Showing papers in "Tree Physiology in 2011"

To what extent is altitudinal variation of functional traits driven by genetic adaptation in European oak and beech

Abstract: The phenotypic responses of functional traits in natural populations are driven by genetic diversity and phenotypic plasticity. These two mechanisms enable trees to cope with rapid climate change. We studied two European temperate tree species (sessile oak and European beech), focusing on (i) in situ variations of leaf functional traits (morphological and physiological) along two altitudinal gradients and (ii) the extent to which these variations were under environmental and/or genetic control using a common garden experiment. For all traits, altitudinal trends tended to be highly consistent between species and transects. For both species, leaf mass per area displayed a positive linear correlation with altitude, whereas leaf size was negatively correlated with altitude. We also observed a significant increase in leaf physiological performance with increasing altitude: populations at high altitudes had higher maximum rates of assimilation, stomatal conductance and leaf nitrogen content than those at low altitudes. In the common garden experiment, genetic differentiation between populations accounted for 0-28% of total phenotypic variation. However, only two traits (leaf mass per area and nitrogen content) exhibited a significant cline. The combination of in situ and common garden experiments used here made it possible to demonstrate, for both species, a weaker effect of genetic variation than of variations in natural conditions, suggesting a strong effect of the environment on leaf functional traits. Finally, we demonstrated that intrapopulation variability was systematically higher than interpopulation variability, whatever the functional trait considered, indicating a high potential capacity to adapt to climate change.

Genotypic variability and phenotypic plasticity of cavitation resistance in Fagus sylvatica L. across Europe.

Abstract: Xylem cavitation resistance is a key physiological trait correlated with species tolerance to extreme drought stresses. Little is known about the genetic variability and phenotypic plasticity of this trait in natural tree populations. Here we measured the cavitation resistance of 17 Fagus sylvatica populations representative of the full range of the species in Europe. The trees were grown in three field trials under contrasting climatic conditions. Our findings suggest that the genotypic variability of cavitation resistance is high between genotypes of a given population. By contrast, no significant differences were found for this trait across populations, the mean population cavitation resistance being remarkably constant in each trial. We found a significant site effect and a significant site × population interaction, suggesting that cavitation resistance has a high phenotypic plasticity and that this plasticity is under genetic control. The implications of our findings for beech forest management in a context of climate change are discussed.

Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens)

Abstract: Provenance-specific growth responses to experimentally applied drought and air warming were studied in saplings of three European oak species: Quercus robur, Quercus petraea and Quercus pubescens. Four provenances of each species were grown in large open-top chambers and subjected to four climates: control, periodic drought, air warming or their combination in 3 subsequent years. Overall growth responses were found among species and provenances, with drought reducing shoot height growth and stem diameter growth and air warming stimulating shoot height growth but reducing stem diameter growth and root length growth. Differential growth responses in shoots, stems and roots resulted in altered allometric growth relations. Root length growth to shoot height growth increased in response to drought but decreased in response to air warming. Stem diameter growth to shoot height growth decreased in response to air warming. The growth responses in shoots and stems were highly variable among provenances indicating provenance-specific sensitivity to drought and air warming, but this response variability did not reflect local adaptation to climate conditions of provenance origin. Shoot height growth was found to be more sensitive to drought in provenances from northern latitudes than in provenances from southern latitudes, suggesting that genetic factors related to the postglacial immigration history of European oaks might have interfered with selective pressure at provenance origins.

Root carbon reserve dynamics in aspen seedlings: does simulated drought induce reserve limitation?

Abstract: 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.

Nitrogen-fixing legume tree species for the reclamation of severely degraded lands in Brazil.

Abstract: The main challenges faced in the reclamation of severely degraded lands are in the management of the systems and finding plant species that will grow under the harsh conditions common in degraded soils. This is especially important in extremely adverse situations found in some substrates from mining activities or soils that have lost their upper horizons. Under these conditions, recolonization of the area by native vegetation through natural succession processes may be extremely limited. Once the main physical and chemical factors restrictive to plant growth are corrected or attenuated, the introduction of leguminous trees able to form symbioses with nodulating N 2 -fixing bacteria and arbuscular mycorrhizal fungi constitutes an efficient strategy to accelerate soil reclamation and initiate natural succession. These symbioses give the legume species a superior capacity to grow quickly in poor substrates and to withstand the harsh conditions presented in degraded soils. In this article we describe several successful results in Brazil using N 2 -fixing legume tree species for reclamation of areas degraded by soil erosion, construction and mining activities, emphasizing the potential of the technique to recover soil organic matter levels and restore ecosystem biodiversity and other environmental functions.

Elevated CO2 enhances leaf senescence during extreme drought in a temperate forest

Abstract: In 2007, an extreme drought and acute heat wave damaged ecosystems across the southeastern US, including a 19-year-old Liquidambar styraciflua L. (sweetgum) tree plantation exposed to long-term elevated CO2 treatments. Stem sap velocities in trees exposed to ambient (A) or elevated (E) CO2 were analyzed to assess potential interactions between CO2 and these weather extremes. Leaf temperature (Tleaf) and net carbon uptake (GPP) were modeled based on patterns of sap velocity to estimate indirect impacts of CO2-reduced transpiration on premature leaf senescence. Elevated CO2 reduced sap flow by 28% during early summer, and by up to 45% late in the drought during record-setting high air temperatures. Canopy transpiration and conductance declined more rapidly in ECO2 plots, resulting in ECO2 Tleaf up to 45 C, which was 1-2 C greater than ACO2 Tleaf. Pre-drought GPP was ~7% greater in ECO2 plots, then declined to 30% less than ACO2 GPP as the drought progressed. Leaf abscission peaked during this period, and was 30% greater for ECO2 trees. While ECO2 can reduce leaf-level water use under droughty conditions, acute drought or heat conditions may induce excessive stomatal closure that could offset benefits of ECO2 to temperate forest species during extreme weather events.

Effects of potassium supply on limitations of photosynthesis by mesophyll diffusion conductance in Carya cathayensis.

Abstract: Potassium (K) influences the photosynthesis process in a number of ways; however, the mechanisms underlying the photosynthetic response to differences in K supply are not well understood. Concurrent measurements of gas exchange and chlorophyll fluorescence were made to investigate the effect of K nutrition on photosynthetic efficiency and mesophyll conductance (g(m)) in hickory seedlings (Carya cathayensis Sarg.) in a greenhouse. The results show that leaf K concentrations < 0.7-0.8% appeared to limit the leaf net CO2 assimilation rate (A), and that the relative limitation of photosynthesis due to g(m) and stomatal conductance (g(s)) decreased with increasing supplies of K. However, a sensitivity analysis indicated that A was most sensitive to the maximum carboxylation rate of Rubisco (V(c,max)) and the maximum rate of electron transport (J(max)). These results indicate that the photosynthetic rate is primarily limited by the biochemical processes of photosynthesis (V(c,max) and J(max)), rather than by g(m) and g(s) in K-deficient plants. Additionally, g(m) was closely correlated with g(s) and the leaf dry mass per unit area (M(A)) in hickory seedlings, which indicates that decreased g(m) and g(s) may be a consequence of leaf anatomical adaptation.

Growth, physiological and molecular traits in Salicaceae trees investigated for phytoremediation of heavy metals and organics

Abstract: Worldwide, there are many large areas moderately contaminated with heavy metals and/or organics that have not been remediated due to the high cost and technical drawbacks of currently available technologies. Methods with a good potential for coping with these limitations are emerging from phytoremediation techniques, using, for example, specific amendments and/or plants selected from various candidates proven in several investigations to be reasonably efficient in extracting heavy metals from soil or water, or in co-metabolizing organics with bacteria flourishing or inoculated in their rhizospheres. Populus and Salix spp., two genera belonging to the Salicaceae family, include genotypes that can be considered among the candidates for this phytoremediation approach. This review shows the recent improvements in analytical tools based on the identification of useful genetic diversity associated with classical growth, physiological and biochemical traits, and the importance of plant genotype selection for enhancing phytoremediation efficiency. Particularly interesting are studies on the application of the phytoremediation of heavy metals and of chlorinated organics, in which microorganisms selected for their degradation capabilities were bioaugmented in the rhizosphere of Salicaceae planted at a high density for biomass and bioenergy production.

Temperature signals contribute to the timing of photoperiodic growth cessation and bud set in poplar.

Abstract: Bud set, the cornerstone delimiting the seasonal growth period in trees, is the dynamic net result of the often photoperiod-controlled growth cessation and the subsequent bud formation. Here, we show that in hybrid poplar, the critical day length for growth cessation and the duration of bud formation each vary with local climatic conditions in identical genotypes. The detailed dissection of bud set suggests temperature as one additional environmental factor that modifies the sensitivity to day-length signals at growth cessation and influences the duration of bud formation in poplar. The ability of perennial plants to integrate additional environmental signals with photoperiod signaling may add to short-term acclimatization to the predicted longer growing seasons in future climates.

Tree responses to drought

Abstract: With global climate change, drought may become more com-mon in the future (IPCC 2007). Several factors will promote more frequent droughts: earlier snowmelt, higher temperatures and higher variability in precipitation. For ecosystems where the water cycle is dominated by snowmelt, warmer tempera-tures bring earlier melt (Stewart et al. 2005) and longer, drier snow-free periods (Westerling et al. 2006). Higher tempera-tures will also increase vapor pressure deficit if evapotranspira-tion does not also increase. Greater variability in precipitation has two implications for plant water balance: longer periods without water, and less captured in the soil in the more intense storms.Many recent tree mortality events have been linked to drought (Allen et al. 2010), but how much do we really know about the drought responses of trees? Physiological responses of trees to low soil water potential vary, but have been fairly well characterized (McDowell et al. 2008)—they vary from concomitantly lower leaf water potential (anisohydric response) to maintenance of leaf water potential above a threshold (isohydric response) with various strategies in between. Many tree species shed leaves during a strong drought, reducing transpiration and photosynthesis, and adjust partitioning to roots and storage (McDowell et al. 2008). Drought also ‘weakens’ trees and makes them more susceptible to insect attacks and pathogens (McDowell et al. 2008). Growth can be reduced through impairment of cell division and cell expansion (Hsiao 197 3), which occurs at a lower water stress threshold than does photosynthetic inhibi-tion (Hsiao et al. 1976). In fact, a variety of physiological processes respond at different plant water potentials (Hsiao 1973, Ditmarova et al. 2010), so that the severity of the drought will influence the physiological response. Drought is also relative—species adapted to low precipitation climates can survive low soil water potentials that would kill or seriously wound trees in more mesic climates and trees in mesic cli-mates might suffer from ‘drought’ that would be normal to trees in xeric climates. Finally, we know that there are many mechanisms of drought tolerance, and that many of these involve coordination within the whole tree.Two papers in this issue try to untangle some of the complex and variable responses of trees to drought. Larcheveque et al. (2011 ) droughted two different hybrid poplar saplings (each with

The long way down—are carbon and oxygen isotope signals in the tree ring uncoupled from canopy physiological processes?

Abstract: The carbon (δ(13)C) and oxygen (δ(18)O) stable isotope composition is widely used to obtain information on the linkages between environmental drivers and tree physiology over various time scales. The tree-ring archive can especially be exploited to reconstruct inter- and intra-annual variation of both climate and physiology. There is, however, a lack of information on the processes potentially affecting δ(13)C and δ(18)O on their way from assimilation in the leaf to the tree ring. As a consequence, the aim of this study was to trace the isotope signals in European beech (Fagus sylvatica L.) from leaf water (δ(18)O) and leaf assimilates (δ(13)C and δ(18)O) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for δ(13)C and δ(18)O analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for δ(18)O analysis were taken approximately every 2 weeks during the growing season of 2007. The δ(13)C and δ(18)O samples from the tree rings were dated intra-annually by monitoring the tree growth with dendrometers. δ(18)O in the phloem organic matter and tree-ring whole wood was not positively related to leaf water evaporative enrichment and δ(18)O of canopy organic matter pools. This finding implies a partial uncoupling of the tree-ring oxygen isotopic signal from canopy physiology. At the same time, internal carbon storage and remobilization physiology most likely prevented δ(13)C in tree-ring whole wood from being closely related to intra-annual variation in environmental drivers. Taking into account the post-photosynthetic isotope fractionation processes resulting in alterations of δ(13)C and δ(18)O not only in the tree ring but also in phloem carbohydrates, as well as the intra-annual timing of changes in the tree internal physiology, might help to better understand the meaning of the tree-ring isotope signal not only intra- but also inter-annually.

Genome-scale transcriptome analysis of the desert poplar, Populus euphratica

Abstract: Populus euphratica is well-adapted to extreme desert environments and is an important model species for studying the effects of abiotic stresses on trees. Here we present the first deep transcriptomic analysis of this species. To maximize representation of conditional transcripts, mRNA was obtained from living tissues of desert-grown trees and two types of callus (salt-stressed and unstressed). De novo assembly generated 86,777 Unigenes using Solexa sequence data. These sequences covered 92% of previously reported P. euphratica expressed sequence tags (ESTs) and 90% of the TIGR poplar ESTs, and a total of 58,499 high-quality unique sequences were annotated by BLAST similarity searches against public databases. We found that 27% of the total Unigenes were differentially expressed (up- or down-regulated) in response to salt stress in P. euphratica callus. These differentially expressed genes are mainly involved in transport, transcription, cellular communication and metabolism. In addition, we found that numerous putative genes involved in ABA regulation and biosynthesis were also differentially regulated. This study represents the deepest transcriptomic and gene-annotation analysis of P. euphratica to date. The genetic knowledge acquired should be very useful for future studies of the molecular adaptation of this tree species to abiotic stress and facilitate genetic manipulation of other poplar species.

Hydraulic patterns and safety margins, from stem to stomata, in three eastern US tree species

Abstract: Adequate water transport is necessary to prevent stomatal closure and allow for photosynthesis. Dysfunction in the water transport pathway can result in stomatal closure, and can be deleterious to overall plant health and survival. Although much is known about small branch hydraulics, little is known about the coordination of leaf and stem hydraulic function. Additionally, the daily variations in leaf hydraulic conductance (K(leaf)), stomatal conductance and water potential (Ψ(L)) have only been measured for a few species. The objective of the current study was to characterize stem and leaf vulnerability to hydraulic dysfunction for three eastern U.S. tree species (Acer rubrum, Liriodendron tulipifera and Pinus virginiana) and to measure in situ daily patterns of K(leaf), leaf and stem Ψ, and stomatal conductance in the field. Sap flow measurements were made on two of the three species to compare patterns of whole-plant water use with changes in K(leaf) and stomatal conductance. Overall, stems were more resistant to hydraulic dysfunction than leaves. Stem P50 (Ψ resulting in 50% loss in conductivity) ranged from -3.0 to -4.2 MPa, whereas leaf P50 ranged from -0.8 to -1.7 MPa. Field Ψ(L) declined over the course of the day, but only P. virginiana experienced reductions in K(leaf) (nearly 100% loss). Stomatal conductance was greatest overall in P. virginiana, but peaked midmorning and then declined in all three species. Midday stem Ψ in all three species remained well above the threshold for embolism formation. The daily course of sap flux in P. virginiana was bell-shaped, whereas in A. rubrum sap flux peaked early in the morning and then declined over the remainder of the day. An analysis of our data and data for 39 other species suggest that there may be at least three distinct trajectories of relationships between maximum K(leaf) and the % K(leaf) at Ψ(min). In one group of species, a trade-off between maximum K(leaf) and % K(leaf) at Ψ(min) appeared to exist, but no trade-off was evident in the other two trajectories.

Hydraulic adjustments underlying drought resistance of Pinus halepensis

Abstract: Drought-induced tree mortality has increased over the last decades in forests around the globe. Our objective was to investigate under controlled conditions the hydraulic adjustments underlying the observed ability of Pinus halepensis to survive seasonal drought under semi-arid conditions. One hundred 18-month saplings were exposed in the greenhouse to 10 different drought treatments, simulating combinations of intensities (fraction of water supply relative to control) and durations (period with no water supply) for 30 weeks. Stomata closed at a leaf water potential (Ψl) of −2.8 MPa, suggesting isohydric stomatal regulation. In trees under extreme drought treatments, stomatal closure reduced CO2 uptake to −1 µmol m −2 s −1 , indicating the development of carbon starvation. A narrow hydraulic safety margin of 0.3 MPa (from stomatal closure to 50% loss of hydraulic conductivity) was observed, indicating a strategy of maximization of CO2 uptake in trees otherwise adapted to water stress. A differential effect of drought intensity and duration was observed, and was explained by a strong dependence of the water stress effect on the ratio of transpiration to evapotranspiration T/ET and the larger partitioning to transpiration associated with larger irrigation doses. Under intense or prolonged drought, the root system became the main target for biomass accumulation, taking up to 100% of the added biomass, while the stem tissue biomass decreased, associated with up to 60% reduction in xylem volume.

Associations between growth, wood anatomy, carbon isotope discrimination and mortality in a Quercus robur forest

Abstract: Observations of forest mortality are increasing globally, but relatively little is known regarding the underlying mechanisms driving these events. Tree rings carry physiological signatures that may be used as a tool for retrospective analyses. We capitalized on a local soil water drainage event in 1982 that resulted in increased mortality within a stand of oak trees (Quercus robur), to examine the underlying physiological patterns associated with survival and death in response to soil water limitations. Pre-dawn water potentials showed more negative values for trees in the process of dying compared with those that survived. We used tree rings formed over the 123 years prior to mortality to estimate productivity from basal area increment (BAI, mm 2 ), multiple xylem hydraulic parameters via anatomical measurements and crown-level gas exchange via carbon isotope discrimination (Δ, ‰). Oaks that died had significantly higher BAI values than trees that survived until the drainage event, after which the BAI of trees that died declined dramatically. Hydraulic diameter and conductivity of vessels in trees that died were higher than in surviving trees until the last 5 years prior to mortality, at which time both groups had similar values. Trees that died had consistently lower Δ values than trees that survived. Therefore, tree mortality in this stand was associated with physiological differences prior to the onset of soil water reduction. We propose that trees that died may have been hydraulically underbuilt for dry conditions, which predisposes them to severe hydraulic constraints and subsequent mortality. Measurements of above-ground/below-ground dry mass partitioning will be critical to future tests of this hypothesis. Based on these results, it is probable that pedunculate oak trees will experience greater future mortality if climate changes cause more severe droughts than the trees have experienced previously.

Different sensitivity of isoprene emission, respiration and photosynthesis to high growth temperature coupled with drought stress in black poplar (Populus nigra) saplings.

Abstract: The effects of the interaction between high growth temperatures and water stress on gas-exchange properties of Populus nigra saplings were investigated. Water stress was expressed as a function of soil water content (SWC) or fraction of transpirable soil water (FTSW). Isoprene emission and photosynthesis (A) did not acclimate in response to elevated temperature, whereas dark (R(n)) and light (R(d)) respiration underwent thermal acclimation. R(d) was ~30% lower than R(n) irrespective of growth temperature and water stress level. Water stress induced a sharp decline, but not a complete inhibition, of both R(n) and R(d). There was no significant effect of high growth temperature on the responses of A, stomatal conductance (g(s)), isoprene emission, R(n) or R(d) to FTSW. High growth temperature resulted in a significant increase in the SWC endpoint. Photosynthesis was limited mainly by CO(2) acquisition in water-stressed plants. Impaired carbon metabolism became apparent only at the FTSW endpoint. Photosynthesis was restored in about a week following rewatering, indicating transient biochemical limitations. The kinetics of isoprene emission in response to FTSW confirmed that water stress uncouples the emission of isoprene from A, isoprene emission being unaffected by decreasing g(s). The different kinetics of A, respiration and isoprene emission in response to the interaction between high temperature and water stress led to rising R(d)/A ratio and amount of carbon lost as isoprene. Since respiration and isoprene sensitivity are much lower than A sensitivity to water stress, temperature interactions with water stress may dominate poplar acclimatory capability and maintenance of carbon homeostasis under climate change scenarios. Furthermore, predicted temperature increases in arid environments may reduce the amount of soil water that can be extracted before plant gas exchange decreases, exacerbating the effects of water stress even if soil water availability is not directly affected.

How does P affect photosynthesis and metabolite profiles of Eucalyptus globulus

Abstract: Phosphorus (P) has multiple effects on plant metabolism, but there are many unresolved questions especially for evergreen trees. For example, we do not know the general effects of P on metabolism, or if P affects photosynthesis via the internal conductance to CO(2) transfer from sub-stomatal cavities to chloroplast or amounts of Rubisco. This study investigates how P deficiency affects seedlings of the evergreen tree Eucalyptus globulus grown for 2.5 months with four nutrient solutions differing in P concentration. To determine why photosynthesis was affected by P supply, Rubisco was quantified by capillary electrophoresis, internal conductance was quantified from gas exchange and carbon isotope discrimination, and biochemical parameters of photosynthesis were estimated from A/C(c) responses. Additional insights into the effect of P on metabolism were provided by gas chromatography-mass spectrometry (GC-MS) metabolite profiling. Larger concentrations of P in the nutrient solution led to significantly faster rates of photosynthesis. There was no evidence that stomatal or internal conductances contributed to the effect of P supply on photosynthesis. The increase in photosynthesis with P supply was correlated with V(cmax), and amounts of P, phosphate and fructose 6-phosphate (6-P). Phosphorous supply affected approximately one-third of the 90 aqueous metabolites quantified by GC-MS, but the effect size was generally smaller than reported for experiments on herbaceous species. Phosphorus deficiency decreased concentrations of phosphate, glucose 6-P and fructose 6-P more than it decreased photosynthesis, suggesting faster turnover of smaller pools of phosphate and phosphorylated intermediates. The effect of P supply on most amino acids was small, with the exception of arginine and glutamine, which increased dramatically under P deficiency. P deficiency had small or non-significant effects on carbohydrates and organic acids of the tricarboxylic acid (TCA) cycle. The small effect of P on carbohydrates, organic acids and (most) amino acids likely reflects a functional homeostasis among C metabolism (glycolysis, TCA and pentose P cycles), rates of photosynthesis and growth. The strong functional homeostasis in E. globulus may reflect a conservative, long-term growth and metabolic strategy of evergreen trees.

Photosynthesis down-regulation precedes carbohydrate accumulation under sink limitation in Citrus.

Abstract: Photosynthesis down-regulation due to an imbalance between sources and sinks in Citrus leaves could be mediated by excessive accumulation of carbohydrates. However, there is limited understanding of the physiological role of soluble and insoluble carbohydrates in photosynthesis regulation and the elements triggering the down-regulation process. In this work, the role of non-structural carbohydrates in the regulation of photosynthesis under a broad spectrum of source-sink relationships has been investigated in the Salustiana sweet orange. Soluble sugar and starch accumulation in leaves, induced by girdling experiments, did not induce down-regulation of the photosynthetic rate in the presence of sinks (fruits). The leaf-to-fruit ratio did not modulate photosynthesis but allocation of photoassimilates to the fruits. The lack of strong sink activity led to a decrease in the photosynthetic rate and starch accumulation in leaves. However, photosynthesis down-regulation due to an excess of total soluble sugars or starch was discarded because photosynthesis and stomatal conductance reduction occurred prior to any significant accumulation of these carbohydrates. Gas exchange and fluorescence parameters suggested biochemical limitations to photosynthesis. In addition, the expression of carbon metabolism-related genes was altered within 24 h when strong sinks were removed. Sucrose synthesis and export genes were inhibited, whereas the expression of ADP-glucose pyrophosphorylase was increased to cope with the excess of assimilates. In conclusion, changes in starch and soluble sugar turnover, but not sugar content per se, could provide the signal for photosynthesis regulation. In these conditions, non-stomatal limitations strongly inhibited the photosynthetic rate prior to any significant increase in carbohydrate levels.

Role of carbohydrate reserves in yield production of intensively cultivated oil olive (Olea europaea L.) trees.

Abstract: Olive (Olea europaea) has a very high tendency for year-to-year deviation in yield (alternate bearing), which has a negative economic impact on the olive oil industry. Among possible reasons for alternate bearing, depletion of stored carbohydrates (CHO) during the On-year (high yield) has often been mentioned. The objective of the present study was to verify the role of CHO reserves, as a cause or effect, in the alternate bearing of intensively cultivated olives. A monthly survey of soluble sugar and starch concentrations in the leaves, branches, bark and roots of On- and Off-trees (cv. Barnea) was carried out during a complete reproductive cycle from November 2005 to October 2006. Carbohydrate concentration in the sapwood was determined in January, as well as an estimate of whole-tree biomass. The trunk and limbs possess the largest portion of CHO reserves. The influence of reduced fruit load on CHO reserves was also investigated. Starch, mannitol and sucrose concentra tions increased from December to March in all tissues, and then declined along with fruit development. Leaves, branches and bark have a significant role in CHO storage, whereas roots accumulated the lowest CHO concentrations. However, fluctuations in reserve content suggested considerable involvement of roots in the CHO budget. Nevertheless, there were no meaningful differences in the annual pattern of CHO concentration between On- and Off-trees. Even a 75–100% reduction in fruit number brought about only a minor, sluggish increase in CHO content, though this was more pronounced in the roots. Carbohydrate reserves were not depleted, even under maximum demands for fruit and oil production. It is concluded that in olives, the status of CHO reserves is not a yield determinant. However, they may play a significant role in the olive’s survival strategy, ensuring tree recovery in the unpredictable semiarid Mediterranean environment. This suggests that CHO reserves in olive act like an active sink, challenging the common concept regarding the regulation of CHO reserves in plants.

Effects of environmental conditions on onset of xylem growth in Pinus sylvestris under drought

Abstract: We determined the influence of environmental factors (air and soil temperature, precipitation, photoperiod) on onset of xylem growth in Scots pine (Pinus sylvestris L.) within a dry inner Alpine valley (750 m a.s.l., Tyrol, Austria) by repeatedly sampling micro-cores throughout 2007-10 at two sites (xeric and dry-mesic) at the start of the growing season. Temperature sums were calculated in degree-days (DD) ≥5 °C from 1 January and 20 March, i.e., spring equinox, to account for photoperiodic control of release from winter dormancy. Threshold temperatures at which xylogenesis had a 0.5 probability of being active were calculated by logistic regression. Onset of xylem growth, which was not significantly different between the xeric and dry-mesic sites, ranged from mid-April in 2007 to early May in 2008. Among most study years, statistically significant differences (P<0.05) in onset of xylem growth were detected. Mean air temperature sums calculated from 1 January until onset of xylem growth were 230 ± 44 DD (mean ± standard deviation) at the xeric site and 205 ± 36 DD at the dry-mesic site. Temperature sums calculated from spring equinox until onset of xylem growth showed somewhat less variability during the 4-year study period, amounting to 144 ± 10 and 137 ± 12 DD at the xeric and dry-mesic sites, respectively. At both sites, xylem growth was active when daily minimum, mean and maximum air temperatures were 5.3, 10.1 and 16.2 °C, respectively. Soil temperature thresholds and DD until onset of xylem growth differed significantly between sites, indicating minor importance of root-zone temperature for onset of xylem growth. Although spring precipitation is known to limit radial growth in P. sylvestris exposed to a dry inner Alpine climate, the results of this study revealed that (i) a daily minimum air temperature threshold for onset of xylem growth in the range 5-6 °C exists and (ii) air temperature sum rather than precipitation or soil temperature triggers start of xylem growth. Based on these findings, we suggest that drought stress forces P. sylvestris to draw upon water reserves in the stem for enlargement of first tracheids after cambial resumption in spring.

Can forest trees compensate for stress-generated growth losses by induced production of volatile compounds?

Abstract: Plants produce a variety of volatile organic compounds (VOCs). Under abiotic and biotic stresses, the number and amount of produced compounds can increase. Due to their long life span and large size, trees can produce biogenic VOCs (BVOCs) in much higher amounts than many other plants. It has been suggested that at cellular and tree physiological levels, induced production of VOCs is aimed at improving plant resistance to damage by reactive oxygen species generated by multiple abiotic stresses. In the few reported cases when biosynthesis of plant volatiles is inhibited or enhanced, the observed response to stress can be attributed to plant volatiles. Reported increase, e.g., in photosynthesis has mostly ranged between 5 and 50%. A comprehensive model to explain similar induction of VOCs under multiple biotic stresses is not yet available. As a result of pathogen or herbivore attack on forest trees, the induced production of VOCs is localized to the damage site but systemic induction of emissions has also been detected. These volatiles can affect fungal pathogens and the arrival rate of herbivorous insects on damaged trees, but also act as signalling compounds to maintain the trophic cascades that may improve tree fitness by improved efficiency of herbivore natural enemies. On the forest scale, biotic induction of VOC synthesis and release leads to an amplified flow of BVOCs in atmospheric reactions, which in atmospheres rich in oxides of nitrogen (NOx) results in ozone formation, and in low NOx atmospheres results in oxidation of VOCs, removal in ozone from the troposphere and the resulting formation of biogenic secondary organic aerosol (SOA) particles. I will summarize recent advances in the understanding of stress-induced VOC emissions from trees, with special focus on Populus spp. Particular importance is given to the ecological and atmospheric feedback systems based on BVOCs and biogenic SOA formation.

Attack on all fronts: functional relationships between aerial and root parasitic plants and their woody hosts and consequences for ecosystems.

Abstract: This review discusses how understanding of functional relationships between parasitic plants and their woody hosts have benefited from a range of approaches to their study. Gross comparisons of nutrient content between infected and uninfected hosts, or parts of hosts, have been widely used to infer basic differences or similarities between hosts and parasites. Coupling of nutrient information with additional evidence of key processes such as transpiration, respiration and photosynthesis has helped elucidate host-parasite relationships and, in some cases, the anatomical nature of their connection and even the physiology of plants in general. For example, detailed analysis of xylem sap from hosts and parasites has increased our understanding of the spatial and temporal movement of solutes within plants. Tracer experiments using natural abundance or enriched application of stable isotopes ((15)N, (13)C, (18)O) have helped us to understand the extent and form of heterotrophy, including the effect of the parasite on growth and functioning of the host (and its converse) as well as environmental effects on the parasite. Nutritional studies of woody hosts and parasites have provided clues to the distribution of parasitic plants and their roles in ecosystems. This review also provides assessment of several corollaries to the host-parasite association.

Down-regulation of glycosyltransferase 8D genes in Populus trichocarpa caused reduced mechanical strength and xylan content in wood

Abstract: Members of glycosyltransferase protein families GT8, GT43 and GT47 are implicated in the biosynthesis of xylan in the secondary cell walls of Arabidopsis. The Arabidopsis mutant irx8 has a 60% reduction in xylan. However, over-expression of an ortholog of Arabidopsis IRX8, poplar PoGT8D, in Arabidopsis irx8 mutant could not restore xylan synthesis. The functions of tree GT8D genes remain unclear. We identified two GT8 gene homologs, PtrGT8D1 and PtrGT8D2, in Populus trichocarpa. They are the only two GT8D members and are abundantly and specifically expressed in the differentiating xylem of P. trichocarpa. PtrGT8D1 transcript abundance was >7 times that of PtrGT8D2. To elucidate the genetic function of GT8D in P. trichocarpa, the expression of PtrGT8D1 and PtrGT8D2 was simultaneously knocked down through RNAi. Four transgenic lines had 85–94% reduction in transcripts of PtrGT8D1 and PtrGT8D2, resulting in 29–36% reduction in stem wood xylan content. Xylan reduction had essentially no effect on cellulose quantity but caused an 11–25% increase in lignin. These transgenics exhibit a brittle wood phenotype, accompanied by increased vessel diameter and thinner fiber cell walls in stem xylem. Stem modulus of elasticity and modulus of rupture were reduced by 17–29% and 16–23%, respectively, and were positively correlated with xylan content but negatively correlated with lignin quantity. These results suggest that PtrGT8Ds play key roles in xylan biosynthesis in wood. Xylan may be a more important factor than lignin affecting the stiffness and fracture strength of wood.

Short-term effects of potassium fertilization on the hydraulic conductance of Laurus nobilis L.

Abstract: This study reports experimental evidence on the effect of short-term potassium fertilization on potassium uptake, tissue concentration and hydraulic conductance of pot-grown laurel plants. Potassium uptake and loading into the xylem of laurel seedlings increased within 24 h after fertilization. Potassium was not accumulated in roots and leaves, but the [K(+)] of xylem sap was 80% higher in fertilized plants (+K) than in potassium-starved plants (-K), as a likely result of recirculation between xylem and phloem. Increased xylem sap [K(+)] resulted in a 45% increase in transpiration rate, a 30% increase in plant hydraulic conductance (K(plant)) and a 120% increase in leaf-specific conductivity of the shoot (k(shoot)). We suggest that this increase was due to ion-mediated up-regulation of xylem hydraulics, possibly caused by the interaction of potassium ions with the pectic matrix of intervessel pits. The enhancement of hydraulic conductance following short-term potassium fertilization is a phenomenon that can be of advantage to plants for maintaining cell turgor, stomatal aperture and gas exchange rates under moderate drought stress. Our data provide additional support for the important role of potassium nutrition in agriculture and forestry.

Tree phenology responses to warming: spring forward, fall back?

Abstract: Some of the strongest evidence that organisms are already responding to climate change is the lengthening of the growing season at midto high latitudes (Menzel and Fabian 1999, Menzel et al. 2006, Ibáñez et al. 2010). Earlier spring bud burst and later autumn senescence have lengthened the growing season by ~11 days since the 1960s (Menzel and Fabian 1999). This response could help mitigate climate change if a longer growing season increases carbon sequestration, since this would provide a negative feedback on rising atmospheric CO2 concentrations by increasing the terrestrial carbon sink. However, while elevated temperatures can delay autumn senescence, temperature is an unreliable cue of when the sequence leading to dormancy should be initiated; late summer and early fall temperatures do not necessarily correlate with the timing of the first frost. Trees need to have completed a series of processes, including growth cessation, bud set and leaf senescence, before the first frost arrives or risk physical damage (see Figure 1). Thus, in many species, day length is used as a signal to begin growth cessation, the first step towards reaching winter dormancy (Wareing 1956). The importance of temperature and day length cues in determining the cessation of tree growth varies between species, with some groups thought to rely exclusively on photoperiod (Pauley and Perry 1954, Wareing 1956, Howe et al. 1996). This dependence on a day length signal has implications for forests in a warming world: while temperatures may increase over time, day length will remain unchanged, limiting the ability of photoperiod-controlled tree species to extend their growing season. In this issue, Rohde et al. (2011) show that, across a latitudinal range of field sites, temperature alters the timing of growth cessation in Populus, a genus where photoperiod alone is often thought to determine the end of the growing season (Pauley and Perry 1954, Howe et al. 1996, Bohlenius et al. 2006, Fracheboud et al. 2009). While warmer temperatures delayed the onset of growth cessation in hybrid poplar, so did cold nights, implying that there was an optimum temperature sum for rapid growth cessation (Rohde et al. 2011). Temperature also affected the time between growth cessation and bud set, with warmer temperatures hastening the process of bud development (Rohde et al. 2011), consistent with results from growth chamber experiments (Mölmann et al. 2005). This work joins other recent studies (Mölmann et al. 2005, Fracheboud et al. 2009, Tanino et al. 2010) that demonstrate that even in trees thought to be photoperiod controlled, temperature can modify the timing of key phenological events. Based on the findings of Rohde et al. (2011), rising temperatures would delay growth cessation in poplar, but accelerate bud development. Other studies in hybrid poplar have found that while treatments with warmer days can indeed delay growth cessation, some combinations of elevated day and night temperature treatments instead led to earlier growth cessation (Kalcsits et al. 2009). While it is therefore unclear whether climate warming will delay or accelerate growth cessation, these differences between studies indicate that the balance between changes in day and night temperature may be critical for predicting changes in tree phenology. Night temperatures appear to have a greater impact on growth cessation and bud set than day temperatures (Kalcsits et al. 2009, Tanino et al. 2010), and since increases in night temperatures are expected to be greater than those in day temperatures, climate warming may alter these processes more rapidly than experiments based on average temperature changes would predict. Temperature can also influence other late season phenological events, including senescence and dormancy. Higher temperatures slowed the speed of chlorophyll degradation during leaf senescence in Populus tremula (Fracheboud et al. 2009). In hybrid poplar, warm nights and low day–night temperature differences promoted deeper winter dormancy and cold hardiness (Kalcsits et al. 2009), implying that the temperature changes predicted by global climate models might enhance dormancy. Commentary

Conifer expansion reduces the competitive ability and herbivore defense of aspen by modifying light environment and soil chemistry

Abstract: Disturbance patterns strongly influence plant community structure. What remains less clear, particularly at a mechanistic level, is how changes in disturbance cycles alter successional outcomes in plant communities. There is evidence that fire suppression is resulting in longer fire return intervals in subalpine forests and that these lengthened intervals increase competitive interactions between aspen and conifer species. We conducted a field and greenhouse study to compare photosynthesis, growth and defense responses of quaking aspen and subalpine fir regeneration under light reductions and shifts in soil chemistry that occur as conifers increase in dominance. The studies demonstrated that aspen regeneration was substantially more sensitive to light and soil resource limitations than that of subalpine fir. For aspen, light reductions and/or shifts in soil chemistry limited height growth, biomass gain, photosynthesis and the production of defense compounds (phenolic glycosides and condensed tannins). Biomass gain and phenolic glycoside concentrations were co-limited by light reduction and changes in soil chemistry. In contrast, subalpine fir seedlings tended to be more tolerant of low light conditions and showed no sensitivity to changes in soil chemistry. Unlike aspen, subalpine fir increased its root to shoot ratio on conifer soils, which may partially explain its maintenance of growth and defense. The results suggest that increasing dominance of conifers in subalpine forests alters light conditions and soil chemistry in a way that places greater physiological and growth constraints on aspen than subalpine fir, with a likely outcome being more successful recruitment of conifers and losses in aspen cover.

Variation in soil water uptake and its effect on plant water status in Juglans regia L. during dry and wet seasons

Abstract: Temporal and spatial variations in the water status of walnut trees (Juglans regia L.) and the soil in which they were growing were traced by analyzing the differences in hydrogen isotopes during spring and summer in a 7-year-old walnut stand. Walnut root dynamics were measured in both dry and wet seasons. Walnut roots were mainly distributed in the upper soil (0–30 c m depth), with around 60% of the total root mass in upper soil layers and 40% in deep soil layers (30–80 c m depth). The upper soil layers contributed 68% of the total tree water requirement in the wet season, but only 47% in the dry season. In the wet season, total roots, living roots and new roots were all significantly more abundant than in the dry season. There were significant differences in pre-dawn branch percentage loss of hydraulic conductance (PLC), pre-dawn leaf water poten tial and transpiration between the dry and wet seasons. Water content in the upper soil layers remarkably influenced xylem water stable-hydrogen isotope (δD) values. Furthermore, there were linear relationships between the xylem water δD value and pre-dawn branch PLC, pre-dawn leaf water potential, transpiration rate and photosynthetic rate. In summary, J. regia was compelled to take a larger amount of water from the deep soil layers in the dry season, but this shift could not prevent water stress in the plant. The xylem water δD values could be used as an indicator to investigate the water stress of plants, besides probing profiles of soil water use. Keyword: branch PLC, gas exchange, Juglans regia, root distribution, stable hydrogen isotopes, water source.

A model of heat transfer in sapwood and implications for sap flux density measurements using thermal dissipation probes

Abstract: A variety of thermal approaches are used to estimate sap flux density in stems of woody plants. Models have proved valuable tools for interpreting the behavior of heat pulse, heat balance and heat field deformation techniques, but have seldom been used to describe heat transfer dynamics for the heat dissipation method. Therefore, to better understand the behavior of heat dissipation probes, a model was developed that takes into account the thermal properties of wood, the physical dimensions and thermal characteristics of the probes, and the conductive and convective heat transfer that occurs due to water flow in the sapwood. Probes were simulated as aluminum tubes 20 mm in length and 2 mm in diameter, whereas sapwood, heartwood and bark each had a density and water fraction that determined their thermal properties. Base simulations assumed a constant sap flux density with sapwood depth and no wounding or physical disruption of xylem beyond the 2 mm diameter hole drilled for probe installation. Simulations across a range of sap flux densities showed that the dimensionless quantity k [defined as (ΔT(m) -ΔT)/ΔT, where ΔT(m) is the temperature differential (ΔT) between the heated and unheated probe under zero-flow conditions] was dependent on the thermal conductivity of the sapwood. The relationship between sap flux density and k was also sensitive to radial gradients in sap flux density and to xylem disruption near the probe. Monte Carlo analysis in which 1000 simulations were conducted while simultaneously varying thermal conductivity and wound diameter revealed that sap flux density and k showed considerable departure from the original calibration equation used with this technique. The departure was greatest for variation in sap flux density typical of ring-porous species. Depending on the specific combination of thermal conductivity and wound diameter, use of the original calibration equation resulted in an 81% under- to 48% overestimation of sap flux density at modest flux rates. Future studies should verify these simulations and assess their utility in estimating sap flux density for this widely used technique.

Higher growth temperatures decreased net carbon assimilation and biomass accumulation of northern red oak seedlings near the southern limit of the species range

Abstract: If an increase in temperature will limit the growth of a species, it will be in the warmest portion of the species distribution. Therefore, in this study we examined the effects of elevated temperature on net carbon assimilation and biomass production of northern red oak (Quercus rubra L.) seedlings grown near the southern limit of the species distribution. Seedlings were grown in chambers in elevated CO2 (700 µmol mol −1 ) at three temperature conditions, ambient (tracking diurnal and seasonal variation in outdoor temperature), ambient +3 °C and ambient +6 °C, which produced mean growing season temperatures of 23, 26 and 29 °C, respectively. A group of seedlings was also grown in ambient [CO2] and ambient temperature as a check of the growth response to elevated [CO2]. Net photosynthesis and leaf respiration, photosynthetic capacity (Vcmax, Jmax and triose phosphate utilization (TPU)) and chlorophyll fluorescence, as well as seedling height, diameter and biomass, were mea sured during one growing season. Higher growth temperatures reduced net photosynthesis, increased respiration and reduced height, diameter and biomass production. Maximum net photosynthesis at saturating [CO2] and maximum rate of electron transport (Jmax) were lowest throughout the growing season in seedlings grown in the highest temperature regime. These parameters were also lower in June, but not in July or September, in seedlings grown at +3 °C above ambient, compared with those grown in ambient temperature, indicating no impairment of photosynthetic capacity with a moderate increase in air temperature. An unusual and potentially important observation was that foliar respiration did not acclimate to growth temperature, resulting in substantially higher leaf respiration at the higher growth temperatures. Lower net carbon assimilation was correlated with lower growth at higher temperatures. Total biomass at the end of the growing season decreased in direct proportion to the increase in growth temperature, declining by 6% per 1 °C increase in mean growing season temperature. Our observations suggest that increases in air temperature above current ambient conditions will be detrimental to Q. rubra seedlings growing near the southern limit of the species range.

Effects of calcium on seed germination, seedling growth and photosynthesis of six forest tree species under simulated acid rain

Abstract: We selected six tree species, Pinus massoniana Lamb., Cryptomeria fortunei Hooibr. ex Otto et Dietr., Cunninghamia lanceolata (Lamb.) Hook., Liquidambar formosana Hance, Pinus armandii Franch. and Castanopsis chinensis Hance, which are widely distributed as dominant species in the forest of southern China where acid deposition is becoming more and more serious in recent years. We investigated the effects and potential interactions between simulated acid rain (SiAR) and three calcium (Ca) levels on seed germination, radicle length, seedling growth, chlorophyll content, photosynthesis and Ca content in leaves of these six species. We found that the six species showed different responses to SiAR and different Ca levels. Pinus armandii and C. chinensis were very tolerant to SiAR, whereas the others were more sensitive. The results of significant SiAR × Ca interactions on different physiological parameters of the six species demonstrate that additional Ca had a dramatic rescue effect on the seed germination and seedling growth for the sensitive species under SiAR. Altogether, we conclude that the negative effects of SiAR on seed germination, seedling growth and photosynthesis of the four sensitive species could be ameliorated by Ca addition. In contrast, the physiological processes of the two tolerant species were much less affected by both SiAR and Ca treatments. This conclusion implies that the degree of forest decline caused by long-term acid deposition may be attributed not only to the sensitivity of tree species to acid deposition, but also to the Ca level in the soil.