leaves probably supported FWU. We observed particularly high FWU and respective high ΔΨ in older leaves of F. sylvatica, possibly aided by trichomes.

Alpine dwarf shrubs show high proportions of nonfunctional xylem: Visualization and quantification of species-specific patterns.

Xylem conductive capacity is a key determinant of plant hydraulic function and intimately linked to photosynthesis and productivity, but can be impeded by temporary or permanent conduit dysfunctions. Here we show that persistent xylem dysfunctions in unstressed plants are frequent in Alpine dwarf shrubs and occur in various but species-specific cross-sectional patterns. Combined synchrotron micro-computed tomography (micro-CT) imaging, xylem staining, and flow measurements in saturated samples of six widespread Ericaceae species evidence a high proportion (19%-50%) of hydraulically nonfunctional xylem areas in the absence of drought stress, with regular distribution of dysfunctions between or within growth rings. Dysfunctions were only partly reversible and reduced the specific hydraulic conductivity to 1.38 to 3.57 ×10-4 m2 s-1 MPa-1 . Decommission of inner growth rings was clearly related to stem age and a higher vulnerability to cavitation of older rings, while the high proportion of nonfunctional conduits in each annual ring needs further investigations. The lower the xylem fraction contributing to the transport function, the higher was the hydraulic efficiency of conducting xylem areas. Improved understanding of the functional lifespan of xylem elements and the prevalence and nature of dysfunctions is critical to correctly assess structure-function relationships and whole-plant hydraulic strategies.

Juniperus communis populations exhibit low variability in hydraulic safety and efficiency.

The performance and distribution of woody species strongly depend on their adjustment to environmental conditions based on genotypic and phenotypic properties. Since more intense and frequent drought events are expected due to climate change, xylem hydraulic traits will play a key role under future conditions, and thus, knowledge of hydraulic variability is of key importance. In this study, we aimed to investigate the variability in hydraulic safety and efficiency of the conifer shrub Juniperus communis based on analyses along an elevational transect and a common garden approach. We studied (i) juniper plants growing between 700 and 2000 m a.s.l. Innsbruck, Austria, and (ii) plants grown in the Innsbruck botanical garden (Austria) from seeds collected at different sites across Europe (France, Austria, Ireland, Germany and Sweden). Due to contrasting environmental conditions at different elevation and provenance sites and the wide geographical study area, pronounced variation in xylem hydraulics was expected. Vulnerability to drought-induced embolisms (hydraulic safety) was assessed via the Cavitron and ultrasonic acoustic emission techniques, and the specific hydraulic conductivity (hydraulic efficiency) via flow measurements. Contrary to our hypothesis, relevant variability in hydraulic safety and efficiency was neither observed across elevations, indicating a low phenotypic variation, nor between provenances, despite expected genotypic differences. Interestingly, the provenance from the most humid and warmest site (Ireland) and the northernmost provenance (Sweden) showed the highest and the lowest embolism resistance, respectively. The hydraulic conductivity was correlated with plant height, which indicates that observed variation in hydraulic traits was mainly related to morphological differences between plants. We encourage future studies to underlie anatomical traits and the role of hydraulics for the broad ecological amplitude of J. communis.

Insights from in vivo micro-CT analysis: testing the hydraulic vulnerability segmentation in Acer pseudoplatanus and Fagus sylvatica seedlings.

The seedling stage is the most susceptible one during a tree's life. Water relations may be crucial for seedlings due to their small roots, limited water buffers and the effects of drought on water transport. Despite obvious relevance, studies on seedling xylem hydraulics are scarce as respective methodical approaches are limited. Micro-CT scans of intact Acer pseudoplatanus and Fagus sylvatica seedlings dehydrated to different water potentials (Ψ) allowed the simultaneous observation of gas-filled versus water-filled conduits and the calculation of percentage loss of conductivity (PLC) in stems, roots and leaves (petioles or main veins). Additionally, anatomical analyses were performed and stem PLC measured with hydraulic techniques. In A. pseudoplatanus, petioles showed a higher Ψ at 50% PLC (Ψ50 -1.13MPa) than stems (-2.51 MPa) and roots (-1.78 MPa). The main leaf veins of F. sylvatica had similar Ψ50 values (-2.26 MPa) to stems (-2.74 MPa) and roots (-2.75 MPa). In both species, no difference between root and stems was observed. Hydraulic measurements on stems closely matched the micro-CT based PLC calculations. Micro-CT analyses indicated a species-specific hydraulic architecture. Vulnerability segmentation, enabling a disconnection of the hydraulic pathway upon drought, was observed in A. pseudoplatanus but not in the especially shade-tolerant F. sylvatica. Hydraulic patterns could partly be related to xylem anatomical traits.

Xylem Sap Surface Tension May Be Crucial for Hydraulic Safety.

The surface tension (γ) of xylem sap plays a key role in stabilizing air-water interfaces at the pits between water- and gas-filled conduits to avoid air seeding at low water potentials. We studied seasonal changes in xylem sap γ in Picea abies and Pinus mugo growing at the alpine timberline. We analyzed their vulnerability to drought-induced embolism using solutions of different γ and estimated the potential effect of seasonal changes in γ on hydraulic vulnerability. In both species, xylem sap γ showed distinct seasonal courses between about 50 and 68 mn m-1 Solutions with low γ caused higher vulnerability to drought-induced xylem embolism. The water potential at 50% loss of hydraulic conductivity in P. abies and P. mugo was -3.35 and -3.86 MPa at γ of 74 mn m-1 but -2.11 and -2.09 MPa at 45 mn m-1 This indicates up to about 1 MPa seasonal variation in 50% loss of hydraulic conductivity. The results revealed pronounced effects of changes in xylem sap γ on the hydraulic safety of trees in situ. These effects also are relevant in vulnerability analyses, where the use of standard solutions with high γ overestimates hydraulic safety. Thus, γ should be considered carefully in hydraulic studies.

Static and dynamic bending has minor effects on xylem hydraulics of conifer branches (Picea abies, Pinus sylvestris).

The xylem hydraulic efficiency and safety is usually measured on mechanically unstressed samples, although trees may be exposed to combined hydraulic and mechanical stress in the field. We analysed changes in hydraulic conductivity and vulnerability to drought-induced embolism during static bending of Picea abies and Pinus sylvestris branches as well as the effect of dynamic bending on the vulnerability. We hypothesized this mechanical stress to substantially impair xylem hydraulics. Intense static bending caused an only small decrease in hydraulic conductance (-19.5 ± 2.4% in P. abies) but no shift in vulnerability thresholds. Dynamic bending caused a 0.4 and 0.8 MPa decrease of the water potential at 50 and 88% loss of conductivity in P. sylvestris, but did not affect vulnerability thresholds in P. abies. With respect to applied extreme bending radii, effects on plant hydraulics were surprisingly small and are thus probably of minor eco-physiological importance. More importantly, results indicate that available xylem hydraulic analyses (of conifers) sufficiently reflect plant hydraulics under field conditions.

Uptake of water via branches helps timberline conifers refill embolized xylem in late winter.

Xylem embolism is a limiting factor for woody species worldwide. Conifers at the alpine timberline are exposed to drought and freeze-thaw stress during winter, which induce potentially lethal embolism. Previous studies indicated that timberline trees survive by xylem refilling. In this study on Picea abies, refilling was monitored during winter and spring seasons and analyzed in the laboratory and in situ experiments, based on hydraulic, anatomical, and histochemical methods. Refilling started in late winter, when the soil was frozen and soil water not available for the trees. Xylem embolism caused up to 86.2% ± 3.1% loss of conductivity and was correlated with the ratio of closed pits. Refilling of xylem as well as recovery in shoot conductance started in February and corresponded with starch accumulation in secondary phloem and in the mesophyll of needles, where we also observed increasing aquaporin densities in the phloem and endodermis. This indicates that active, cellular processes play a role for refilling even under winter conditions. As demonstrated by our experiments, water for refilling was thereby taken up via the branches, likely by foliar water uptake. Our results suggest that refilling is based on water shifts to embolized tracheids via intact xylem, phloem, and parenchyma, whereby aquaporins reduce resistances along the symplastic pathway and aspirated pits facilitate isolation of refilling tracheids. Refilling must be taken into account as a key process in plant hydraulics and in estimating future effects of climate change on forests and alpine tree ecosystems.

Damage in needle tissues after infection with Chrysomyxa rhododendri increases cuticular conductance of Picea abies in winter.

Chrysomyxa rhododendri is a rust which infects Picea abies growing near the alpine timberline. Attacked needles are normally shed, but few remain on shoots. We hypothesised that these needles increase transpiration of Picea during winter. Partly damaged, completely damaged and healthy needles of an infected tree as well as healthy needles of a resistant tree were compared in a microscopy analysis, and needle conductance of shoots was measured gravimetrically. Despite needle shedding, more than 6% of needles remaining on infected tree shoots were damaged. Partly damaged needles showed local brownish areas in the periphery and completely damaged needles necrotic parenchyma and epidermal tissues. Cuticular conductance of affected shoots was up to 25.23 +/- 2.75 mmol m(-2) s(-1) at moderate water potential and thus twofold higher than in the resistant tree. Needle shedding reduces negative effects of Chrysomyxa infections during summer, but remaining damaged needles impair tree water relations in winter.

Hydraulic efficiency and safety of leader shoots and twigs in Norway spruce growing at the alpine timberline.

Xylem within trees varies in its hydraulic efficiency and safety. Trees at the alpine timberline were expected to exhibit a hydraulic architecture protecting the leader shoot from winter embolism. Hydraulic and related anatomical parameters were compared as well as seasonal courses of winter embolism in leader shoots and twigs of Norway spruce trees growing at 2000 m. Leader shoots had a 1.4-fold higher specific hydraulic conductivity (ks) as well as a 4.9-fold higher leaf specific conductivity (kl) than side twigs. Vulnerability to drought-induced embolism was lower in leader shoots with a 50% loss of conductivity occurring at a water potential (Psi 50) 0.7 MPa lower than in twigs. Higher ks and kl were related to 1.2-fold wider tracheid diameters in leader shoots. Lower vulnerability corresponded to smaller pit dimensions but not to wood density. High ks and kl reflect the hydraulic dominance of the leader shoot, which is important for its water supply during summer. Low vulnerability protects the leader shoot from embolism during the winter season. In field measurements at the timberline during the winter of 2001/2002, conductivity losses of up to 56% were observed only in twigs while leader shoots showed little or no embolism. Results demonstrate that leader shoot xylem is both hydraulically efficient and safe.