Katline Charra-Vaskou

Uptake of Water via Branches Helps Timberline Conifers Refill Embolized Xylem in Late Winter

Timberline conifers, which exhibit potentially lethal winter embolism, refill stem xylem with water taken up via branches and water transport to isolated, embolized conduits by active, cellular processes. 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.

Are needles of Pinus pinaster more vulnerable to xylem embolism than branches? New insights from X‐ray computed tomography

Plants can be highly segmented organisms with an independently redundant design of organs. In the context of plant hydraulics, leaves may be less embolism resistant than stems, allowing hydraulic failure to be restricted to distal organs that can be readily replaced. We quantified drought-induced embolism in needles and stems of Pinus pinaster using high-resolution computed tomography (HRCT). HRCT observations of needles were compared with the rehydration kinetics method to estimate the contribution of extra-xylary pathways to declining hydraulic conductance. High-resolution computed tomography images indicated that the pressure inducing 50% of embolized tracheids was similar between needle and stem xylem (P50 needle xylem u2009=u2009-3.62u2009MPa, P50 stem xylem u2009=u2009-3.88u2009MPa). Tracheids in both organs showed no difference in torus overlap of bordered pits. However, estimations of the pressure inducing 50% loss of hydraulic conductance at the whole needle level by the rehydration kinetics method were significantly higher (P50 needle u2009=u2009-1.71u2009MPa) than P50 needle xylem derived from HRCT. The vulnerability segmentation hypothesis appears to be valid only when considering hydraulic failure at the entire needle level, including extra-xylary pathways. Our findings suggest that native embolism in needles is limited and highlight the importance of imaging techniques for vulnerability curves.

Freeze-Thaw Stress: Effects of Temperature on Hydraulic Conductivity and Ultrasonic Activity in Ten Woody Angiosperms

Gas segregation and air seeding are both involved in embolism development in angiosperms. Freeze-thaw events can affect plant hydraulics by inducing embolism. This study analyzed the effect of temperature during the freezing process on hydraulic conductivity and ultrasonic emissions (UE). Stems of 10 angiosperms were dehydrated to a water potential at 12% percentage loss of hydraulic conductivity (PLC) and exposed to freeze-thaw cycles. The minimal temperature of the frost cycle correlated positively with induced PLC, whereby species with wider conduits (hydraulic diameter) showed higher freeze-thaw-induced PLC. Ultrasonic activity started with the onset of freezing and increased with decreasing subzero temperatures, whereas no UE were recorded during thawing. The temperature at which 50% of UE were reached varied between −9.1°C and −31.0°C across species. These findings indicate that temperatures during freezing are of relevance for bubble formation and air seeding. We suggest that species-specific cavitation thresholds are reached during freezing due to the temperature-dependent decrease of water potential in the ice, while bubble expansion and the resulting PLC occur during thawing. UE analysis can be used to monitor the cavitation process and estimate freeze-thaw-induced PLC.

Hydraulic efficiency and safety of vascular and non-vascular components in Pinus pinaster leaves

Leaves, the distal section of the soil-plant-atmosphere continuum, exhibit the lowest water potentials in a plant. In contrast to angiosperm leaves, knowledge of the hydraulic architecture of conifer needles is scant. We investigated the hydraulic efficiency and safety of Pinus pinaster needles, comparing different techniques. The xylem hydraulic conductivity (k(s)) and embolism vulnerability (P(50)) of both needle and stem were measured using the cavitron technique. The conductance and vulnerability of whole needles were measured via rehydration kinetics, and Cryo-SEM and 3D X-ray microtomographic observations were used as reference tools to validate physical measurements. The needle xylem of P. pinaster had lower hydraulic efficiency (k(s)u2009=u20092.0u2009×u200910(-4) m(2) MPa(-1) s(-1)) and safety (P(50)u2009=u2009-u20091.5 MPa) than stem xylem (k(s)u2009=u20097.7u2009×u200910(-4) m(2) MPa(-1) s(-1); P(50)u2009=u2009-u20093.6 tou2009-u20093.2 MPa). P(50) of whole needles (both extra-vascular and vascular pathways) wasu2009-u20090.5 MPa, suggesting that non-vascular tissues were more vulnerable than the xylem. During dehydration tou2009-u20093.5 MPa, collapse and embolism in xylem tracheids, and gap formation in surrounding tissues were observed. However, a discrepancy in hydraulic and acoustic results appeared compared with visualizations, arguing for greater caution with these techniques when applied to needles. Our results indicate that the most distal parts of the water transport pathway are limiting for hydraulics of P. pinaster. Needle tissues exhibit a low hydraulic efficiency and low hydraulic safety, but may also act to buffer short-term water deficits, thus preventing xylem embolism.

Winter at the alpine timberline causes complex within-tree patterns of water potential and embolism in Picea abies

Winter temperatures at the alpine timberline cause ice formation in the xylem of conifers blocking water uptake as well as water shifts within the axes system. This amplifies drought stress that, in combination with freeze-thaw events, causes embolism. This study focussed on within-tree patterns of water potential (psi) and embolism in Norway spruce [Picea abies (L.) Karst.]. At five sampling dates in midwinter, psi was determined at numerous positions in the crown of three trees, and at the end of March, the extent of embolism in representative sections of the axes system was analysed. Until 14 March, mean psi decreased to -3.77 +/- 0.11 MPa with less negative psi in exposed crown parts. On 30 March, psi was -1.60 +/- 0.06 MPa, while loss of conductivity reached up to 100%. Conductivity losses increased with exposition and were highest in the smallest tree. The observed complex within-tree patterns of psi and embolism were caused by ice blockages and differences in stress intensities within the xylem. High conductivity losses despite moderate psi in exposed crown parts indicated freeze-thaw events to be a major inducer of winter embolism. Tree size may play a critical role for winter water relations as trees profit from water stored in the stem and in crown parts below the snow cover.

Drought and frost resistance of trees: a comparison of four species at different sites and altitudes

ContextDrought and frost resistances are key factors for the survival and distribution of tree species.AimsIn this study, the vulnerability to drought-induced embolism and frost resistance of four species were analysed, whereby different sites and altitudes were compared and seasonal variation was considered.MethodsFagus sylvatica L., Sorbus aucuparia L., Picea abies L. Karst and Larix decidua Mill samples were harvested at high and low altitude sites in France and Austria, respectively, and sampling occurred in winter and summer. Pressure at 50% loss of conductivity (P50), specific hydraulic conductivity (ks) and temperature lethal for 50% of cells (LT50) were determined, and soluble carbohydrate and starch content were quantified.ResultsNo site-, altitude- or season-specific trend in P50 was observed, except for S. aucuparia, which showed P50 to decrease with altitude. Within regions, ks tended to decrease with altitudes. LT50 was between −48.4°C (winter) and −9.4°C (summer) and more negative in Tyrolean trees. Starch content was overall lower and carbohydrate content higher in winter than in summer, no site-specific or altitudinal trend was observed.ConclusionStudied species obviously differed in their strategies to withstand to frost and drought, so that site-related, altitudinal and seasonal patterns varied.

The hydraulic conductivity of the xylem in conifer needles (Picea abies and Pinus mugo)

Main resistances of the plant water transport system are situated in leaves. In contrast to angiosperm leaves, knowledge of conifer needle hydraulics and of the partitioning of resistances within needles is poor. A new technique was developed which enabled flow-meter measurements through needles embedded in paraffin and thus quantification of the specific hydraulic conductivity (K(s)) of the needle xylem. In Picea abies, xylem K(s) of needle and axes as well as in needles of different age were compared. In Pinus mugo, resistance partitioning within needles was estimated by measurements of xylem K(s) and leaf conductance (K(leaf), measured via rehydration kinetics). Mean K(s) in P. abies needles was 3.5×10(-4) m(2) s(-1) MPa(-1) with a decrease in older needles, and over all similar to K(s) of corresponding axes xylem. In needles of P. mugo, K(s) was 0.9×10(-4) m(2) s(-1) MPa(-1), and 24% of total needle resistance was situated in the xylem. The results indicate species-specific differences in the hydraulic efficiency of conifer needle xylem. The vascular section of the water transport system is a minor but relevant resistance in needles.

Monitoring of freezing dynamics in trees: a simple phase shift causes complexity.

Monitoring of freezing in trees via nondestructive methods revealed complex spatial and temporal freezing patterns that promote internal water shifts and cavitation events. During winter, trees have to cope with harsh conditions, including extreme freeze-thaw stress. This study focused on ice nucleation and propagation, related water shifts and xylem cavitation, as well as cell damage and was based on in situ monitoring of xylem (thermocouples) and surface temperatures (infrared imaging), ultrasonic emissions, and dendrometer analysis. Field experiments during late winter on Picea abies growing at the alpine timberline revealed three distinct freezing patterns: (1) from the top of the tree toward the base, (2) from thin branches toward the main stem’s top and base, and (3) from the base toward the top. Infrared imaging showed freezing within branches from their base toward distal parts. Such complex freezing causes dynamic and heterogenous patterns in water potential and probably in cavitation. This study highlights the interaction between environmental conditions upon freezing and thawing and demonstrates the enormous complexity of freezing processes in trees. Diameter shrinkage, which indicated water fluxes within the stem, and acoustic emission analysis, which indicated cavitation events near the ice front upon freezing, were both related to minimum temperature and, upon thawing, related to vapor pressure deficit and soil temperature. These complex patterns, emphasizing the common mechanisms between frost and drought stress, shed new light on winter tree physiology.