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Little is known about the ability of trees to recover hydraulic conductance (k) within a growing season by regrowth or refilling of embolized conduits. Recovery of k lost to drought or other causes would prevent chronic reductions in gas exchange and productivity. To test recovery ability we conducted a 2-year experiment (2014-15) on a cohort of aspen ramets (Populus tremuloides, Michx.). Whole-tree k was measured from mid-June through September from sapflow (Q) and pre-dawn and mid-day xylem pressure. We induced embolism in the treatment group with high air pressure delivered by a split pressure chamber sealed around the main trunk. Successful treatments reduced k and Q by 50% or more without causing rapid desiccation. The majority of trees recovered following treatment, rising to control levels of k and Q between 12 and 17 days. Failure to recover was correlated with drier climate conditions. The growing-season recovery of k was attributed to refilling of embolized vessels, based on the absence of diameter growth. Pre-dawn xylem pressures during recovery were similar to the threshold needed to passively collapse emboli. Successful recovery during the 2-year study was consistent with no reduction in cumulative Q or canopy area in treatment vs controls. However, non-recovering trees in 2014 exhibited lower basal area growth at the start of the 2015 growing season, suggesting a linkage between recovery ability and productivity. This study provides evidence for the potential of trees to recover xylem function by refilling during the growing season. [ABSTRACT FROM AUTHOR]

The article focuses in a study conducted on a whole-tree model to analyze the link between leaf gas exchange, sugar utilization and soil water uptake and xylem and phloem fluxes. According to the study, the photosynthesis rate is maximized by the plant within the constraints imposed by meteorological conditions, soil water availability and phloem's sugar transport.

The occurrence of amphistomatic leaves (stomata on both surfaces) versus hypostomatic leaves (stomata limited to the lower or abaxial surface) has strong associations with environment. Amphistomy provides the advantage of higher conductance of CO2 for photosynthesis, however, unless the stomata on both leaf surfaces can be independently controlled in response to environmental cues, amphistomy may lead to inefficient gas exchange. While previous studies have found evidence that stomata can operate independently across and between surfaces of dorsiventral leaves, we investigate whether an independent stomatal response can be induced for isobilateral leaves by largely natural conditions. Here, we exposed surfaces of isobilateral, amphistomatic Eucalyptus globulus Labill. leaves to natural diurnal variation in differential evaporative demand, using leaf orientation to drive differences in irradiance and heat load on leaf surfaces. We identified preferential closure of stomata on the surface exposed to higher irradiation (and therefore evaporative demand) during the afternoon under natural conditions and similarly induced differential stomatal closure under experimental conditions in the laboratory. The differential response confirms that sufficient hydraulic isolation exists for independent stomatal response to occur between surfaces of amphistomatic, isobilateral leaves, and importantly, we show that natural conditions can induce surface-specific stomatal closure. [ABSTRACT FROM AUTHOR]

Recently, three types of cavitation: (i) expanding gradually; (ii) expanding-exploding, becoming a long-shaped bubble-lengthening by degrees; (iii) suddenly exploding and fully filling the conduit instantly, were proposed. Directed by this theory, experiments were performed using light microscopy to study the natural drying processes of xylem sections of Platycladus orientalis (L.) Franco. Three different phenomena of gas filling process in conduits were captured by replaying recorded videos. The first phenomenon is that a bubble emerging in a conduit expands and elongates gradually to fill the conduit. The second phenomenon is that a bubble emerging in a conduit expands gradually, and then suddenly becomes long-shaped, and extends continuously. The third phenomenon is that a bubble instantly fully fills a conduit. This paper suggests in these experiments that after losing the bulk water of a section, as the water stress of that section became more severe, the water pressures of different conduits of the section were not necessarily the same, and as time went on, the water pressures decreased constantly. Considering some practical factors, the three phenomena captured in our experiment are explained by our theory. [ABSTRACT FROM AUTHOR]

Boreal trees experience repeated freeze-thaw cycles annually. While freezing has been extensively studied in trees, the dynamic responses occurring during the freezing and thawing remain poorly understood. At freezing and thawing, rapid changes take place in the water relations of living cells in needles and in stem. While freezing is mostly limited to extracellular spaces, living cells dehydrate, shrink and their osmotic concentration increases. We studied how the freezing-thawing dynamics reflected on leaf gas exchange, chlorophyll fluorescence and xylem and living bark diameter changes of Scots pine (Pinus sylvestris L.) saplings in controlled experiments. Photosynthetic rate quickly declined following ice nucleation and extracellular freezing in xylem and needles, almost parallel to a rapid shrinking of xylem diameter, while that of living bark followed with a slightly longer delay. While xylem and living bark diameters responded well to decreasing temperature and water potential of ice, the relationship was less consistent in the case of increasing temperature. Xylem showed strong temporal swelling at thawing suggesting water movement from bark. After thawing xylem diameter recovered to a pre-freezing level but living bark remained shrunk. We found that freezing affected photosynthesis at multiple levels. The distinct dynamics of photosynthetic rate and stomatal conductance reveals that the decreased photosynthetic rate reflects impaired dark reactions rather than stomatal closure. Freezing also inhibited the capacity of the light reactions to dissipate excess energy as heat, via non-photochemical quenching, whereas photochemical quenching of excitation energy decreased gradually with temperature in agreement with the gas exchange data. [ABSTRACT FROM AUTHOR]

Xylem hydraulic conductance varies in response to changes in sap solute content, and in particular of potassium (K+) ion concentration. This phenomenon, known as the ‘ionic effect’, is enhanced in embolized stems, where it can compensate for cavitation-induced loss of hydraulic conductance. Previous studies have shown that in well-watered laurel plants (Laurus nobilis L.), potassium concentration of the xylem sap and plant hydraulic conductance increased 24 h after fertilization with KCl. The aim of this work was to test whether water-stressed laurel plants, grown under low potassium availability, could recover earlier from stress when irrigated with a KCl solution instead of potassium-free water. Two-year-old potted laurel seedlings were subjected to water stress by suspending irrigation until leaf conductance to water vapour (gL) dropped to ∼30% of its initial value and leaf water potential (ψL) reached the turgor loss point (ψTLP). Plants were then irrigated either with water or with 25 mM KCl and monitored for water status, gas exchange and plant hydraulics recovery at 3, 6 and 24 h after irrigation. No significant differences were found between the two experimental groups in terms of ψL, gL, plant transpiration, plant hydraulic conductance or leaf-specific shoot hydraulic conductivity. Analysis of xylem sap potassium concentration showed that there were no significant differences between treatments, and potassium levels were similar to those of potassium-starved but well-watered plants. In conclusion, potassium uptake from the soil solution and/or potassium release to the xylem appeared to be impaired in water-stressed plants, at least up to 24 h after relief from water stress, so that fertilization after the onset of stress did not result in any short-term advantage for recovery from drought. [ABSTRACT FROM AUTHOR]

Woody species hydraulically vulnerable to xylem cavitation may experience daily xylem embolism. How such species cope with the possibility of accumulated embolism is unclear. In this study, we examined seven temperate woody species to assess the hypothesis that low cavitation resistance (high vulnerability to cavitation) is compensated by high recovery performance via vessel refilling. We also evaluated leaf functional and xylem structural traits. The xylem recovery index (XRI), defined as the ratio of xylem hydraulic conductivity in plants rewatered after soil drought to that in plants under moist conditions, varied among species. The xylem water potential causing 50% loss of hydraulic conductivity (Ψ50) varied among the species studied, whereas only a slight difference was detected with respect to midday xylem water potential (Ψmin), indicating smaller hydraulic safety margins (Ψmin - Ψ50) for species more vulnerable to cavitation. Cavitation resistance (|Ψ50|) was negatively correlated with XRI across species, with cavitation-vulnerable species showing a higher performance in xylem recovery. Wood density was positively correlated with cavitation resistance and was negatively correlated with XRI. These novel results reveal that coordination exists between cavitation resistance and xylem recovery performance, in association with wood functional traits such as denser wood for cavitation-resistant xylem and less-dense but water-storable wood for refillable xylem. These findings provide insights into long-term maintenance of water transport in tree species growing under variable environmental conditions. [ABSTRACT FROM AUTHOR]

Predawn leaf water potential (LWP), the LWP between 09:00 and 10:30 h (termed minimum LWP), stem xylem hydraulic conductivity, foliar nitrogen, leaf gas exchange and leaf traits were measured on the same days in adults and 1-year-old to 7-year-old resprouts that had regrown after removing all the aboveground shoots. Height growth and accumulation of aboveground biomass quickly decreased with resprout age and there was no difference between 7-year-old resprouts and the uncut adults. Predawn LWP showed no significant difference between resprouts and adults, but the minimum LWP decreased gradually from −2.0 MPa in 1-year-old resprouts to −3.0 MPa in 7-year-old resprouts. The decrease in minimum LWP was associated with increased hydraulic resistance, as indicated by the gradual decrease in leaf area-specific hydraulic conductivity (KL) and sapwood area-specific hydraulic conductivity (KS) and the associated increase in stem native percentage loss of hydraulic conductivity in older than 2-year-old resprouts. The leaf nitrogen content per unit area (Narea) also decreased steadily from 3.6 g m−2 in 1-year-old resprouts to 1.7 g m−2 in 7-year-old resprouts. With the decline in LWP and Narea, the rate of leaf photosynthesis per unit area (Aarea) decreased from 20 μ mol m−2 s−1 in 1-year-old resprouts to 11 μ mol m−2 s−1 in 7-year-old resprouts. In adults, although KS decreased further compared with 7-year-old resprouts, the minimum LWP, KL, Narea and the rate of photosynthesis increased by 0.3 MPa, 29, 34 and 23%, respectively. The results show that a progressive loss of stem hydraulic conductivity and a steady decrease in foliar nitrogen with age were associated with a decrease in the photosynthetic rate of Caragana korshinskii Kom. resprouts, possibly changing the allocation of photosynthetic assimilates and slowing resprout height growth. [ABSTRACT FROM AUTHOR]

In a greenhouse study we quantified the gradual change of gas exchange, water relations and root reserves of aspen (Populus tremuloides Michx.) seedlings growing over a 3-month period of severe water stress. The aim of the study was to quantify the complex interrelationship between growth, water and gas exchange, and root carbon (C) dynamics. Various growth, gas exchange and water relations variables in combination with root reserves were measured periodically on seedlings that had been exposed to a continuous drought treatment over a 12-week period and compared with well-watered seedlings. Although gas exchange and water relations parameters significantly decreased over the drought period in aspen seedlings, root reserves did not mirror this trend. During the course of the experiment roots of aspen seedlings growing under severe water stress showed a two orders of magnitude increase in sugar and starch content, and roots of these seedlings contained more starch relative to sugar than those in non-droughted seedlings. Drought resulted in a switch from growth to root reserves storage which indicates a close interrelationship between growth and physiological variables and the accumulation of root carbohydrate reserves. Although a severe 3-month drought period created physiological symptoms of C limitation, there was no indication of a depletion of root C reserve in aspen seedlings. [ABSTRACT FROM AUTHOR]

Six Populus deltoides Bartr. ex Marsh. × P. nigra L. genotypes were selected to investigate whether stem xylem anatomy correlated with gas exchange rates, water-use efficiency (WUE) and growth performance. Clonal copies of the genotypes were grown in a two-plot common garden test under contrasting water regimes, with one plot maintained irrigated and the other one subjected to moderate summer water deficit. The six genotypes displayed a large range of xylem anatomy, mean vessel and fibre diameter varying from about 40 to 60 μm and from 7.5 to 10.5 μm, respectively. Decreased water availability resulted in a reduced cell size and an important rise in vessel density, but the extent of xylem plasticity was both genotype and trait dependent. Vessel diameter and theoretical xylem-specific hydraulic conductivity correlated positively with stomatal conductance, carbon isotope discrimination and growth performance-related traits and negatively with intrinsic WUE, especially under water deficit conditions. Vessel diameter and vessel density measured under water deficit conditions correlated with the relative losses in biomass production in response to water deprivation; this resulted from the fact that a more plastic xylem structure was generally accompanied by a larger loss in biomass production. [ABSTRACT FROM PUBLISHER]

We studied relationships among branch hydraulic conductivity, xylem embolism, stomatal conductance (gs), foliar nitrogen (N) concentration and specific leaf area (SLA) of seven tree species growing at four temperate woodland sites spanning a 464–1350 mm rainfall gradient. Specifically, we examined the question: are gs and foliar N concentration coordinated with branch hydraulic conductivity and, if so, what are the implications for carbon assimilation? Area-based, light-saturated photosynthetic rate (Aa) was uniquely and positively correlated with gs and foliar N concentration. Multiple regression analyses showed that, when variability in SLA was controlled for, the (positive) partial slope for each predictor remained significant. In contrast, there was a negative correlation between gs and foliar N concentration such that, for any given Aa, leaves with a high gs allocated less N to foliage than leaves with a low gs. Foliar N concentration was negatively correlated with branch hydraulic conductivity, whereas gs was positively correlated with branch hydraulic conductivity. These relationships were also significant when variability in leaf area to sapwood area ratio, gs and SLA were controlled for in a multiple regression, suggesting that the relationships were unique and independent of other confounding factors. Trees with low water transport capacity were able to support a high Aa by increasing investment in foliar N. Resource substitution occurred such that there was a trade-off between gs and foliar N in relation to branch hydraulic conductivity. High Aa could be sustained through either a high branch hydraulic conductivity and hence high gs and a low allocation to foliar N, or the effect of a low branch hydraulic conductivity and hence low gs could be offset by a high allocation to foliar N. The results are discussed in relation to mechanisms for minimizing the negative effects of limited water availability on carbon gain. [ABSTRACT FROM AUTHOR]