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Seasonal development of cambium and secondary xylem in poplar# characteristics of cambium and the accumulation of secondary xylem cells of fast-growing Populus×euramericana cv. ‘74 /76’ are described using the traditional wood anatomy method during the active phase. The relationship between seasonal cambial activity and secondary xylem development are discussed. Results showed that the cambium presented a seasonal cyclical 10 pattern of activity and dormancy. The accumulation of secondary xylem cells was closely related to the features of cambium, in particular to the layers of cambium. Keywords: Wood science and technology; Cambium; Secondary xylem; Active phase 0 Introduction 15 The cambium of trees in the temperate zone presents a seasonal cyclical pattern of activity and dormancy [1]. During this periodicity, cambial cells and secondary xylem reveal changes in structure [2, 3]. Many researchers have focused on studies related to cambial activity and have researched the anatomical, cytological, and histochemical changes that occur in the cambium [4]. Many studies have dealt with the cellular changes associated with cambium periodicity, especially 20 changes in the cytoplasm [5-7]. Numerous experiments have also demonstrated that studies on the cambial activity, especially in xylem formation, provide not only data about the age of trees, but also clues about the possible factors that control the growth of the trees. This kind of data is also useful in dendrochronology, in forecasting timber and biomass yield, and in determining forest dynamics [8-11]. Studies about the seasonal development of secondary xylem and phloem in tropical 25 trees, mainly Indian species, have been published [12-16]. Marcati et al. [17]studied the cambial activity and annual rhythm of xylem production of Cedrela fissilis (Meliaceae) from the standpoint of phenology and climatic factors. The relationship between seasonal cambial activity, xylem and phloem development, and phenology in schizolobium parahyba, is that they all describe a fast growing tree from southeastern Brazil [18]. 30 Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignifications, and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture, and composition [19]. Populus is one 35 of the fastest growing tree species, which is why it is used extensively in reforestation projects. It is evident that the ability to improve wood properties depends on a fundamental understanding of the processes of xylem cell production and differentiation [20]. The morphology of secondary xylem cells is determined in part by the dimensions of the cambial initials from which they are derived, and in part by the developmental changes that occur during differentiation 40 [21]. Changes in cambial anatomy are reported to occur with stem diameter, height, and season [22]. Previous studies have focused mainly on mature wood (secondary xylem) variation in cell morphology [23], but such features are not involved with the quantity changes during the active phase. In this study, cambial activity of Populus×euram ericana cv. ‘74 /76’ and the relationship of this activity with phenology and annual climatic variations were examined. 45 1 Experimental 1.1 Materials Healthy plants of two-year old fast-growing Populus×euramericana cv.‘74/76’ grown in a plantation of Xiaotangshan in Beijing’s Changping district (40°17″N, 116°39″E; Beijing, China) with the same diameter at breast height were chosen and marked. Plant materials were collected 50 1.3 m above the trunk and sampled once a week from April to October in 2010. On each occasion, blocks of about 10 mm3 including phloem, cambium, and xylem cells were immediately immersed in fixative formalin–acetic acid–alcohol. Upon returning to the laboratory, they were placed in the same fresh fixative under a slight vacuum for 30 min. Following vacuum, these pieces were fixed in fresh fixative and preserved at 4°C. 55 1.2 Methods Cross slices with a thickness of 10 μm, including phloem, cambium, and xylem, were cut on a sliding microtome, and then stained with 1% safranine of alcohol solution for 2 hours. In the next stage, the slices were dehydrated with gradient alcohol, from 50%, 70%, 85%, 95%, 100% into xylene, and then cemented with neutral gum. After that, an image analysis system and a 60 research microscope (Olympus BH-2) were used to make measurements of cambium number, radial and tangential diameter of cambium, number of immature xylem and wood fiber, and number of mature xylem and wood fiber, according to quantitative anatomy conventional methods of wood. With reference to the method that Cui et al. [24] described, the numbers of three kinds of cells were counted, respectively, in accordance with the following morphology standard, and then 65 variance analysis and drawings were made for better analyzing. (1) Cambium cells, or cambial zone, radial rows of fusiform and ray initials, including phloem and xylem mother cells, which are located between xylem and phloem, that have a radial diameter that is two times less than or equal to the smallest cell along the radial direction. (2) Immature xylem cells, secondary xylem suffering/underdifferentiation, which are located inside of cambium, that have a radial diameter 70 that is greater than that of the biggest cambium cell, less than or equal to that of the mature xylem cells, and have a cell wall that is thinner than that of mature xylem without secondary wall. (3) Mature xylem cells, which have the largest radial diameter with the thickest secondary wall and deeper colors. 2 Results and Discussion 75 2.1 Seasonal changes of cambial activity and characteristics of cambial zone The changes of cambium and the formation of xylem have a close relationship with the changes of phenology characteristics during the active phase of Populus×euramericana cv. ‘74/76’. Phonological data (anthotaxy elongation, leaf growth, full foliage, yellow leaf, partial defoliation, and defoliation) were collected at one-week intervals during the reproductive period in 80 2010 (Table 1). As it can be seen in Table 1, the range of cambium cells number was from 5 to 12 layers during the seasonal cycle. The trend of cambial cells number first increased then decreased, and the number reached to a peak (11-12 layers) in June and July. The diameter of trees grew from 27.73mm to 36.25mm during the active phase. The active phase of Populus×euramericana cv. ‘74/76’ was from early April to late September in 2010. On April 7th, the cambium was beginning 85 its activity, and the trees were in the budding phenophase. The highest cambial activity was observed between April 30th and September 22nd, when the trees had mature leaves. The beginning of the reduction of cambial activity to a minimum was on September 30th, when the trees underwent partial leaf fall. Tab. 1 Relationship of Cambium 90 Activity with Phenology Sampling time Cambial cell number Diameter at breast height /mm Tissue differentiation Phenological features Different phase of cambial activity Apr.7, 2010 5.38 27.73±0.17 Phloem Anthotaxy elongation Active phase Apr.15, 2010 6.38 27.78±0.30 Phloem Leaf growth Active phase Apr. 22, 2010 6.67 28.32±0.25 Xylem+Phloem Leaf growth Active phase Apr.30 2010 7.10 28.82±0.36 Xylem+Phloem Full foliage Active phase May 7, 2010 9.60 29.10±0.11 Xylem+Phloem Full foliage Active phase May 15, 2010 10.78 29.78±0.27 Xylem+Phloem Full foliage Active phase May 22, 2010 10.17 29.60±0.36 Xylem+Phloem Full foliage Active phase May 30, 2010 10.02 30.59±0.20 Xylem+Phloem Full foliage Active phase Jun. 7, 2010 9.78 30.66±0.22 Xylem+Phloem Full foliage Active phase Jun. 15, 2010 11.12 31.66±0.31 Xylem+Phloem Full foliage Active phase Jun. 22, 2010 10.47 31.73±0.29 Xylem+Phloem Full foliage Active phase Jun. 30, 2010 11.52 31.83±0.40 Xylem+Phloem Full foliage Active phase Jul. 7, 2010 10.51 32.08±0.25 Xylem+Phloem Full foliage Active phase Jul. 15, 2010 9.20 32.04±0.49 Xylem+Phloem Full foliage Active phase Jul. 22, 2010 8.53 32.51±0.41 Xylem+Phloem Full foliage Active phase Jul. 30, 2010 11.56 32.67±0.27 Xylem+Phloem Full foliage Active phase Aug. 7, 2010 8.53 33.28±0.30 Xylem+Phloem Full foliage Active phase Aug. 15, 2010 9.51 33.63±0.13 Xylem+Phloem Full foliage Active phase Aug. 22, 2010 8.27 33.97±0.15 Xylem+Phloem Full foliage Active phase Aug. 30, 2010 11.61 34.16±0.43 Xylem+Phloem Full foliage Active phase Sep. 7, 2010 10.22 34.16±0.32 Xylem+Phloem Full foliage Active phase Sep. 15, 2010 9.09 34.66±0.37 Xylem Full foliage Active phase Sep. 22, 2010 7.10 35.18±0.33 Xylem Full foliage Active phase Sep. 30, 2010 9.06 35.40±0.40 Xylem Yellow leaf Dormant phase Oct. 7, 2010 9.34 35.51±0.18 - Partial defoliation Dormant phase Oct. 15, 2010 6.61 35.95±0.13 - Partial defoliation Dormant phase Oct. 22, 2010 5.56 36.20±0.25 - Defoliation Dormant phase Oct.30, 2010 5.71 36.25±0.44 - Defoliation Dormant phase a Cambium was in the beginning of its activity in April 2010 (April 7, Bar=20μm) b Cambium started to differentiate into immature xylem cells (May 7, Bar=50μm) c The differentiation capacity of cambium reached to peak (July 15, Bar=50μm) d The immature xylem cells began to reduce (September 7, Bar=50μm) e Cambium was in dormant phase in October 2010 (October 7, Bar=50μm) f There were no immature xylem cells (October 15, Bar=100μm) Fig. 1 Microstructure of cross section of Populus×euramericana cv. ‘74/76’ in 2010 According to Figs. 1, on April 7th (Fig. 1, a), the cambial cells (from 5 to 7 layers of fusiform 95 cells) were swelling and showed few periclinal divisions, indicating the beginning of cambium cz cz cz cz cz cz Xd Xd activity. When the cambium was active, there were a large number of immature xylem cells differentiating. On May 7th, cambial activity was just beginning, and approximately 4 to 10 layers of xylem immature cells and 6 to 11 layers of cells in the cambial zone were visible (Fig. 1, b). On July 15th, the cambium had 7 to 15 layers of fusiform cells and many layers (10 to 25) of immature xylem cells, indicating high cambial activity (Fig. 1, c). On September 100 7th, the cambium was reducing its activity to a minimum, with 5 to 10 layers of cells in cambium and 7 to 12 layers of immature xylem cells (Fig. 1, d). When the cambium was entered in dormant phase, the cambial zone was narrow, with relatively thick radial walls and surrounded by no layers or only a few (1 to 2) layers of immature secondary xylem. Dormant cambium is shown in Fig. 5. On October 7th (Fig. 105 1, e) and October 15th (Fig. 1, f), the cambium was dormant and contained 5 to 7 cells in the radial 学术论文网Tag： |
