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Elucidating the genetic factors underlying long-term biological processes remains challenging since the relevant genes and their effects may vary across different developmental stages. In this study, we carried out a large-scale field trial of the progeny of an F1 full-sib pedigree of Salix suchowensis and measured plant height and ground diameter periodically over a time course of 240 days. With the obtained data, we characterized plant growth rhythms and performed time-sequential analyses of quantitative trait loci underlying the dynamic growth of the plants. The dynamic mapping of quantitative trait loci revealed that stem height and ground diameter were under the control of four quantitative trait loci, and the effects of these quantitative trait loci varied greatly throughout the growth process, in which two quantitative trait loci were found to exert a pleiotropic effect determining the correlation between stem height and ground diameter. The analysis of candidate genes in the target genetic intervals showed that the pleiotropic effect of the two quantitative trait loci arises from the colocalization of genes with independent effects on stem height and ground diameter. Further examination of the expression patterns of the candidate genes indicated that height and circumference growth involve different activities of leaf and cambium tissues. This study provides unprecedented information to help us understand the dynamic growth of plants and presents an applicable strategy for elucidating the genetic mechanism underlying a long-term biological process by using plant growth as an example. [ABSTRACT FROM AUTHOR]

Bark chloroplasts play important roles in carbon balancing by recycling internal stem CO2 into assimilated carbon. The photosynthetic response of bark chloroplasts to interior stem environments has been studied recently in woody plants. However, the molecular regulatory mechanisms underlying specific characteristics of bark photosynthesis remain unclear. To address this knowledge gap, differences in the structure, photosynthetic activity and protein expression profiles between bark and leaf chloroplasts were investigated in Salix matsudana in this study. Bark chloroplasts exhibited broader and lower grana stacks and higher levels of starch relative to leaf chloroplasts. Concomitantly, decreased oxygen evolution rates and decreased saturated radiation point were observed in bark chloroplasts. Furthermore, a total of 293 differentially expressed proteins (DEPs) were identified in bark and leaf chloroplast profile comparisons. These DEPs were significantly enriched in photosynthesis-related biological processes or Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with photosynthesis. All 116 DEPs within the KEGG pathways associated with photosynthesis light reactions were downregulated in bark chloroplasts, including key proteins responsible for chlorophyll synthesis, light energy harvesting, nonphotochemical quenching, linear electron transport and photophosphorylation. Interestingly, seven upregulated proteins involved in dark reactions were identified in bark chloroplasts that comprised two kinds of malic enzymes typical of C4-type photosynthesis. These results provide comprehensive proteomic evidence to understand the low photochemical capability of bark chloroplasts and suggest that bark chloroplasts might fix CO2 derived from malate decarboxylation. [ABSTRACT FROM AUTHOR]

Salix matsudana Koidz is a low cadmium (Cd)-accumulating willow, whereas its cultivated variety, Salix matsudana var. matsudana f. umbraculifera Rehd. is a high Cd-accumulating and tolerant willow (HCW). The physiological and molecular mechanisms underlying differential Cd accumulation and tolerance in the two Salix species are poorly understood. Here, we confirmed that the differential Cd translocation capacity from roots to the shoots leads to the differential Cd accumulation in their aboveground parts between these two willow genotypes. Cadmium accumulation happens preferentially in the transport pathway, and Cd is mainly located in the vacuolar, cell wall and intercellular space in HCW bark by cadmium location analysis at tissue and subcellular levels. Comparative transcriptome analysis revealed that higher expressions of several metal transporter genes (ATP-binding cassette transporters, K+ transporters/channels, yellow stripe-like proteins, zinc-regulated transporter/iron-regulated transporter-like proteins, etc.) are involved in root uptake and translocation capacity in HCW; meanwhile, ascorbate–glutathione metabolic pathways play essential roles in Cd detoxification and higher tolerance of the Cd-accumulator HCW. These results lay the foundation for further understanding the molecular mechanisms of Cd accumulation in woody plants and provide new insights into molecular-assisted-screening woody plant varieties for phytoremediation. [ABSTRACT FROM AUTHOR]

Globally, soil salinization is becoming increasingly prevalent, due to local hydrogeologic phenomena, climate change and anthropogenic activities. This has significantly curtailed current world food production and limits future production potential. In the prairie region of North America, sulfate salts, rather than sodium chloride, are often the predominant cause of soil degradation. In order to amend soil quality, revegetate salt-affected sites and recover economic loss associated with soil salinization, the establishment of short-rotation coppice plantations with willows (Salix spp.) has been suggested as a possible solution. To screen for the best candidates for such an application, 20 hybrid and 16 native willow genotypes were treated with three different salt conditions for 3 months. The treatments were designed to reflect the salt composition and concentrations on North American prairies. Under moderate salinity treatment (7 dS m−1), hybrid willows had better growth, as they established quickly while managing salt transport and mineral nutrition balance. However, native willows showed higher potential for long-term survival under severe salinity treatment (14 dS m−1), showing a lower sodium:potassium ratio in roots and better photosynthetic performance. Two native willow genotypes with high osmotic and salinity tolerance indices, specifically LAR-10 and MJW-9, are expected to show superior potential for remediating salt-affected sites. In addition, we observed significantly higher sulfate/sulfur concentrations in both leaf and root tissues in response to the severe salinity treatment, shedding light on the effect of sulfate salinity on sulfate uptake, and potentially sulfur metabolism in plants. [ABSTRACT FROM AUTHOR]

There are increasing concerns about trace metal levels such as copper (Cu) in industrial sites and the broader environment. Different studies have highlighted the role of mycorrhizal associations in plant tolerance to trace metals, modulating some of the plant metabolic and physiological responses. In this study, we investigated the role of the symbiotic association between Rhizophagus irregularis and Salix purpurea L. in modulating plant responses under Cu stress. We measured Cu accumulation, oxidative stress-related, photosynthetic-related and hydraulic traits, for non-inoculated (non-arbuscular mycorrhizal fungi) and inoculated saplings exposed to different Cu concentrations. We found that S. purpurea is a suitable option for phytoremediation of Cu, acting as a phytostabilizer of this trace metal in its root system. We observed that the symbiotic association modulates a broad spectrum of metabolic and physiological responses in S. purpurea under Cu conditions, including (i) a reduction in gas exchange associated with chlorophyll content changes and (ii) the sequestration of Cu into the cell walls, modifying vessels anatomy and impacting leaf specific conductivity (KL) and root hydraulic conductance (LP). Upholding KL and LP under Cu stress might be related to a dynamic Aquaporin gene regulation of PIP1;2 along with an up-regulation of TIP2;2 in the roots of inoculated S. purpurea. [ABSTRACT FROM AUTHOR]

The authors discuss the role played by the arbuscular mycorrhizal fungi (AMF) to the tolerance of plants to soil copper (Cu) contamination. Topics discussed include the significance of Cu as micronutrient for plants, the response of plants to Cu stress such as cell differentiation, plant growth, and physiological processes, and the accumulation of Cu in willows.

Willows are highly diverse catkin-bearing trees and shrubs of the genus Salix. They occur in many growth forms, from tall trees to creeping alpines, and successfully occupy a wide variety of ecological niches. Shrubby willows (sub-genus Vetrix) have many characteristics that render them suited to cultivation in much faster growth cycles than conventional forestry. They respond well to coppicing, can be propagated vegetatively as cuttings and achieve rapid growth with low fertilizer inputs. As a result, willows grown as short rotation coppice are now among the leading commercially grown biomass crops in temperate regions. However, although willows have a long history of cultivation for traditional uses, their industrial use is relatively recent and, compared with major arable crops, they are largely undomesticated. Breeding programmes initiated to improve willow as a biomass crop achieved a doubling of yields within a period of <15 years. These advances were made by selecting for stem characteristics (height and diameter) and coppicing response (shoot number and shoot vigour), as well as resistance to pests, diseases and environmental stress, with little or no knowledge of the genetic basis of these traits. Genetics and genomics, combined with extensive phenotyping, have substantially improved our understanding of the basis of biomass traits in willow for more targeted breeding via marker-assisted selection. Here, we present the strategy we have adopted in which a genetic-based approach was used to dissect complex traits into more defined components for molecular breeding and gene discovery. [ABSTRACT FROM AUTHOR]

An Agrobacterium tumefaciens-mediated transformation method was developed for Salix matsudana Koidz. using mature seeds as starting material. Multiple shoots were induced directly from embryonic shoot apices of germinating seeds. Although thidiazuron, 6-benzylaminopurine and zeatin induced multiple shoot induction with high frequency, zeatin (4.5 μM) was more effective for elongation of shoots and roots. The binary vector pCAMBIA1303, which contained neomycin phosphotransferase as a selectable marker gene and β-glucuronidase as a reporter gene, was used for transformation. Factors affecting transformation efficiency were examined for optimization of the procedure. Up to 35 of 180 seeds regenerated kanamycin-resistant shoots under optimal transformation conditions as follows: seeds were precultured for 4 days, apices of embryonic shoots were removed and infected with A. tumefaciens strain LBA4404 grown to a cell density equivalent (OD600) of 0.6, and then the infected explants were cultivated at 21 °C for 4 days. Storage of seeds at –20 °C for as long as 3 years had no significant effect on the induction of kanamycin-resistant shoots. Using this method, transgenic plants were obtained within ∼5 months with a transformation frequency of 7.2%. Analysis by polymerase chain reaction (PCR) showed that 36.4–93.8% of plants from all 13 tested kanamycin-resistant lines were PCR positive. Several ‘escapes’ were eliminated by a second round of selection. PCR, Southern blot and reverse transcriptase–PCR analyses of selected transgenic individuals 2 years after cutting propagation confirmed the successful generation of stable transformants. Our method, which minimizes the duration of axenic culture, may provide an alternative procedure for transformation of other recalcitrant Salix species. [ABSTRACT FROM PUBLISHER]

Carefully managed tree plantations offer an opportunity for sustainable biomass production. In recent years, the responses of the Salicaceae to environmental constraints have increasingly been investigated at different levels of biological integration, giving rise to a physiological approach to the function of trees in environmental restoration and monitoring. Significant progress has been achieved by the poplar and willow community in understanding targeted characteristics of complex tree stress responses. The Fifth International Poplar Symposium brought together experts in this area, with the main objective being to improve, coordinate and communicate existing national research on the biological and environmental dimension of multifunctional poplar and willow plantations. The secondary objective was to develop a network of research scientists and extension workers to provide scientific support for subjects interested in using fast-growing poplar and willow species for tree-related environmental projects. The ultimate goal was to build up services for the multipurpose tree plantation network on local-level management in order to obtain maximized benefits from tree crops. The purpose was also to maximize the synergy between local knowledge and global-level processes that require information on multipurpose tree crop production. [ABSTRACT FROM PUBLISHER]

Many willow species (genus: Salix) co-occur within habitats (α-diversity) and across the landscape (β-diversity) throughout North America. This high diversity is challenging to explain because closely related species often share similar functional traits and thus experience heightened competition and shared pest and pathogen susceptibility. To investigate whether traits related to drought survival are important in maintaining diversity, we conducted an experimental dry-down on six willow species in a greenhouse. We compared species' growth rates, stem and leaf hydraulics, leaf function and dieback and examined potential associations between their drought responses and habitat affinities. Habitat affinities were characterized based on species occurrence in randomly established field plots in central Minnesota. Overall, species that occur in drier, more seasonally variable habitats tended to have higher water-use efficiency, and faster growth rates than species from wetter habitats. However, the greatest difference in drought survival strategies was found between two species with similar habitat affinities. We conclude that differences in willow species could be important in both driving habitat differentiation and permitting temporal differentiation in resource utilization within habitats. Therefore, species' water-use strategies could be important in maintaining both α- and β-diversity across the landscape. [ABSTRACT FROM AUTHOR]

Six commercial willow (Salix spp.) varieties were examined to investigate the effects of genotype and environment on spring and autumn phenology and the relationships between phenology, shoot growth and leaf nitrogen (N) retranslocation. The willows were field-grown under different irrigation and fertilization in central Sweden. Two independent data sets of bud-burst, leaf unfolding duration, growth cessation and the timing of leaf abscission were assessed, and the biomass and leaf N data from the end of the first cutting cycle were used. Specific hypotheses were that (1) spring phenology has a greater effect on the shoot biomass production than autumn phenology; (2) later bud-burst is associated with more rapid leaf unfolding; (3) the timing of leaf abscission has a greater effect on the shoot biomass production than height growth cessation; and (4) later leaf fall is associated with poorer leaf N retranslocation. Bud-burst date varied by 19 and 39 days in the 2 years and leaf unfolding duration varied by 13 and 38 days. Growth cessation varied by 2.5 weeks and completion of leaf abscission (> 90% of leaves shed) by more than 3 weeks between the genotypes and treatments. Bud-burst date was inversely correlated with leaf unfolding duration (R2 = 0.96). Significant effects of the duration of leafy period (bud-burst to leaf abscission) and bud-burst date on shoot growth were found. Delayed growth cessation and leaf abscission were generally associated with a greater biomass production, but especially the relationship between growth cessation and biomass was weak. The results show that the timing of bud-burst and leaf abscission is more important for willow biomass production than growth cessation. Delayed leaf abscission has a negative effect on leaf N retranslocation and increases the N losses. The results have implications for the breeding of perennial energy crops. [ABSTRACT FROM PUBLISHER]

Using the non-invasively ion-selective microelectrode technique, flux profiles of K+, Na+ and H+ in mature roots and apical regions, and the effects of Ca2+ on ion fluxes were investigated in salt-tolerant poplar species, Populus euphratica Oliver and salt-sensitive Populus simonii × (P. pyramidalis + Salix matsudana) (Populus popularis 35-44, P. popularis). Compared to P. popularis, P. euphratica roots exhibited a greater capacity to retain K+ after exposure to a salt shock (SS, 100 mM NaCl) and a long-term (LT) salinity (50 mM NaCl, 3 weeks). Salt shock-induced K+ efflux in the two species was markedly restricted by K+ channel blocker, tetraethylammonium chloride, but enhanced by sodium orthovanadate, the inhibitor of plasma membrane (PM) H+-ATPase, suggesting that the K+ efflux is mediated by depolarization-activated (DA) channels, e.g., KORCs (outward rectifying K+ channels) and NSCCs (non-selective cation channels). Populus euphratica roots were more effective to exclude Na+ than P. popularis in an LT experiment, resulting from the Na+/H+ antiport across the PM. Moreover, pharmacological evidence implies that the greater ability to control K+/Na+ homeostasis in salinized P. euphratica roots is associated with the higher H+-pumping activity, which provides an electrochemical H+ gradient for Na+/H+ exchange and simultaneously decreases the NaCl-induced depolarization of PM, thus reducing Na+ influx via NSCCs and K+ efflux through DA-KORCs and DA-NSCCs. Ca2+ application markedly limited salt-induced K+ efflux but enhanced the apparent Na+ efflux, thus enabling the two species, especially the salt-sensitive poplar, to retain K+/Na+ homeostasis in roots exposed to prolonged NaCl treatment. [ABSTRACT FROM PUBLISHER]

Growth and water-use parameters of four willow (Salix spp.) clones grown in a moderate drought regime or with ample water supply were determined to characterize their water-use efficiency, drought resistance and capacity for drought acclimation. At the end of the 10-week, outdoor pot experiment, clonal differences were observed in: (1) water-use efficiency of aboveground biomass production (WUE); (2) resistance to xylem cavitation; and (3) stomatal conductance to leaf-specific, whole-plant hydraulic conductance ratio (gst/KP; an indicator of water balance). Across clones and regimes, WUE was positively correlated with the assimilation rate to stomatal conductance ratio (A/gst), a measure of instantaneous water-use efficiency. Both of these water-use efficiency indicators were generally higher in drought-treated trees compared with well-watered trees. However, the between-treatment differences in (shoot-based) WUE were smaller than expected, considering the differences in A/gst for two of the clones, possibly because plants reallocated dry mass from shoots to roots when subject to drought. Higher root hydraulic conductance to shoot hydraulic conductance ratios (KR/KS) during drought supports this hypothesis. The same clones were also the most sensitive to xylem cavitation and, accordingly, showed the strongest reduction in gst/KP in response to drought. Drought acclimation was manifested in decreased gst, gst/KP, osmotic potential and leaf area to vessel internal cross-sectional area ratio, and increased KR, KP and WUE. Increased resistance to stem xylem cavitation in response to drought was observed in only one clone. It is concluded that WUE and drought resistance traits are inter-linked and that both may be enhanced by selection and breeding. [ABSTRACT FROM AUTHOR]

Pot experiments are often performed to assess plant physiological traits and relationships among growth traits under controlled environments. However, the reliability of pot studies for predicting the growth and performance of trees in the field has rarely been rigorously assessed. We evaluated the suitability of pot experiments for predicting field performance, measured as shoot biomass production, by investigating determinants of growth in hybrid willows (Salix spp.) grown under various environmental conditions in the field, and by comparing the data with the results from a corresponding pot study. Biomass production in six hybrid willow clones, bred for use as bio-fuels, was assessed in three field trials located in central and southeastern Sweden throughout the first 3-year cutting cycle. The determinants of biomass productivity, measured as biomass allocation and nitrogen (N) economy, were identified in one of the field trials. Key traits for shoot biomass production in the field were total leaf area and total amount of N; plant N losses by shed leaves were only partly controlled by leaf-litter N concentration. These key traits were also obtained from the pot study and related to shoot biomass production and abscission-leaf N loss in the field. Total leaf area and total N pool of plants grown in pot experiments were good predictors of long-term biomass production in the field, whereas shoot biomass production, specific leaf area and tissue N concentration of pot-grown plants were less suitable as predictors of field performance. Relationships between the key traits and shoot biomass production were clone-specific, indicating the need for analysis of growth traits at the clone level if field performance of trees is to be evaluated based on data from pot studies. Nutrient loss components are important for tree performance in the long term and evaluations of nutrient loss characteristics at the individual-tree level should focus on nutrient pools lost rather than on nutrient concentrations in abscised plant parts. [ABSTRACT FROM AUTHOR]

An increased risk of frost is expected during the growing season, as climate warming increases spring temperatures in the Arctic. Because deciduous species have a growth season limited in length and also have generally larger conduit volumes, they are more likely than evergreens to be injured by freeze–thaw-induced cavitation during the growing season. To test whether growth at elevated temperature increases susceptibility to freeze–thaw damage, we grew a deciduous arctic shrub species (Salix pulchra Cham.) in simulated Alaskan summer temperatures and at 5 °C above the ambient simulation (+5 °C plants) in controlled environments. Stem specific hydraulic conductivity (ks) and leaf stomatal conductance (gs) were measured in plants grown at both temperatures before and after a freeze treatment simulating a mid-season frost. Before the freeze treatment, specific xylem conductivity was 2.5 times higher and stomatal conductances were 1.3 times higher in +5 °C plants than in ambient-grown plants. Reductions in hydraulic conductivity and stomatal conductance as a result of the freeze were 3.5 and 1.8 times greater respectively in +5 °C plants than in ambient-grown plants. Many of the +5 °C plants showed extensive leaf damage. Plants grown in the two treatments also differed in comparative xylem anatomy; +5 °C plants had larger vessel diameters (25.4 versus 22.6 μm) and higher vessel densities (71 versus 67.4 vessels mm−2) than ambient-grown plants. Our results suggest that higher growing season temperatures will increase the susceptibility of arctic deciduous shrubs to frost damage, which may offset their competitive growth advantage. [ABSTRACT FROM PUBLISHER]

The hypothesis that fast-growing breeds of willow (Salix spp.) are more sensitive to nutrient and water stress and less efficient in nutrient- and water-use than slower-growing natural willow clones was tested. Cuttings of a natural clone of S. viminalis L. collected in Sweden (L78183) and a hybrid clone of S. schwerinii E. Wolf. × S. viminalis L. (“Tora”) were grown outdoors in pots under various experimental conditions in a full-factorial design. The experimental conditions included three fertilization, two irrigation and two temperature regimes. Classical growth analysis techniques, based on an initial and a final harvest, were used as a screening method, together with calculation of intrinsic water-use efficiency (foliar carbon isotope ratio; δ13C). In addition, nitrogen-use efficiency was calculated as the product of nitrogen productivity and mean residence time of nitrogen on an annual basis. There were significant differences in plant structural parameters (leaf area ratio, specific leaf area) and water-use efficiency between the clones. Furthermore, several clone × treatment interaction effects on various growth parameters indicated that the clones adapted to specific environments in different ways. “Tora” plants produced up to 25% more shoot biomass than plants of the natural clone in response to high rates of fertilization and irrigation, whereas clone ranking was reversed in most other treatments. The results support the hypothesis that fast-growing hybrids are more sensitive to nutrient and water stress than slower-growing natural clones. The hypothesis that natural clones have higher resource-use efficiency than fast-growing hybrids was supported with respect to water, but not nitrogen. [ABSTRACT FROM PUBLISHER]

Excised leaves and roots of willow (Salix dasyclados Wimm.) accumulated abscisic acid (ABA) in response to desiccation. The accumulation of ABA was greater in young leaves and roots than in old leaves and roots. In mature leaves, ABA accumulation was related to the severity and duration of the desiccation treatment. Water loss equal to 12% of initial fresh weight caused the ABA content of mature leaves to increase measurably within 30 min and to double in 2.5 h. The drying treatment caused significant (P = 0.05) reductions in leaf water potential and stomatal conductance. Recovery of leaf water potential to the control value occurred within 10 min of rewatering the dehydrated leaves, but recovery of stomatal conductance took an hour or longer, depending on the interval between dehydration and rewatering. The addition of ABA to the transpiration stream of well-watered excised leaves was sufficient to cause partial stomatal closure within 1 h and, depending on ABA concentration, more or less complete stomatal closure within 3 h. When the ABA solution was replaced with water, stomatal conductance increased at a rate inversely related to the concentration of the ABA solution with which the leaves had been supplied. [ABSTRACT FROM PUBLISHER]

Root tips of intact willow (Salix dasyclados Wimm., Clone 81-090) plants were partially dried by exposure to ambient greenhouse air and then kept in water-vapor-saturated air for up to 3 days. The drying treatment increased abscisic acid (ABA) concentrations in both the root tips subjected to drying and in the xylem sap, while it reduced leaf stomatal conductance and leaf extension rate. Despite the decrease in stomatal conductance, leaf water potentials were unaffected by the root drying treatment, indicating that the treatment reduced hydraulic conductivity between roots and foliage. After roots subjected to drying were returned to a nutrient solution or excised, ABA concentrations in the remaining roots and in the xylem sap, stomatal conductance of mature leaves and extension rate of unfolding leaves all returned to values observed in control plants. The 4-fold increase in xylem sap ABA concentration following the root drying treatment was not solely the result of reduced sap flow, and thus may be considered a potential cause, not merely a consequence, of the observed reduction in stomatal conductance. [ABSTRACT FROM PUBLISHER]

Fast-growing willow clones (six clones of Salix viminalis L. and one clone each of S. viminalis × S. schwerenii E. Wolf and S. dasyclados Wimm.) were compared with respect to growth rhythm and frost hardening in the fall. Frost resistance of stem tissues was assessed by controlled freezing followed by analysis of chlorophyll fluorescence and scoring of visible cambial discoloration. The fluorescence method proved superior to scoring based on visible cambial discoloration because it was more rapid and less subjective, but needed calibration against cambial damage. Frost hardening in mature parts of stems did not start until growth cessation was initiated in the shoot apices, irrespective of whether growth cessation occurred early or late in the fall. Frost resistance varied because of clonal variations in: (1) pre-hardening frost resistance; (2) timing of growth cessation and hence start of frost hardening; and (3) rate of frost hardening. Compared with coastal and southern clones, continental and northern clones started hardening earlier, and a continental clone proceeded through hardening more rapidly at a given temperature. A cross between a continental and coastal clone was intermediate in timing. The pre-hardening frost resistance, however, was unrelated to both growth and frost hardening characteristics. [ABSTRACT FROM PUBLISHER]