Tracheary elements from calli of Japanese horse chestnut (Aesculus turbinata) form perforation-like structures.

MAIN CONCLUSION: Calli derived from young leaves of Aesculus turbinata contained tracheary elements with large pores that resembled perforations of vessel elements. The differentiation of tracheary elements in vitro provides a useful system for detailed analysis of xylem cell differentiation. To examine the mechanism of formation of cell wall structures, new differentiation systems are required that allows us to induce highly organized structures, such as perforations. In this study, we developed such a system in which we were able to induce formation of tracheary elements with perforations, using calli of a hardwood, Aesculus turbinata. Young leaves of A. turbinata were placed on modified MS medium that contained 5 µM 2,4-dichlorophenoxyacetic acid (2,4-D) and 5 µM benzyladenine (BA). Tracheary elements were induced in calli derived from young leaves of A. turbinata. Some tracheary elements formed broad areas of secondary wall with typical features of secondary xylem. Other tracheary elements formed spiral thickenings, which are typical features of vessel elements in secondary xylem of A. turbinata. Approximately 10% of tracheary elements formed large pores that resembled perforations of vessel elements and various types of the perforation plate were observed. Addition of NAA and brassinolide to the induction medium enhanced the differentiation of tracheary elements in calli of A. turbinata. Newly induced tracheary elements also formed typical features of secondary xylem such as perforations of the vessel elements. Our model system might be useful in efforts to understand the mechanisms of formation of highly organized structures in tracheary elements in secondary xylem.

Artificially lignified cell wall catalyzed by peroxidase selectively localized on a network of microfibrils from cultured cells.

MAIN CONCLUSION: An artificial lignified cell wall was synthesized in three steps: (1) isolation of microfibrillar network; (2) localization of peroxidase through immunoreaction; and (3) polymerization of DHP to lignify the cell wall. Artificial woody cell wall synthesis was performed following the three steps along with the actual formation in nature using cellulose microfibrils extracted from callus derived from Cryptomeria japonica. First, we constructed a polysaccharide network on a transmission electron microscopy (TEM) grid. The preparation method was optimized by chemical treatment, followed by mechanical fibrillation to create a microfibrillated network. Morphology was examined by TEM, and chemical characterization was by Fourier transform infrared (FTIR) spectroscopy. Second, we optimized the process to place peroxidase on the microfibrils via an immunoreaction technique. Using a xyloglucan antibody, we could ensure that gold particles attached to the secondary antibodies were widely and uniformly localized along with the microfibril network. Third, we applied the peroxidase attached to secondary antibodies and started to polymerize the lignin on the grid by simultaneously adding coniferyl alcohol and hydrogen peroxide. After 30 min of artificial lignification, TEM observation showed that lignin-like substances were deposited on the polysaccharide network. In addition, FTIR spectra revealed that the bands specific for lignin had increased, demonstrating the successful artificial formation of woody cell walls. This approach may be useful for studying woody cell wall formation and for producing made-to-order biomaterials.

Changes in cambial activity are related to precipitation patterns in four tropical hardwood species grown in Indonesia.

PREMISE: Cambial activity in some tropical trees varies intra-annually, with the formation of xylem rings. Identification of the climatic factors that regulate cambial activity is important for understanding the growth of such species. We analyzed the relationship between climatic factors and cambial activity in four tropical hardwoods, Acacia mangium, Tectona grandis, Eucalyptus urophylla, and Neolamarckia cadamba in Yogyakarta, Java Island, Indonesia, which has a rainy season (November-June) and a dry season (July-October). METHODS: Small blocks containing phloem, cambium, and xylem were collected from main stems in January 2014, October 2015 and October 2016, and examined with light microscopy for cambial cell division, fusiform cambial cells, and expanding xylem cells as evidence of cambial activity. RESULTS: During the rainy season, when precipitation was high, cambium was active. By contrast, during the dry season in 2015, when there was no precipitation, cambium was dormant. However, in October 2016, during the so-called dry season, cambium was active, cell division was conspicuous, and a new xylem ring formation was initiated. The difference in cambial activity appeared to be related to an unusual pattern of precipitation during the typically dry months, from July to October, in 2016. CONCLUSIONS: Our results indicate that low or absent precipitation for 3 to 4 months induces cessation of cambial activity and temporal periodicity of wood formation in the four species studied. By contrast, in the event of continuing precipitation, cambial activity in the same trees may continue throughout the year. The frequency pattern of precipitation appears to be an important determinant of wood formation in tropical trees.

Stem gravitropism and tension wood formation in Acacia mangium seedlings inclined at various angles.

Background and Aims: In response to a gravitational stimulus, angiosperm trees generally form tension wood on the upper sides of leaning stems in order to reorientate the stems in the vertical direction. It is unclear whether the angle of inclination from the vertical affects tension wood formation. This study was designed to investigate negative gravitropism, tension wood formation and growth eccentricity in Acacia mangium seedlings inclined at different angles. Methods: Uniform seedlings of A. mangium were artificially inclined at 30°, 45°, 60° and 90° from the vertical and harvested, with non-inclined controls, 3 months later. We analysed the effects of the angle of inclination on the stem recovery angle, the anatomical features of tension wood and radial growth. Key Results: Smaller inclination angles were associated with earlier stem recovery while stems subjected to greater inclination returned to the vertical direction after a longer delay. However, in terms of the speed of negative gravitopism towards the vertical, stems subjected to greater inclination moved more rapidly toward the vertical. There was no significant difference in terms of growth eccentricity among seedlings inclined at different angles. The 30°-inclined seedlings formed the narrowest region of tension wood but there were no significant differences among seedlings inclined at 45°, 60° and 90°. The 90°-inclined seedlings formed thicker gelatinous layers than those in 30°-, 45°- and 60°-inclined seedlings. Conclusion: Our results suggest that the angle of inclination of the stem influences negative gravitropism, the width of the tension wood region and the thickness of gelatinous layers. Larger amounts of gelatinous fibres and thicker gelatinous layers might generate the higher tensile stress required for the higher speed of stem-recovery movement in A. mangium seedlings.

Heterorhizy and fine root architecture of rabbiteye blueberry (Vaccinium virgatum) softwood-cuttings.

All fine root systems consist of individual fine roots. Individual roots have morphological, anatomical, and functional heterogeneity (heterorhizy). Heterorhizy plays crucial roles in plant ecosystems. However, in many species, the heterorhizy and fine root system architecture based on individual root units are unclear. This study investigated heterorhizy along the root system architecture of Vaccinium virgatum Ait (rabbiteye blueberry) softwood-cuttings (propagated from annual shoots in growing season) using protoxylem groups (PGs), a classification according to the number of protoxylem poles, as an indicator of individual root traits. Individual roots of rabbiteye blueberry varied from monarch to heptarch. The frequency of roots with larger number of PGs decreased but those with smaller number of PGs increased from adventitious roots toward lateral roots with different branching levels. This architecture were stable among cultivars, collecting position of the cuttings, or indole acetic acids treatment. Individual root sizes and secondary growth were positively correlated with the PGs. These results indicate that branching itself strongly and broadly controls individual root traits. The individual roots were classified into two types: monarch and diarch roots with small size and lacking secondary growth (thought to be hair roots in core Ericaceae) and triarch or more PG roots with large size and showing secondary growth. These heterogeneous individual roots responded differently to the experimental factors. In particular, elongation of the large roots significantly contributed to increased total root length. These results mean that heterorhizic plasticity is a determinant of root system development and heterorhizic variation exists even under practical cutting condition. In conclusion, we demonstrated heterorhizy of rabbieye blueberry cuttings based on the strong relationships of PG, individual root morphology and growth potential, and root system architecture. This study also supports strong connection between root morphology and functional roles intermediated by the PG.

Agatharesinol biosynthesis-related changes of ray parenchyma in sapwood sticks of Cryptomeria japonica during cell death.

MAIN CONCLUSION: The work demonstrates a relationship between the biosynthesis of the secondary metabolite, agatharesinol, and cytological changes that occur in ray parenchyma during cell death in sapwood sticks of Cryptomeria japonica under humidity-regulated conditions. To characterize the death of ray parenchyma cells that accompanies the biosynthesis of secondary metabolites, we examined cell death in sapwood sticks of Cryptomeria japonica under humidity-regulated conditions. We monitored features of ray parenchyma cells, such as viability, the morphology of nuclei and vacuoles, and the amount of starch grains. In addition, we analyzed levels of agatharesinol, a heartwood norlignan, by gas chromatography-mass spectrometry in the same sapwood sticks. Dramatic changes in the amount of starch grains and in the level of agatharesinol occurred simultaneously. Therefore, the biosynthesis of agatharesinol appeared to originate from the breakdown of starch. Furthermore, we observed the expansion of vacuoles in ray parenchyma cells prior to other cytological changes at the final stage of cell death. In our experimental system, we were able to follow the process of cell death and to demonstrate relationships between cytological changes and the biosynthesis of a secondary metabolite during the death of ray parenchyma cells.

Cavitation of intercellular spaces is critical to establishment of hydraulic properties of compression wood of Chamaecyparis obtusa seedlings.

BACKGROUND AND AIMS: When the orientation of the stems of conifers departs from the vertical as a result of environmental influences, conifers form compression wood that results in restoration of verticality. It is well known that intercellular spaces are formed between tracheids in compression wood, but the function of these spaces remains to be clarified. In the present study, we evaluated the impact of these spaces in artificially induced compression wood in Chamaecyparis obtusa seedlings. METHODS: We monitored the presence or absence of liquid in the intercellular spaces of differentiating xylem by cryo-scanning electron microscopy. In addition, we analysed the relationship between intercellular spaces and the hydraulic properties of the compression wood. KEY RESULTS: Initially, we detected small intercellular spaces with liquid in regions in which the profiles of tracheids were not rounded in transverse surfaces, indicating that the intercellular spaces had originally contained no gases. In the regions where tracheids had formed secondary walls, we found that some intercellular spaces had lost their liquid. Cavitation of intercellular spaces would affect hydraulic conductivity as a consequence of the induction of cavitation in neighbouring tracheids. CONCLUSIONS: Our observations suggest that cavitation of intercellular spaces is the critical event that affects not only the functions of intercellular spaces but also the hydraulic properties of compression wood.

Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers.

BACKGROUND AND AIMS: In temperate regions, trees undergo annual cycles of cambial growth, with periods of cambial activity and dormancy. Environmental factors might regulate the cambial growth, as well as the development of cambial derivatives. We investigated the effects of low temperature by localized cooling on cambial activity and latewood formation in two conifers, Chamaecyparis obtusa and Cryptomeria japonica. METHODS: A plastic rubber tube that contained cooled water was wrapped around a 30-cm-wide portion of the main stem of Chamaecyparis obtusa and Cryptomeria japonica trees during seasons of active cambium. Small blocks were collected from both cooled and non-cooled control portions of the stems for sequential observations of cambial activity and for anatomical measurements of cell morphology by light microscopy and image analysis. KEY RESULTS: The effect of localized cooling was first observed on differentiating tracheids. Tracheids narrow in diameter and with significantly decreased cambial activity were evident 5 weeks after the start of cooling in these stems. Eight weeks after the start of cooling, tracheids with clearly diminished diameters and thickened cell walls were observed in these stems. Thus, localized low temperature induced narrow diameters and obvious thickening of secondary cell walls of tracheids, which were identified as latewood tracheids. Two months after the cessation of cooling, a false annual ring was observed and cambium became active again and produced new tracheids. In Cryptomeria japonica, cambial activity ceased earlier in locally cooled portions of stems than in non-cooled stems, indicating that the cambium had entered dormancy sooner in the cooled stems. CONCLUSIONS: Artificial cooling of stems induced latewood formation and cessation of cambial activity, indicating that cambium and its derivatives can respond directly to changes in temperature. A decrease in the temperature of the stem is a critical factor in the control of cambial activity and xylem differentiation in trees.

Effect of epicuticular wax crystals on the localization of artificially deposited sub-micron carbon-based aerosols on needles of Cryptomeria japonica.

Elucidation of the mechanism of adsorption of particles suspended in the gas-phase (aerosol) to the outer surfaces of leaves provides useful information for understanding the mechanisms of the effect of aerosol particles on the growth and physiological functions of trees. In the present study, we examined the localization of artificially deposited sub-micron-sized carbon-based particles on the surfaces of needles of Cryptomeria japonica, a typical Japanese coniferous tree species, by field-emission scanning electron microscopy. The clusters (aggregates) of carbon-based particles were deposited on the needle surface regions where epicuticular wax crystals were sparsely distributed. By contrast, no clusters of the particles were found on the needle surface regions with dense distribution of epicuticular wax crystals. Number of clusters of carbon-based particles per unit area showed statistically significant differences between regions with sparse epicuticular wax crystals and those with dense epicuticular wax crystals. These results suggest that epicuticular wax crystals affect distribution of carbon-based particles on needles. Therefore, densely distributed epicuticular wax crystals might prevent the deposition of sub-micron-sized carbon-based particles on the surfaces of needles of Cryptomeria japonica to retain the function of stomata.

The effects of localized heating and disbudding on cambial reactivation and formation of earlywood vessels in seedlings of the deciduous ring-porous hardwood, Quercus serrata.

BACKGROUND AND AIMS: The networks of vessel elements play a vital role in the transport of water from roots to leaves, and the continuous formation of earlywood vessels is crucial for the growth of ring-porous hardwoods. The differentiation of earlywood vessels is controlled by external and internal factors. The present study was designed to identify the limiting factors in the induction of cambial reactivation and the differentiation of earlywood vessels, using localized heating and disbudding of dormant stems of seedlings of a deciduous ring-porous hardwood, Quercus serrata. METHODS: Localized heating was achieved by wrapping an electric heating ribbon around stems. Disbudding involved removal of all buds. Three treatments were initiated on 1 February 2012, namely heating, disbudding and a combination of heating and disbudding, with untreated dormant stems as controls. Cambial reactivation and differentiation of vessel elements were monitored by light and polarized-light microscopy, and the growth of buds was followed. KEY RESULTS: Cambial reactivation and differentiation of vessel elements occurred sooner in heated seedlings than in non-heated seedlings before bud break. The combination of heating and disbudding of seedlings also resulted in earlier cambial reactivation and differentiation of first vessel elements than in non-heated seedlings. A few narrow vessel elements were formed during heating after disbudding, while many large earlywood vessel elements were formed in heated seedlings with buds. CONCLUSIONS: The results suggested that, in seedlings of the deciduous ring-porous hardwood Quercus serrata, elevated temperature was a direct trigger for cambial reactivation and differentiation of first vessel elements. Bud growth was not essential for cambial reactivation and differentiation of first vessel elements, but might be important for the continuous formation of wide vessel elements.

Structural and mechanical roles of wood polymer assemblies in softwood revealed by gradual removal of polysaccharides or lignin.

A deep understanding of the inherent roles of wood polymers such as cellulose, hemicelluloses, and lignin in the hierarchical structure of wood is of key importance for advancing functional wood-based materials but is currently lacking. To address this gap, we clarified the underexplored contributions of wood polymer assemblies to the structural support and compressive properties of wood by chemically removing polysaccharides or lignin from wood blocks of a conifer Cryptomeria japonica. Compositional and structural evaluations revealed that cellulose, hemicelluloses, and lignin contributed to the dimensional stability of wood, especially that the polysaccharide network at cell corners sustained the honeycomb cell structure. Wood polymer assemblies featuring the anatomical structure of wood were also evaluated in terms of compressive properties. The modulus and strength reflected the density and anisotropy, whereas fracture behavior was well characterized by each wood polymer assembly through the classification of stress-strain curves based on principal component analysis. The difference in fracture behaviors indicated that the rigid lignin and flexible cellulose assemblies, possibly mediated by hemicelluloses, complementarily determine the unique compressive response of wood. These findings enable the adjustment of wood functionality and the selection of composite components for wood modification while inspiring the development of novel wood applications.

Direct mapping of morphological distribution of syringyl and guaiacyl lignin in the xylem of maple by time-of-flight secondary ion mass spectrometry.

Lignin, one of the main structural polymer of plant cell walls, varies in amount and monomeric composition among tissue and cell types, as well as among plant species. However, few analytical methods are available that can conveniently and accurately determine the morphological distribution of lignin units at the cellular level. In this report, we used time-of-flight secondary ion mass spectrometry (TOF-SIMS) to directly map guaiacyl (G) and syringyl (S) lignin units in several successive growth rings of the maple xylem. TOF-SIMS imaging and a semiquantitative approach revealed clear difference in the annual distribution of lignins between the fiber and vessel. While the vessel walls were constantly G-rich with varied S/G ratios through a growth ring, the fibers showed fairly regular annual distribution of lignins in which the earlywood was S-rich with an almost constant S/G ratio and the latewood was G-rich resulting from a decrease of the S unit. The reliability of TOF-SIMS results was demonstrated by its high correlation with the results of thioacidolysis on radial distribution of the S/G ratio in several contiguous tree rings and also in the latewood and earlywood of each ring. These results indicate that TOF-SIMS allows direct visualization of lignin composition in plant tissues.

In vitro induction of secondary xylem-like tracheary elements in calli of hybrid poplar (Populus sieboldii × P. grandidentata).

The formation of tracheary elements was induced in calli derived from petioles of hybrid poplar (Populus sieboldii × P. grandidentata) after 10 days of culture on medium that lacked auxin but contained 1 µM brassinolide. Some differentiated cells formed broad regions of cell walls and bordered pits, which are typical features of tracheary elements of secondary xylem. Other differentiated cells resembled tracheary elements of primary xylem, with spiral or reticulate thickening of cell walls. The tracheary elements that developed in calli were formed within cell clusters. This induction system provides a new model for studies of the mechanism of differentiation of secondary xylem cells in vitro.

Gibberellin mediates the development of gelatinous fibres in the tension wood of inclined Acacia mangium seedlings.

BACKGROUND AND AIMS: Gibberellin stimulates negative gravitropism and the formation of tension wood in tilted Acacia mangium seedlings, while inhibitors of gibberellin synthesis strongly inhibit the return to vertical growth and suppress the formation of tension wood. To characterize the role of gibberellin in tension wood formation and gravitropism, this study investigated the role of gibberellin in the development of gelatinous fibres and in the changes in anatomical characteristics of woody elements in Acacia mangium seedlings exposed to a gravitational stimulus. METHODS: Gibberellin, paclobutrazol and uniconazole-P were applied to the soil in which seedlings were growing, using distilled water as the control. Three days after the start of treatment, seedlings were inclined at 45 ° to the vertical and samples were harvested 2 months later. The effects of the treatments on wood fibres, vessel elements and ray parenchyma cells were analysed in tension wood in the upper part of inclined stems and in the opposite wood on the lower side of inclined stems. KEY RESULTS: Application of paclobutrazol or uniconazole-P inhibited the increase in the thickness of gelatinous layers and prevented the elongation of gelatinous fibres in the tension wood of inclined stems. By contrast, gibberellin stimulated the elongation of these fibres. Application of gibberellin and inhibitors of gibberellin biosynthesis had only minor effects on the anatomical characteristics of vessel and ray parenchyma cells. CONCLUSIONS: The results suggest that gibberellin is important for the development of gelatinous fibres in the tension wood of A. mangium seedlings and therefore in gravitropism.

Regulation of cambial activity in relation to environmental conditions: understanding the role of temperature in wood formation of trees.

The timing of cambial reactivation plays an important role in determination of the amount and quality of wood and the environmental adaptivity of trees. Environmental factors, such as temperature, influence the growth and development of trees. Temperatures from late winter to early spring affect the physiological processes that are involved in the initiation of cambial cell division and xylem differentiation in trees. Cumulative elevated temperatures from late winter to early spring result in earlier initiation of cambial reactivation and xylem differentiation in tree stems and an extended growth period. However, earlier cambial reactivation increases the risk for frost damage because the cold tolerance of cambium decreases after cambial reactivation. The present review focuses on temperature regulation on the timing of cambial reactivation and xylem differentiation in trees, and also highlights recent advances in our understanding of seasonal changes in the cold stability of microtubules in trees. The review also summarizes the present understanding of the relationships between the timing of cambial reactivation, the start of xylem differentiation and changes in levels of storage materials in trees, as well as an attempt to identify the source of energy for cell division and differentiation. A better understanding of the mechanisms that regulate wood formation in trees and the influence of environmental conditions on such mechanisms should help in efforts to improve and enhance the exploitation of wood for commercial applications and to prepare for climatic change.

Visualization of the mycelia of wood-rotting fungi by fluorescence in situ hybridization using a peptide nucleic acid probe.

White rot fungus, Phanerochaete chrysosporium, and brown rot fungus, Postia placenta, grown on agar plates, were visualized by fluorescence in situ hybridization (FISH) using a peptide nucleic acid (PNA) probe. Mycelia grown on wood chips were also clearly detected by PNA-FISH following blocking treatment. To the best of our knowledge, this is the first report on the visualization of fungi in wood by FISH.

Cold stability of microtubules in wood-forming tissues of conifers during seasons of active and dormant cambium.

The cold stability of microtubules during seasons of active and dormant cambium was analyzed in the conifers Abies firma, Abies sachalinensis and Larix leptolepis by immunofluorescence microscopy. Samples were fixed at room temperature and at a low temperature of 2-3°C to examine the effects of low temperature on the stability of microtubules. Microtubules were visible in cambium, xylem cells and phloem cells after fixation at room temperature during seasons of active and dormant cambium. By contrast, fixation at low temperature depolymerized microtubules in cambial cells, differentiating tracheids, differentiating xylem ray parenchyma and phloem ray parenchyma cells during the active season. However, similar fixation did not depolymerize microtubules during cambial dormancy in winter. Our results indicate that the stability of microtubules in cambial cells and cambial derivatives at low temperature differs between seasons of active and dormant cambium. Moreover, the change in the stability of microtubules that we observed at low temperature might be closely related to seasonal changes in the cold tolerance of conifers. In addition, low-temperature fixation depolymerized microtubules in cambial cells and differentiating cells that had thin primary cell walls, while such low-temperature fixation did not depolymerize microtubules in differentiating secondary xylem ray parenchyma cells and tracheids that had thick secondary cell walls. The stability of microtubules at low temperature appears to depend on the structure of the cell wall, namely, primary or secondary. Therefore, we propose that the secondary cell wall might be responsible for the cold stability of microtubules in differentiating secondary xylem cells of conifers.

DWARF50 (D50), a rice (Oryza sativa L.) gene encoding inositol polyphosphate 5-phosphatase, is required for proper development of intercalary meristem.

Rice internodes are vital for supporting high-yield panicles, which are controlled by various factors such as cell division, cell elongation and cell wall biosynthesis. Therefore, formation and regulation of the internode cell-producing intercalary meristem (IM) are important for determining the shape of internodes. To understand the regulation of internode development, we analysed a rice dwarf mutant, dwarf 50 (d50). Previously, we reported that parenchyma cells in the elongated internodes of d50 ectopically deposit cell wall phenolics. In this study, we revealed that D50 encodes putative inositol polyphosphate 5-phosphatase (5PTase), which may be involved in phosphoinositide signalling required for many essential cellular functions, such as cytoskeleton organization, endocytosis and vesicular trafficking in eukaryotes. Analysis of the rice genome revealed 20 putative 5PTases including D50. The d50 mutation induced abnormally oriented cell division, irregular deposition of cell wall pectins and thick actin bundles in the parenchyma cells of the IM, resulting in abnormally organized cell files of the internode parenchyma and dwarf phenotype. Our results suggest that the putative 5PTase, encoded by D50, is essential for IM formation, including the direction of cell division, deposition of cell wall pectins and control of actin organization.

A rapid decrease in temperature induces latewood formation in artificially reactivated cambium of conifer stems.

BACKGROUND AND AIMS: Latewood formation in conifers occurs during the later part of the growing season, when the cell division activity of the cambium declines. Changes in temperature might be important for wood formation in trees. Therefore, the effects of a rapid decrease in temperature on cellular morphology of tracheids were investigated in localized heating-induced cambial reactivation in Cryptomeria japonica trees and in Abies firma seedlings. METHODS: Electric heating tape and heating ribbon were wrapped on the stems of C. japonica trees and A. firma seedlings. Heating was discontinued when 11 or 12 and eight or nine radial files of differentiating and differentiated tracheids had been produced in C. japonica and A. firma stems, respectively. Tracheid diameter, cell wall thickness, percentage of cell wall area and percentage of lumen area were determined by image analysis of transverse sections and scanning electron microscopy. KEY RESULTS: Localized heating induced earlier cambial reactivation and xylem differentiation in stems of C. japonica and A. firma as compared with non-heated stems. One week after cessation of heating, there were no obvious changes in the dimensions of the differentiating tracheids in the samples from adult C. japonica. In contrast, tracheids with a smaller diameter were observed in A. firma seedlings after 1 week of cessation of heating. Two or three weeks after cessation of heating, tracheids with reduced diameters and thickened cell walls were found. The results showed that the rapid decrease in temperature produced slender tracheids with obvious thickening of cell walls that resembled latewood cells. CONCLUSIONS: The results suggest that a localized decrease in temperature of stems induces changes in the diameter and cell wall thickness of differentiating tracheids, indicating that cambium and its derivatives can respond directly to changes in temperature.

Gibberellin is required for the formation of tension wood and stem gravitropism in Acacia mangium seedlings.

BACKGROUND AND AIMS: Angiosperm trees generally form tension wood on the upper sides of leaning stems. The formation of tension wood is an important response to gravitational stimulus. Gibberellin appears to be involved in the differentiation of secondary xylem, but it remains unclear whether gibberellin plays a key role in the formation of tension wood and plant gravitropism. Therefore, a study was designed to investigate the effects of gibberellin and of inhibitors of the synthesis of gibberellin, namely paclobutrazole and uniconazole-P, on the formation of tension wood and negative stem gravitropism in Acacia mangium seedlings. METHODS: Gibberellic acid (GA(3)), paclobutrazole and uniconazole-P were applied to seedlings via the soil in which they were growing. Distilled water was applied similarly as a control. Three days after such treatment, seedlings were tilted at an angle of 45° from the vertical, and samples of stems were collected for analysis 2 weeks, 2 months and 6 months after tilting. The effects of treatments on the stem recovery degree (Rº) were analysed as an index of the negative gravitropism of seedlings, together the width of the region of tension wood in the upper part of inclined stems. KEY RESULTS: It was found that GA(3) stimulated the negative gravitropism of tilted seedling stems of A. mangium, while paclobutrazole and uniconazole-P inhibited recovery to vertical growth. Moreover, GA(3) stimulated the formation of tension wood in tilted A. mangium seedlings, while paclobutrazole and uniconazole-P strongly suppressed the formation of tension wood, as assessed 2 weeks after tilting. CONCLUSIONS: The results suggest that gibberellin plays an important role at the initial stages of formation of tension wood and in stem gravitropism in A. mangium seedlings in response to a gravitational stimulus.