毛白杨维管形成层带细胞超微结构的季节性变化

木材(次生木质部)是树木形成层细胞分化的产物,形成层的活动方式不仅影响木材的产量,而且影响木材的结构和性质.利用透射电子显微镜观察了生长在北京地区的毛白杨(Populus tomentosa Carr.)枝条形成层带细胞一个完整活动周期的超微结构变化.在木质部母细胞完全恢复活动之前,形成层纺锤状原始细胞的分裂和韧皮部细胞的分化已经开始.枝条上芽的展开和幼叶的生长可能决定了形成层带细胞的这种活动方式.透射电镜观察更清楚地揭示了树木形成层细胞在活动初期的分化特点.活动期形成层细胞中的大液泡在进入休眠期后逐渐分成许多小液泡分散在细胞质中.随着液泡融合逐渐消失的深色蛋白类物质又重新充满了大部分液泡.油滴和淀粉颗粒的年变化情况同液泡中的蛋白类物质基本相似.无论在活动期还是休眠期,形成层纺锤形细胞的质膜上都发现有许多可能与物质运输有关的小泡状内折.由核膜、内质网和高尔基体及其分泌小泡组成的细胞内膜系统,在形成层活动周期的不同阶段,其形态和分布明显不同,尤其在形成层细胞的恢复活动及其衍生木质部细胞次生壁的沉积过程中发挥着重要作用.整个活动周期中,形成层纺锤形细胞的径向壁都比弦向壁厚,处在休眠期的形成层带细胞,其径向壁与弦向壁的差别则更明显.形成层恢复活动时,径向壁上特别是与弦向壁相连的角隅处出现部分自溶现象.细胞壁特别是径向壁的变薄是形成层细胞恢复活动的重要特征.     

Seasonal development of secondary xylem and phloem in <Emphasis Type="Italic">Schizolobium parahyba</Emphasis> (Vell.) Blake (Leguminosae: Caesalpinioideae)

The cambial activity and periodicity of secondary xylem and phloem formation have been less studied in tropical tree species than in temperate ones. This paper describes the relationship between seasonal cambial activity, xylem and phloem development, and phenology in Schizolobium parahyba, a fast growing semideciduous seasonal forest tree from southeastern Brazil. From 2002 to 2003, wood samples were collected periodically and phenology and climate were recorded monthly in the same period. S. parahyba forms annual growth increments in wood, delimited by narrow initial parenchyma bands. The reduction of the cambial activity to a minimum correlates to the dry season and leaf fall. The higher cambial activity correlates to the wet season and the presence of mature leaves. In phloem, a larger conductive region was observed in the wet season, when the trees were in full foliage. The secondary phloem did not exhibit any incremental zone marker; however, we found that the axial parenchyma tends to form irregular bands.     

Fluctuations of cambial activity in relation to precipitation result in annual rings and intra-annual growth zones of xylem and phloem in teak (Tectona grandis) in Ivory Coast

Background and Aims Teak forms xylem rings that potentially carry records of carbon sequestration and climate in the tropics. These records are only useful when the structural variations of tree rings and their periodicity of formation are known. Methods The seasonality of ring formation in mature teak trees was examined via correlative analysis of cambial activity, xylem and phloem formation, and climate throughout 1·5 years. Xylem and phloem differentiation were visualized by light microscopy and scanning electron microscopy. Key Results A 3 month dry season resulted in semi-deciduousness, cambial dormancy and formation of annual xylem growth rings (AXGRs). Intra-annual xylem and phloem growth was characterized by variable intensity. Morphometric features of cambium such as cambium thickness and differentiating xylem layers were positively correlated. Cambium thickness was strongly correlated with monthly rainfall (R(2) = 0·7535). In all sampled trees, xylem growth zones (XGZs) were formed within the AXGRs during the seasonal development of new foliage. When trees achieved full leaf, the xylem in the new XGZs appeared completely differentiated and functional for water transport. Two phloem growth rings were formed in one growing season. Conclusions The seasonal formation pattern and microstructure of teak xylem suggest that AXGRs and XGZs can be used as proxies for analyses of the tree history and climate at annual and intra-annual resolution.     

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.     

Opposite patterns in the annual distribution and time-course of endogenous abscisic acid and indole-3-acetic acid in relation to the periodicity of cambial activity in Eucommia ulmoides Oliv

The seasonal change of free abscisic acid (ABA) and indole-3-acetic acid (IAA) and their relationship with the cambial activity in Eucommia ulmoides trees were investigated by ABA and IAA immunolocalization using primary polyclonal and rhodamine-red fluorescing secondary antibodies, ABA and IAA quantification using high performance liquid chromatography (HPLC), and systematic monitoring of vascular cell layers production. ABA and IAA clearly displayed opposite annual distribution patterns. In the active period (AP), both immunolocalization and HPLC detected an abrupt decrease of ABA, reaching its lowest level in the summer. During dormancy, ABA started increasing in the first quiescence (Q1) (autumn), peaked in the rest (winter), and gradually decreased from the onset of the second quiescence (Q2) (the end of winter). IAA showed a reverse pattern to that of ABA: it sharply increased in AP, but noticeably decreased from the commencement of Q1. Longitudinally, the ABA distribution increased apico-basally, contrasting with IAA. Laterally, most of the ABA was located in mature vascular tissues, whereas the IAA essentially occurred in the cambial region. The concomitant IAA-ABA distribution and seasonal changes in vascular tissues greatly correlated with xylem and phloem cell production, and late wood differentiation and maturation. Interestingly, the application of exogenous ABA to quiescent E. ulmoides branches, in a water-culture system, inhibited external IAA action on cambial activity reactivation. These results suggest that, in E. ulmoides, ABA and IAA might probably interact in the cambial region. The annual cambial activity could be influenced by an IAA:ABA ratio; and ABA might play a key role in vascular cambium dormancy in higher plants. The relationship between hormonal changes and the (particular) annual life cycle of E. ulmoides is also discussed.     

Cambial activity and annual rhythm of xylem production of elephant apple tree (<Emphasis Type="Italic">Dillenia indica</Emphasis> Linn.) in relation to phenology and climatic factor growing in sub-tropical wet forest of northeast India

The interrelationship between phenological events, climatic factors, periodicity of cambial activity and seasonal production of xylem was examined in Dillenia indica L. (Dilleniaceae) growing in sub-tropical wet forest of Meghalaya state, India. The reactivation of cambial activity was seen in the first week of May, 15 days after sprouting of new leaves and buds. The activity of cambium and xylem production gradually declined toward December and ceased from January to April end. There was correlation between leaf fall and cambial dormancy. It was evident from the correlation and regression analysis, the relationship between cambial activity, xylem production with climatic factors, the monthly mean minimum temperature plays an important role for the cambial activity and xylem production rather than influence by rainfall and relative humidity in D. indica L. The data were discussed in the light of cambial activity, xylem production and phenological events.     

Cambial activity and annual rhythm of xylem production of Pinus kesiya Royle ex. Gordon (Pinaceae) in relation to phenology and climatic factors growing in sub-tropical wet forest of North East India

The interrelationship between phenological events, climatic factors, periodicity of cambial activity and seasonal production of xylem was examined in Pinus kesiya Royle ex. Gordon growing in sub-tropical wet forest of Meghalaya state, India. Reactivation of dormant cambium occurs after sprouting of new needles during the middle of February. Since the formation of reproductive cones takes place simultaneously with vegetative bud break and needle formation, cone formation could also lead to the enhancement of cambial activity. The activity of cambium and xylem production decline gradually towards November and cease from end of December to end of January. There was no correlation between needle fall and cambial activity. Due to the production of three flushes of new needles and branches in a year the tree never becomes completely leafless. It was evident from correlation and regression analysis that the annual course of average temperature plays an important role for the reactivation of vascular cambium after dormancy. The differentiation of xylem elements correlated with mean temperature in the first place and secondly with precipitation. Increase in length of fusiform initials and their derivatives could be correlated with relative humidity, precipitation and mean maximum temperature. Dormancy was imposed by low temperature and less precipitation. The data are discussed in the light of cambial activity, xylem production and phenological events.     

CAMBIAL ACTIVITY IN ALASKAN WHITE SPRUCE

Cambial activity in white spruce stems in Alaska was observed during a 2-year period in 50–60-year-old natural stands. Mitotic index was used as a measure of the rate of periclinal division of fusiform cells in the cambial zone. Anticlinal divisions were relatively rare, averaging only one per 27S periclinal divisions in most stems. Mitotic index, at any given time, appeared uniform throughout the cambial zone of an internode, among internodes of the same tree, and even among trees growing at markedly different rates. Diurnal variation in mitotic index was observed. There were three distinct growing season periods: early, grand, and late. Early period activity was characterized by reactivation of periclinal division, erratic mitotic indices, and an approximate doubling of the number of cambial zone cells per radial file (NCZ). Production of the first new xylem and phloem elements marked the beginning of the grand period. Rate of cell production in the cambial zone remained about equal to derivative production for the next 45–50 days, when about 80 % of annual xylem and phloem increment occurred. There was a drop in NCZ at the beginning of the late period coincident with a decline in mitotic index, and NCZ soon dropped to the dormant level. Complete termination of cambial activity was gradual, extending through late August and perhaps into September.     

Induction of cambial reactivation by localized heating in a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata)

BACKGROUND AND AIMS: The timing of cambial reactivation plays an important role in the control of both the quantity and the quality of wood. The effect of localized heating on cambial reactivation in the main stem of a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata) was investigated. METHODS: Electric heating tape (20-22 degrees C) was wrapped at one side of the main stem of cloned hybrid poplar trees at breast height in winter. Small blocks were collected from both heated and non-heated control portions of the stem for sequential observations of cambial activity and for studies of the localization of storage starch around the cambium from dormancy to reactivation by light microscopy. KEY RESULTS: Cell division in phloem began earlier than cambial reactivation in locally heated portions of stems. Moreover, the cambial reactivation induced by localized heating occurred earlier than natural cambial reactivation. In heated stems, well-developed secondary xylem was produced that had almost the same structure as the natural xylem. When cambial reactivation was induced by heating, the buds of trees had not yet burst, indicating that there was no close temporal relationship between bud burst and cambial reactivation. In heated stems, the amount of storage starch decreased near the cambium upon reactivation of the cambium. After cambial reactivation, storage starch disappeared completely. Storage starch appeared again, near the cambium, during xylem differentiation in heated stems. CONCLUSIONS: The results suggest that, in deciduous diffuse-porous hardwood poplar growing in a temperate zone, the temperature in the stem is a limiting factor for reactivation of phloem and cambium. An increase in temperature might induce the conversion of storage starch to sucrose for the activation of cambial cell division and secondary xylem. Localized heating in poplar stems provides a useful experimental system for studies of cambial biology.     

Cambial Growth Season of Brevi-Deciduous Brachystegia spiciformis Trees from South Central Africa Restricted to Less than Four Months

We investigate cambial growth periodicity in Brachystegia spiciformis, a dominant tree species in the seasonally dry miombo woodland of southern Africa. To better understand how the brevi-deciduous (experiencing a short, drought-induced leaf fall period) leaf phenology of this species can be linked to a distinct period of cambial activity, we applied a bi-weekly pinning to six trees in western Zambia over the course of one year. Our results show that the onset and end of cambial growth was synchronous between trees, but was not concurrent with the onset and end of the rainy season. The relatively short (three to four months maximum) cambial growth season corresponded to the core of the rainy season, when 75% of the annual precipitation fell, and to the period when the trees were at full photosynthetic capacity. Tree-ring studies of this species have found a significant relationship between annual tree growth and precipitation, but we did not observe such a correlation at intra-annual resolution in this study. Furthermore, a substantial rainfall event occurring after the end of the cambial growth season did not induce xylem initiation or false ring formation. Low sample replication should be taken into account when interpreting the results of this study, but our findings can be used to refine the carbon allocation component of process-based terrestrial ecosystem models and can thus contribute to a more detailed estimation of the role of the miombo woodland in the terrestrial carbon cycle. Furthermore, we provide a physiological foundation for the use of Brachystegia spiciformis tree-ring records in paleoclimate research.     

Participation of auxin and abscisic acid in the regulation of seasonal variations in cambial activity and xylogenesis

Summary The current notion that hormonal level and cell response are clearly correlated has often been challenged recently. During the period of cambial activity, auxin content seems to control the intensity of mitosis and some features of the resulting wood, but not the duration of the active period itself. During cambial rest, the indole-3-acetic acid (IAA) level often remains high in the cambium, but the cell sensitivity to auxin is low. The decrease of auxin transport in autumn is sometimes interpreted as a major qualitative change affecting the pattern of transport, and sometimes as a secondary change occurring later than rest onset. The causes of the seasonal variation of cambial response remain unknown. A hypothesis is proposed that accounts for the structural-functional changes occurring in cambial cells during the onset of dormancy. Abscisic acid (ABA) may reduce wood production and xylem cell enlargement in late summer. An important amount of ABA may be present in the cambial zone in autumn after drought stress and in spring in the young growing shoot. Changes in ABA level do not appear to be clearly correlated with the different steps of cambial rest and activity. Beyond the role of ABA as a stress mediator, its participation in the annual regulation of cambial activity remains unclear. Its distribution in the most alkaline compartments may account for the particularities of its seasonal activity. The involvement of IAA and ABA in cambial growth is discussed within the scope of a possible annual alternation of two different metabolisms in the cambial cell.Abbreviations ABA abscisic acid - DPA dihydrophaseic acid - GA gibberellic acid - GC-MS gas chromatography-mass spectrometry - IAA indole-3-acetic acid - PA phaseic acid - RNA ribonucleic acid - SICM single ion current monitoring - SIM selected ion monitoring     

Concomitant analysis of cambial abscisic acid and cambial growth activity in poplar

Endogenous levels of cambial region abscisic acid (ABA) were quantified by immunoassay and assessed together with cambial growth activity in poplar (Populus nigra L. × P. maximowiczii Henry, clone Kamabuchi) over the course of a growing season. The level of cambial region ABA increased from spring to late-summer but decreased sharply in autumn. Cambial growth activity, measured as the radial number of undifferentiated cambial cells and enlarging xylem cells, also increased from spring to summer and decreased sharply in autumn, indicating the onset of cambial dormancy. Exogenous ABA, applied laterally to poplar stems at two times within the growing season, enhanced cambial growth activity, as the radial number of undifferentiated cambial cells increased in ABA-treated trees subsequent to the two application times. Xylem cell development was also affected by exogenous ABA as fibre length increased significantly in ABA-treated trees at both application times. The positive correlation of cambial region ABA and cambial growth activity as well as the positive effects of exogenous ABA application thereon sheds new light on the role of this hormonal growth regulator.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN ACER NEGUNDO

Functional sieve elements are present year-round in the secondary phloem of the trunk of Acer negundo L., the box elder tree. Judging from numerous collections made between May, 1962, and May, 1964, the seasonal cycle of phloem development is as follows: cambial activity and new phloem differentiation begin in late March or early April; xylem differentiation begins about a month later and is completed in most trees in late August. At the time of cessation of cambial activity most of the relatively wide sieve elements of the current season's increment are mature. However, numerous groups of narrow, immature sieve elements and companion cells located on the outer margin of the cambial zone do not reach maturity until fall and winter. By the time of cambial reactivation in spring, most, if not all, of these narrow elements are mature. Some of the sieve elements which reach maturity either shortly after cessation of cambial activity or during dormancy become non-functional within 6 weeks after resumption of cambial activity in spring, while others remain functional until mid-August. For the phloem increment of a given year, cessation of function begins in September with the accumulation of definitive callose on the sieve plates of the first-formed sieve elements and spreads to all but the last-formed ones by the end of December.     

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.     

Seasonal development of phloem in Siberian larch stems

The seasonal development of phloem in the stems of Siberian larch (Larix sibirica Ldb.) was studied over two seasons on 50–60-year-old trees growing in a natural stand in the Siberian forest-steppe zone. Trees at the age of 20–25 years were used to study metabolites in differentiating and mature phloem elements, cambial zone, and radially growing xylem cells in the periods of early and late wood formation. The development of the current-year phloem in the stems of 50–60-year-old trees started, depending on climatic conditions, in the second-third decades of May, 10–20 days before the xylem formation, and ended together with the shoot growth cessation in late July. Monitoring of the seasonal activity of cambium producing phloem sieve cells and the duration of their differentiation compared to the xylem derivatives in the cambium demonstrated that the top production of phloem and xylem cells could coincide or not coincide during the season, while their differentiation activity was always in antiphase. Sieve cells in the early phloem are separated from those in the late phloem by a layer of tannin-containing cells, which are formed in the period when late xylem formation starts. The starch content in the structural elements of phloem depends on the state of annual xylem layer development. The content of low molecular weight carbohydrates, amino acids, organic acids, and phenols in phloem cells, cambial zone, and xylem derivatives of the cambium depends on the cell type and developmental stage as well as on the type of forming wood (early or late) differing by the cell wall parameters and, hence, by the requirement for assimilates. Significant differences in the dynamics of substances per dry weight and cell were observed during cell development.     

Structural Changes in the Vascular Cambium of Pinus strobus L. during an Annual Cycle

The changes in the ultrastructure of cambial cells of easternwhite pine (Pinus strobus L.) during an annual cycle are observedand recorded as are relationships of cambial cells during dormancyand at resumption of cambial activity. Cambial activity wasresumed late in March or early in April, when a few cells dividedpericlinally. Cambial activity reached a maximum during thelatter part of May with 15 to 20 undifferentiated cells present.In July it declined markedly, and the number of undifferentiatedcells equalled that of the dormant period. The xylem and phloemtissue cells produced late in the annual cycle overwinteredat varying developmental stages. In October cambial cells structurallyresembled dormant cells. The number of dormant cells in easternwhite pine cambium varied from 6 to 10. Active cells were characterizedby a large central vacuole, by an abundance of all cell organelles,and by thin cell walls. Dormant cells were characterized bynumerous small vacuoles, by structurally and quantitativelymodified cell organelles, and by relatively thick cell walls.     

Seasonal variation in the ultrastructure of the cambium in young stems of willow (Salix viminalis) in relation to phenology

The growth period of Salix viminalis L. (clone 683) plants near Stockholm, Sweden, (59.5°N, 18.3°E) started in April with flowering and ended in October with abscission of the shoot tips. Cell divisions in the vascular cambium started almost two months before sprouting and ceased at about the same time as the elongation growth of the shoots. Phloem cells were apparently produced before flowering, while new xylem production started at the time of flushing. Cytodifferentiation in immature xylem continued until November. Thick-walled cells with protoplasm were observed adjacent to xylem mother cells in the cambium during the winter. The number of radially arranged cells in the cambial zone increased from 3–4 during dormancy to about 18 during the mitotic maximum in July. Seasonal variation was apparent in vacuolization, wall thickness and presence of storage material in the cells. Lipid bodies and protein bodies occurred in both fusiform and ray initials, while starch was observed in ray initials, ray cells and in the phloem. In September the ultrastructure of the cambium showed anatomical features characteristic for both active and dormant cells. Dictyosomes with vesicles and rough ER were present in thick-walled cells that contained lipid bodies and starch granules. Nuclear divisions in the cambium ended in October.     

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.     

The Annual Rhythm of Activity of the Lateral Meristems (Cambium and Phellogen) in Cupressus sempervirens L

The annual rhythm of cambial activity in Cupressus sempervirensseems to be endogenous and fits well the Mediterranean climaticrhythm. Under outdoor conditions the cambium begins its activitytowards the autumn when temperatures drop and water is lesslimiting. The cambium continues to be active during the mildMediterranean winter and enters dormancy towards the beginningof the dry summer. A constant supply of water is not enoughto avoid cambial dormancy but may result in earlier onset anda prolonged period of activity. Relationships between xylemand phloem production, between shoot and root, and between cambialand phellogen activity are discussed. Cupressus sempervirens L., cypress, cambium, phellogen, annual rhythm, wood production