A Radial Concentration Gradient of Indole-3-Acetic Acid Is Related to Secondary Xylem Development in Hybrid Aspen

The radial distribution pattern of indole-3-acetic acid (IAA) was determined across the developing tissues of the cambial region in the stem of hybrid aspen (Populus tremula L. x Populus tremuloides Michx). IAA content was measured in consecutive tangential cryo-sections using a microscale mass spectrometry technique. Analysis was performed with wild-type and transgenic trees with an ectopic expression of Agrobacterium tumefaciens IAA-biosynthetic genes. In all tested trees IAA was distributed as a steep concentration gradient across the developing tissues of the cambial region. The peak level of IAA was within the cambial zone, where cell division takes place. Low levels were reached in the region where secondary wall formation was initiated. The transgenic trees displayed a lower peak level and a wider radial gradient of IAA compared with the wild type. This alteration was related to a lower rate of cambial cell division and a longer duration of xylem cell expansion in the transgenic trees, resulting in a decreased xylem production and a larger fiber lumen area. The results indicate that IAA has a role in regulating not only the rate of physiological processes such as cell division, but also the duration of developmental processes such as xylem fiber expansion, suggesting that IAA functions as a morphogen, conveying positional information during xylem development.     

INCREASE IN CELL WALL SURFACE AREA DURING ENLARGEMENT OF CAMBIAL DERIVATIVES IN ABIES CONCOLOR

Wilson , Brayton F. (U. California, Berkeley). Increase in cell wall surface area during enlargement of cambial derivatives in Abies eoncolor . Amer. Jour. Bot. 50(1): 95–102. Illus. 1963.— Dimensions of fusiform cells (tracheids and sieve cells) and ray cells were measured from samples of the 1960 xylem and phloem increment of 5 trees felled at monthly intervals from April through July, 1960. Calculations using these measurements gave the magnitude, direction and rate of increase in cell wall surface area during enlargement. Although 14 times more tracheids than sieve cells were produced, both cell types enlarged mostly in a radial direction (up to 400%) at the same rate (20–33 × 10³μ² wall surface area/day) to the same final size. Fusiform cambial cells doubled their wall area between successive periclinal divisions. Calculations showed that ⅞ of this increase was in the radial walls of the daughter cells at a rate comparable to that in enlarging tracheids and sieve cells; the other ⅞ was from cell plate formation at an estimated rate of 187–327 × 10³μ²/day. Enlargement of derivatives in the radial direction largely determined the amount of increase in wall area. Besides radial enlargement, tracheids also elongated (up to 13%) and phloem cells enlarged tangentially (sieve cells up to 36%; pholem ray cells up to 60%). The relationships of enlarging tracheids and xylem ray cells are discussed, and it is suggested that slippage may occur between the developing walls.     

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.     

The role of plant hormones in higher plant cellular differentiation. II. Experiments with the vascular cambium, and sclereid and tracheid differentiation in the pine, Pinus contorta

In sterile-cultured explants of stems of the pine Pinus contorta Dougl., fusiform cambial cells differentiated entirely into axial parenchyma cells when exogenous indol-3yl-acetic acid (IAA) was omitted. The normal appearance of the cambial zone was maintained when IAA was included in the medium. The IAA-maintained stability of cambial structure suggests physiological rather than epigenetic control over vascular cambium structure. IAA was essential for the occurrence of callus growth in stem explants. Callus growth was similar in appearance and extent in winter- and summer-explanted material. Tracheids differentiated in explants only when actively differentiating tracheids were already present at the moment of explanting, suggesting the absence of factors necessary for tracheid differentiation in over-wintering tissues. Sclereid differentiation, which normally does not occur in phloem or xylem development in P. contorta, occurred in callus derived from active cambial explants. The sclereids were identical to sclereids which differentiated in pith of intact stems. The possibility that sclereid and tracheid differentiation may be fundamentally similar types of gene expression is discussed. Growth of P. contorta trees in continuous darkness resulted in extensive compression-wood tracheid differentiation in the upright main stem. Normal-wood tracheids differentiated in similar trees grown in light. More tracheids differentiated in light than in darkness. This apparently is the first report of induction of compression-wood tracheid differentiation in the absence of hormone treatment or tilting of trees. Different types and numbers of tracheids differentiated at different position in two-year-old disbudded defoliated stem cuttings of P. contorta in response to apically supplied IAA. No evidence for new tracheid differentiation was seen in control cuttings; however, the results suggest that neither cambial cell division nor tracheid differentiation were actually initiated by IAA. Directed transport of additional regulatory factors toward areas of high IAA concentration is formulated as a hypothesis to explain these observations. Gibberellic acid, (S)-abscisic acid and IAA inhibited tracheid differentiation when individually supplied to basal ends of P. contorta cuttings predisposed to differentiate new tracheids. Experiments with single intact needles on Pinus cembroides var. monophylla cuttings confirmed a previous interpretation that the mature pine needle, rather than the short-shoot apical meristem at its base, promotes tracheid differentiation in the stem.     

Effects of environmental factors on wood formation in larch (Larix sibirica Ldb.) stems

 Effects of temperature and precipitation on xylem cell production by the cambium, radial cell expansion and secondary wall thickening in larch stems have been studied. The observations were carried out over two seasons on ten 50- to 60-year-old trees, growing in central Siberia and chosen according to growth rate (the number of cells in radial rows of each of two of the preceding seasons was equal). The data on the number of cells in differentiation zones and mature xylem along radial rows of tracheids, radial and tangential sizes of tracheids and their lumina were used for calculating cambial activity, the rates and durations of cell development in the zones, and both the thickness and cross-sectional areas of tracheid walls. The mean day air temperature, mean maximum diurnal and mean minimum nocturnal temperatures as well as precipitation have been shown by correlation and regression analyses to affect differentially separate stages of tracheid differentiation. Throughout all the seasons it was temperature that had the main influence on the initial divisions in the xylem, radial cell expansion and biomass accumulation. However, the levels of such an effect on separate stages of cytogenesis were different, especially the influence of nocturnal temperature on xylem cell production by cambium and primary wall growth. The optimum values of temperature and precipitation for cell production by cambium, for radial cell expansion and secondary wall thickening have been calculated. These optimum values of the first and second processes proved to be practically equal, while the last differs considerably in response to temperature. The data are discussed in connection with formation of early and late tracheids. Received: 3 July 1996 / Accepted: 7 February 1997     

Shoot hydraulic characteristics, plant water status and stomatal response in olive trees under different soil water conditions

Aims

To evaluate the impact of the amount and distribution of soil water on xylem anatomy and xylem hydraulics of current-year shoots, plant water status and stomatal conductance of mature ‘Manzanilla’ olive trees.

Methods

Measurements of water potential, stomatal conductance, hydraulic conductivity, vulnerability to embolism, vessel diameter distribution and vessel density were made in trees under full irrigation with non-limiting soil water conditions, localized irrigation, and rain-fed conditions.

Results

All trees showed lower stomatal conductance values in the afternoon than in the morning. The irrigated trees showed water potential values around ?1.4 and ?1.6 MPa whereas the rain-fed trees reached lower values. All trees showed similar specific hydraulic conductivity (K _s) and loss of conductivity values during the morning. In the afternoon, K _s of rain-fed trees tended to be lower than of irrigated trees. No differences in vulnerability to embolism, vessel-diameter distribution and vessel density were observed between treatments.

Conclusions

A tight control of stomatal conductance was observed in olive which allowed irrigated trees to avoid critical water potential values and keep them in a safe range to avoid embolism. The applied water treatments did not influence the xylem anatomy and vulnerability to embolism of current-year shoots of mature olive trees.     

Concentrations of oxygen and indole-3-acetic acid in cambial region during latewood formation and dormancy development in Picea abies stems

To manipulate the occurrence of latewood formation and cambial dormancy in Picea abies (L.) Karst. stems, potted seedlings were transferred from the natural environment on 9 July, when tracheids early in the transition between earlywood and latewood were being produced, and cultured for up to 5 weeks in a controlled environment chamber having: (1) Warm LD, (25/15C during day/night) and long (16 h) photoperiod, (2) Warm SD, (25/15C) and short (8 h) photoperiod, or (3) Cold SD, (18/8°C) and short (8 h) photoperiod. In Warm LD trees, the radial enlargement of primary-walled derivatives on the xylem side of the cambium, as well as xylem production, continued at the same magnitude throughout the experiment. In Warm SD and Cold SD trees, the radial enlargement of primary-walled derivatives declined and the cambium entered dormancy, both developments occurring faster in the Warm SD trees. The concentrations of indole-3-acetic acid (IAA) was higher in developing xylem tissue than in cambium+phloem tissues, but did not vary with environmental treatment or decrease during the experimental period. The O2 concentration in the cambial region followed the order of Cold SD>Warm SD>Warm LD trees and was <5%, the threshold for the inhibition of IAA-induced proton secretion, for the first 3 weeks in Warm SD and Warm LD trees. Thus, neither latewood formation nor cambial dormancy can be attributed to decreased IAA in the cambial region. Nor does lower O2 concentration in the cambial region appear to be inhibiting the IAA action that is associated with cambial growth.     

Relationship between the earlywood-to-latewood transition and changes in levels of stored starch around the cambium in locally heated stems of the evergreen conifer <Emphasis Type="Italic">Chamaecyparis pisifera</Emphasis>

Key message

We observed the formation of latewood tracheids with narrow diameters and thick walls and the disappearance of stored starch around the cambium on the locally heated region of stems in evergreen conifer Chamaecyparis pisifera during winter cambial dormancy.

Abstract

Wood formation is controlled by cambial cell division, which determines the quantity and quality of wood. We investigated the factors that control cambial activity and the formation of new tracheids in locally heated stems of the evergreen conifer Chamaecyparis pisifera. Electric heating tape was wrapped around one side of the stem, at breast height, of two trees in 2013 and two in 2014. Pairs of stems were locally heated in winter, and small blocks were collected from heated and non-heated regions of stems. Cambial activity and levels of stored starch around the cambium were investigated by microscopy. Cambial reactivation and xylem differentiation occurred earlier in heated than in non-heated regions. New cell plates were formed after 14–18 days of heating. After a few layers of tracheids with large diameters and thin walls had formed, cell division and cell enlargement during differentiation were inhibited. Tracheids with narrow diameters and thick walls, defining those as latewood, were formed near the cambium, and finally, four to six layers of tracheids were induced. After cambial reactivation, amounts of stored starch started to decrease and starch disappeared completely from phloem and xylem cells that were located near the cambium during the differentiation of heated regions. Our results suggest that an increase in temperature induces the conversion of stored starch to soluble sugars for continuous cambial cell division and earlywood formation. By contrast, a shortage of stored starch might be responsible for inhibition of cambial activity and induction of the formation of latewood tracheids.

     

The Resumption of Cambial Activity and the Change of Peroxidase Isozymes in Broussonetia papyrifera

Five Broussonetia papyrifera (L.) Vent. trees were selected in a natural stand located on the campus of Peking University, Beijing, China. The trees were ca. 5-6 years old, 3-4 m tall,and had diameters of about 3 cm measured 1.2 m above ground level. They were samplied at monthly intervals between January 28 and March 25, then at ten-day intervals between March 25 and May 20,1991. On each occasion, one 3-year-old shoot was cut from the tree. Two blocks (about 1 cm ×1 cm) contained peridern,phloem,cambium and wood with more than one annual ring were cut from every shoot,fixed in FAA,and then were prepared for anatomical studies. And on each occasion,7 layers of tissues (from periderm to mature xylem)were scraped off from the shoots and 100 mg of separate tissues were randomly extracted in 0.1 ml of 20% sucrose. The extracts were used for isoelectric-focusing in polyacrylamide gel slabs (85 mm × 60 mm × 1 mm). Benziding and odianisidine was used as substrate. After electrophoresis the gel slabs were placed in the substrate buffer until the isozyme bands were visible. Owing to the ring-porous structure of the wood of Broussonetia papyrifera, the cambial activity was comparable with that in the most ring-porous dicots. The cambium activity started about ten days before bud sprouting. On April 4,the dormant cambial zone consisted of ca. 4 cell layers. The trees did not sprout until April 16,but ca. 2 cell layers of immature xylem and phloem were formed concomitantly. Ten days later, 8-9 cell layers of xylem and ca. 5 cell layers of phloem were formed. The formation of immature phloem cells continued to increase slowly between April 4 and May 20, whereas that of immature xylem cells increased rapidly between April 4 and April 26,and then decreased between April 26 and May 20. It was suggested that differentiation of immature xylem into mature xylem lasted ca. 10 days,whereas that of immature phloem into mature one lasted ca. 20 days. There were totally 6 peroxidase isozyme bands in dormant cambial region and functional phloem. Variation of zymogram in cambial region occurred before cambial activity activated which is followed by more or less minor changes of bands in all other tissues. These indicated that several significant changes were related to the level of endogenous IAA and differentiation of vascular tissues.     

EFFECTS OF TREE AGE ON SECONDARY XYLEM AND PHLOEM ANATOMY IN STEMS OF GREAT BASIN BRISTLECONE PINE (PINUS LONGAEVA)

Xylem and phloem tissue samples were collected from various-aged Great Basin bristlecone pine (Pinus longaeva D. K. Bailey) stems in southern Utah and southeastern California to determine whether the vascular cambia of older trees produce fewer xylem rays, shorter-lived xylem and phloem ray cells, fewer phloem sieve cells, and a thinner phloem. Increment cores were examined to determine whether ‘aged’ cambia produced narrower tracheids that might reduce water translocation. Sapwood thickness was measured and sapwood growth layers were counted on these cores. Regression and Classification and Regression Tree (CART) analyses of sample data found no age-related changes in cambial products. Phloem and xylem production appeared normal at all ages, with no evidence of cambial malfunction.     

Effects of a 20-day-long dry period on cambial and apical meristem growth in Abies balsamea seedlings

There is general agreement that in many regions additional precipitation with climate change will not be able to balance the increased evaporation rate induced by higher air temperatures, causing periods of intense drought. Although seedlings of Abies balsamea growing in the boreal forest are known for their resistance to harsh environmental conditions, the impact of water stress on their growth still remains largely unexamined. The aim of this study was to investigate growth responses of this species during and after a dry period by monitoring cambial and apical meristem activity at short time scale. Meristem growth was studied from May to October 2005 on seedlings of A. balsamea submitted to a 20-day-long dry period in June–July. Lower rates of shoot lengthening were observed in non-irrigated seedlings only in the first part of the growing season. Irrigated and non-irrigated trees showed the same trend of xylem formation and timings of cell differentiation. Cell production during cambial activity was estimated at about one xylem cell per day thus achieving in 100 days 108 tracheids in the tree ring and a width of 2 mm, with thinner tree rings observed in non-irrigated plants. A reduction of up to 50% in lumen area and cell diameter was observed for the cells produced during the dry period. Response of A. balsamea seedlings to a 20-day-long dry period consisted of good resistance of the cambial meristems during and after water stress, high sensitivity and rapid recovery of cell sizes during water depletion and slow but effective recovery of shoot growth after treatment.     

Effect of local heating and cooling on cambial activity and cell differentiation in the stem of Norway spruce (Picea abies)

BACKGROUND AND AIMS The effect of heating and cooling on cambial activity and cell differentiation in part of the stem of Norway spruce (Picea abies) was investigated. METHODS: A heating experiment (23-25 degrees C) was carried out in spring, before normal reactivation of the cambium, and cooling (9-11 degrees C) at the height of cambial activity in summer. The cambium, xylem and phloem were investigated by means of light- and transmission electron microscopy and UV-microspectrophotometry in tissues sampled from living trees. KEY RESULTS: Localized heating for 10 d initiated cambial divisions on the phloem side and after 20 d also on the xylem side. In a control tree, regular cambial activity started after 30 d. In the heat-treated sample, up to 15 earlywood cells undergoing differentiation were found to be present. The response of the cambium to stem cooling was less pronounced, and no anatomical differences were detected between the control and cool-treated samples after 10 or 20 d. After 30 d, latewood started to form in the sample exposed to cooling. In addition, almost no radially expanding tracheids were observed and the cambium consisted of only five layers of cells. Low temperatures reduced cambial activity, as indicated by the decreased proportion of latewood. On the phloem side, no alterations were observed among cool-treated and non-treated samples. CONCLUSIONS: Heating and cooling can influence cambial activity and cell differentiation in Norway spruce. However, at the ultrastructural and topochemical levels, no changes were observed in the pattern of secondary cell-wall formation and lignification or in lignin structure, respectively.     

Effects of environmental factors on wood formation in Scots pine stems

Summary To find the optimal conditions for growth and development of tracheid walls in Scots pine stems the effects of temperature and precipitation on xylem cell production by the cambium, radial cell expansion and secondary wall thickening have been studied. The observations were carried out on 10 specially chosen 50 to 60-year-old trees, growing in central Siberia, over 2 seasons. The data on the number of cells in differentiation zones and mature xylem along radial rows of tracheids, radial and tangential sizes of tracheids and their lumens were used for calculating cambial activity, the rates and durations of cell development in the zones, and both the thickness and cross sectional areas of tracheid walls. The mean day, mean maximal diurnal and mean minimal nocturnal temperatures have been shown by correlation and regression analyses to affect differentially separate stages of cytogenesis. The temperature influenced the initial division the side of xylem and radial cell expansion mainly in May–June, while the influence of precipitation increased in July–August. Throughout all seasons it was the temperature that had the main influence on the biomass accumulation in cell walls. Optimal values of temperature and precipitation for cell production by cambium, radial cell expansion and secondary wall thickening have been calculated. The data are discussed in connection with productivity and quality of wood.     

Tissue-specific localization of gibberellins and expression of gibberellin-biosynthetic and signaling genes in wood-forming tissues in aspen

Bioactive gibberellins (GAs) are known regulators of shoot growth and development in plants. In an attempt to identify where GAs are formed, we have analyzed the expression patterns of six GA biosynthesis genes and two genes with predicted roles in GA signaling and responses in relation to measured levels of GAs. The analysis was based on tangential sections, giving tissue-specific resolution across the cambial region of aspen trees (Populus tremula). Gibberellin quantification by GC/MS-SRM showed that the bioactive GA1 and GA4 were predominantly located in the zone of expansion of xylem cells. Based on co-localization of the expression of the late GA biosynthesis gene GA 20-oxidase 1 and bioactive GAs, we suggest that de novo GA biosynthesis occurs in the expanding xylem. However, expression levels of the first committed GA biosynthesis enzyme, ent-copalyl diphosphate synthase, were high in the phloem, suggesting that a GA precursor(s) may be transported to the xylem. The expression of the GA signaling and response genes DELLA-like1 and GIP-like1 coincided well with sites of high bioactive GA levels. We therefore suggest that the main role of GA during wood formation is to regulate early stages of xylem differentiation, including cell elongation.     

Cambial sensitivity to rising temperatures by natural condition and artificial heating from late winter to early spring in the evergreen conifer Cryptomeria japonica

Differences in the timing of cambial reactivation and the initiation of xylem differentiation in response to the sum of daily maximum temperatures were studied in two Cryptomeria japonica trees with cambium of different ages under natural and locally heated conditions. In addition, we observed the effects of low temperature on cambial activity. The timing of cambial reactivation and of the initiation of xylem differentiation differed between 55- and 80-year-old cambium under natural conditions. In the 55-year-old cambium, cambial reactivation occurred when the cambial reactivation index (CRI), calculated on the basis of daily maximum temperatures in excess of 10°C, was 94 and 97°C in 2007 and 2008, respectively. In 80-year-old cambium, cambial reactivation occurred when the CRI, calculated on the basis of daily maximum temperatures in excess of 11°C, was 69 and 71°C in 2007 and 2008, respectively. After cambial reactivation in 2007, normal cell division was evident in the cambium even though the minimum temperature had fallen between −2 and −3°C. Under natural conditions, xylem differentiation started 38–44 days after cambial reactivation. In heated stems, the time between cambial reactivation and the initiation of xylem differentiation ranged from 14 to 16 days, a much shorter time than under natural conditions, indicating that continuous exposure to an elevated temperature had induced earlier xylem differentiation. Our observations indicate that the sensitivity to reactivation inducing stimuli of the cambium depends on both the stage of dormancy and tree age of the cambium.     

STRUCTURALLY PRESERVED FOSSIL PLANTS FROM ANTARCTICA. I. ANTARCTICYCAS,GEN. NOV., A TRIASSIC CYCAD STEM FROM THE BEARDMORE GLACIER AREA

Silicified stems with typical cycadalean anatomy are described from specimens collected from the Fremouw Formation (Triassic) in the Transantarctic Mountains of Antarctica. Axes are slender with a large parenchymatous pith and cortex separated by a narrow ring of vascular tissue. Mucilage canals are present in both pith and cortex. Vascular tissue consists of endarch primary xylem, a narrow band of secondary xylem tracheids, cambial zone, and region of secondary phloem. Vascular bundles contain uni- to triseriate rays with larger rays up to 2 mm wide separating the individual bundles. Pitting on primary xylem elements ranges from helical to scalariform; secondary xylem tracheids exhibit alternate circular bordered pits. Traces, often accompanied by a mucilage canal, extend out through the large rays into the cortex where some assume a girdling configuration. A zone of periderm is present at the periphery of the stem. Large and small roots are attached to the stem and are conspicuous in the surrounding matrix. The anatomy of the Antarctic cycad is compared with that of other fossil and extant cycadalean stems.     

XYLEM STRUCTURE IN BOTRYCHIUM DISSECTUM SPRENGEL AND ITS RELEVANCE TO THE TAXONOMIC POSITION OF THE OPHIOGLOSSACEAE

The radially seriate xylem of Botrychium dissectum Sprengel resembles secondary xylem, particularly that of gymnosperms, in many important details. It is derived from a layer of cells which strongly resembles a vascular cambium. Presumptive cambial initials are fusiform, and derivatives are radially seriate. The walls of the initials and derivatives have a beaded appearance when viewed in tangential section. The number of xylem elements increases in seasonal increments. Circular-bordered pit pairs occur where tracheids abut other tracheids, and specialized cross-field pit pairs occur where they abut the radially-aligned parenchyma or rays. Cambial activity in Botrychium differs from that found in seed plants and progymnosperms in not producing secondary phloem. Tracheids are less similar to those known in progymnosperms than previously assumed, and some supposed similarities may be less significant than previously assumed. The significance of these dissimilarities is unclear. The recognition that the bulk of the xylem is secondary and that protoxylem strands are arranged as sympodia suggests that Botrychium may be eustelic rather than siphonostelic.     

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.     

Periodicity of cambial activity in Abies balsamea. I. Effects of temperature and photoperiod on cambial dormancy and frost hardiness

The relationship between from hardiness and growth potential, and their dependence on temperature and photoperiod, was investigated in the one-year-old cambium of balsam fir [Abies balsamea (L.) Mill.]. Six-year-old trees were exposed for 9 weeks to either the natural environment or one of 4 controlled environments in the fall (18 September-18 November), spring (12 April–14 June) and summer (19 July – 19 September). The 4 controlled environments were (1) WS, warm temperature (24/20°C in day/night) + short day (8 h). (2) WL. warm temperature (24/20°C) + long day (8 h + 1 h night break), (3) CS. cold temperature (9/5°C) + short day (8 h) and (4) CL, cold temperature (9/5°C) + long day (8 h + 1 h night break). At the beginning and end of each exposure, cambial activity was measured by recording the number of xylem, cambium and phloem cells, frost hardiness was estimated from the cambium's ability to survive freezing to –40°C, and cambial growth potential was deduced from the duration of the cell cycle and the production of xylem, cambium and phloem cells in cuttings cultured for 4 weeks with exogenous indole-3-acetic acid (IAA) under environmental conditions favourable for cambial activity. In the natural environment, frost hardening began in September and was completed in November, while dehardening occurred when the cambium reactivated. CL, CS, and to a lesser extent WS, promoted hardening in the summer and fall, but did not prevent dehardening in the spring. The cambial growth potential in the natural environment declined from a maximum in April to a low level in June, reached a minimum in September, then increased to a high level in November. This potential was promoted by CL and CS on all dates by WL in the summer and fall. The ratio of xylem to phloem induced by IAA treatment was greatest in June and least in September in cuttings from trees exposed to the natural environment, and was increased by CL and CS in the fall. The cambium in intact branches of trees protected from chilling during the fall and winter resumed cell cycling after less than 9 weeks of dormancy, but produced mostly or only phloem in the subsequent growing period. It is concluded that the frost hardiness of the cambium, the IAA-induced cycling of cambial cells, and IAA-induced xylem to phloem ratio vary independently with season, temperature and photoperiod, and that the periodicity of these processes is regulated endogenously.     

Age-dependent xylogenesis in timberline conifers

Neither anatomical change nor physiological abnormalities have been observed in the cambia of older trees. However, different sensitivity and period of significant responses to climate suggest the existence of some age-related change in the patterns of cambial activity and/or wood cell formation. Here, weekly cambial activity and timing and duration of xylem cell enlargement and wall thickening were compared in adult (50-80 yr) and old (200-350 yr) trees of Larix decidua, Pinus cembra and Picea abies at the Alpine timberline during 2004 and 2005. Timings and durations of xylogenesis differed between adult and old trees, with 2-3 wk shorter cambial activity found in the latter. The delayed onset of cambium division and lower cell production in old trees, with respect to adult trees, led to reductions of 15-20% in the overall duration of xylem differentiation. These results demonstrate that cambial dynamics change during the tree lifespan and that the time window of tree-ring production shortens with age. Variations in the period of xylem growth may be the cause of age-dependent responses to climate. The observed shorter xylogenesis in older plants at the Alpine timberline could be related to a size effect and not just to age per se.