Scientia Silvae Sinicae  2008, Vol. 44 Issue (2): 106-110   PDF    
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Yang Shumin, Jiang Zehui, Ren Haiqing, Furukawa Ikuo
杨淑敏, 江泽慧, 任海青, 古川郁夫
Ecological Anatomy Characteristics of Secondary Xylem Cells of Two Xerophytes in Elaeagnaceae
胡颓子科2种旱生植物次生木质部的生态解剖学特性
Scientia Silvae Sinicae, 2008, 44(2): 106-110.
林业科学, 2008, 44(2): 106-110.

文章历史

Received on: Apr., 14, 2006

作者相关文章

Shumin Yang
Zehui Jiang
Haiqing Ren
Furukawa Ikuo

胡颓子科2种旱生植物次生木质部的生态解剖学特性
杨淑敏1, 江泽慧1, 任海青1, 古川郁夫2     
1. 中国林业科学研究院木材工业研究所 北京 100091;
2. 日本鸟取大学农学部 鸟取湖山 680-8553
摘要:从生态学角度对沙棘和沙枣的木材结构进行对比研究。两树种具有的共同特点为:生长轮明显,半环孔材,具单穿孔,导管间纹孔交互排列,无分隔木纤维,导管较窄,环管管胞和纤维状管胞具螺纹加厚,轴向薄壁组织缺失或很少。所选树种明显区别是射线类型、宽度和叠生排列方式:沙棘射线,轴向薄壁组织细胞和导管分子具有明显的叠生排列方式,但沙枣的叠生排列不规则;沙棘射线为异型,1~2列,沙枣射线为同型,2~5列;并且二者之间数量化指标有差异,沙棘导管频率较大,导管分子较短及导管管孔较小,因此V和M值也较小,更能适应干旱环境。两树种导管分子长和纤维长的水平变动不规律,并且树种间和同一树种个体间差异显著。本文最后对该树种的解剖学特征和沙漠环境的适应性进行了讨论。
关键词沙生灌木    木材生态解剖学    胡颓子科    适应性    
Ecological Anatomy Characteristics of Secondary Xylem Cells of Two Xerophytes in Elaeagnaceae
Yang Shumin1, Jiang Zehui1, Ren Haiqing1, Furukawa Ikuo2     
1. Research Institute of Wood Industry, CAF Beijing100091;
2. Faculty of Agriculture, Tottori University, Japan Koyama Tottori 680-8553
Abstract: The wood anatomy of Hippophae rhamnoide. and Elaeagnus angustifoli. were described and compared from an ecological perspective. Both species showed similar wood structure: distinct growth ring boundaries, semi-ring-porosity, simple perforation plate, alternate intervessel pitting, non-septate fibres and helical thickenings are present while axial parenchyma is absent or rarely present. Typical different characters of two species in Elaeagnaceae are ray type and width and storied structure. In H. rhamnoides, .there is a distinct storied structure in rays, axial parenchyma cells and vessel elements, but irregularly storied in E. angustifolia. Rays are uni-to bi-seriate, heterogeneous in H. rhamnoides. and 2~5-seriate, homogeneous in E. angustifolia. But there were few quantitative differences observed between them. The vessel frequency is larger, vessel element length is shorter and vessel diameter is much narrower in H. rhamnoides. thus lead to a smaller vulnerability and mesomorphy value, indicating adaptive to xeric conditions. The horizontal variation of vessel element and fibre length along the ring number from pith showed irregular tendency. There are significantly difference in vessel element length and fibre length within trees and between species. Furthermore, the relationships between anatomical features and adaptability to desert environments were discussed.
Key words: desert shrubs     ecological wood anatomy     Elaeagnaceae     adaptability    
1 Introduction

Elaeagnaceae distributes in temperate to subtropical regions, consisting of shrubs and small trees. The typical genera are Hippopha. and Elaeagnus. The species of H. rhamnoides. commonly named sea buckthorn, is usually found growing near the coast, often forming thickets on fixed dunes and sea cliffs, and can survive under extreme climate exposure and tolerate poor soils. Environmentally, its strong nitrogen-fixing ability and rapid growth make it a good species for improving soil fertility, controlling erosion, conserving water and stabilizing sand dunes(Li et al., 2003). E. angustifoli.(Russian Olive) thrives in a variety of soil and moisture conditions, including bare mineral substrates due to its low water requirements and a high tolerance for salinity and alkalinity(Olson et al., 1985; 1986). Especially it is valued for nitrogen fixation and thus improves soil fertility, provides shading, increases moisture retention, and reduces soil-borne disease in shelterbelts and windbreaks(Friedrich et al., 1984).

Previous studies have included the data on the secondary xylem of Elaeagnaceae(Fahn et al., 1986; Schweingruber, 1990) based on light microscopy examinations. Zhang et al.(1990) reported anatomical study of H. rhamnoide. from different habitats. Jansen et al.(2000) reported wood anatomy of Elaeagnaceae, including H. rhamnoides. with comment on vestured pits, helical thickening and systematic relationships with other families. However, quantitative data have been not offered for wood in relative references except that Liu et al.(2004) only studied younger shrubs of H. rhamnoides.

The results of this paper were to provide preliminary information of the wood anatomy of selected species and discuss the relationship between anatomical characteristics of secondary xylem and adaptability to arid climate. Furthermore, the horizontal variations in vessel element length and fibre length were studied. The information obtained through this study will be useful for selecting and introducing suitable species to control desert expansion.

2 Materials and methods

The description below is based on two species of Hippophae rhamnoides.and Elaeagnus angustifoli. in Elaeagnaceae obtained from Dengkou Psammophytes Plantation, Inner Mongolia.

Five healthy trees were felled and two discs (2~3 cm thick)from each tree were taken at the height of 20~30 cm above the ground. Some of the discs were immediately fixed in formalin-acetic-alcohol(5:5:90).

Wood samples were softened in 5% glycerin solution, subsequently sectioned with a sliding microtome moving on transverse, radial and tangential surfaces of the disks. Thin sections were stained with safranine, dehydrated in a graded alcohol series and mounted in Canada balsam for light microscope examination. Small blocks exposing transverse, radial and tangential surfaces were respectively prepared according to Exley et al.(1977) for scanning electron microscope (X-650, Hitachi Ltd Tokyo, Japan) observations. Maceration was prepared through soaking in Jeffrey's solution and mounted in glycerin-jelly. Quantitative data are based on 25 measurements of vessel element length and 50 of fibre length. Terminology and methodology follow the IAWA list of microscopic features for hardwood identification(IAWA, 1989).

3 Results 3.1 General wood anatomical descriptions (Plate Ⅰ-1~13)
图版Ⅰ PlateⅠ 1. TS. Growth ring boundaiy is distinct;2. TLS. Axial parenchyma cells, rays and vessel elements are storied;3. RLS. Rays consist of procumbent cells, with square mairginal cells;4. TLS. Helical thickening is present in narrow vessel elements; perforation plate is simple;5. RLS. Intervessel pitting is alternate (viewed from inner side);6. TLS. Vesse-ray pitting is similar to intervessel pitting, alternate;7. TS. Wood is semi-ring porous;8. TLS. Rays are uniseriate or multiseriate;9. RLS. Rays consist of procumbent cells;10. TLS. Perforation plate is exclusively simple;11. TLS. Helical thickening is present in narrow vessel element;12. RLS. Pit apertures oval, slit-like, or coalescent. 13. TLS. Vessel-ray pitting is similar to intewessel pitting, except opposite arrangement.TS: transverse section; TLS: tangential longitudinal section; RLS: radial longitudinal section. LM: light microscope; SEM: scanning electronic microscope.

Wood is ring to semi-ring porous, distinct growth ring boundaries are marked by thick-walled latewood fibres(Plate Ⅰ-1, 7);largest vessels are not directly along growth ring boundary; vessels are partly solitary(about 60%), partly in short tangential multiples of 2~3 or in small clusters in earlywood, but solitary or occasionally pairs in latewood; vessel frequency is 73~113 mm-2; latewood vessels are greatly much smaller and less than earlywood vessels; outline of vessels is oval, round or angular, with 37~60 μm in tangential diameter; vessel element length is 164~182 μm.

Rays are 5~16 mm-1, 165~229 μm in width; rays are heterogeneous, storied, uniseriate or biseriate, and only seldom three cells wide in H. rhamnoides.(Plate Ⅰ-2); homogeneous rays of two sizes are distinct in E. angustifolia. uni- or biseriate, and more than four-seriate (Plate Ⅰ-8), distending in growth ring boundaries. Rays consist of procumbent cells in E. angustifolia, .and with mostly 1~2 rows of square to slightly upright marginal cells in H. rhamnoide. (Plate Ⅰ-3, 9); pit on ray cell wall is round, oval or elongated.Sheath cells present and crystals are not found in both species. Vascular tracheids present in clusters in earlywood and solitary or absent in latewood; helical thickenings are generally throughout body of the narrow vessels and vascular tracheid (Plate Ⅰ-4, 11). Intervessel pitting are vestured, alternate; pit apertures are round to oval, elongated, slit-like or irregularly coalescent, extending over the entire intervessel wall, with 1.5-2.8μm in diameter (Plate Ⅰ-5, 12); vessel-ray pitting is similar to the intervessel pitting in shape and larger than the latter (Plate Ⅰ-6, 13).

801~841μm long nonseptate fibres present and fibre wall present with helical thickenings in H. rhamnoides. fibres in earlywood are frequently thin-walled, while latewood fibres thick-walled with distinctly bordered pits common in radial and tangential walls; vestuered pits are also found in fibres tracheid of both species; sometimes tyloses are found in H. rhamnoides.

Axial parenchyma is absent or extremely rare in both species, apotracheal, diffuse, rarely paratracheal; fusiform parenchyma cells present and consists of 2 cells per parenchyma strand. Simple perforation plates present (Plate Ⅰ-4, 10).

3.2 Ecological wood anatomy

Both species grow in the same environmental conditions and in the same locality, nevertheless the functionally significant characters vary between two species. Vulnerability (mean vessel diameter divided by the mean number of vessels per sq. mm) and mesomorphy (vulnerability multiplied by mean vessel element length) were proposed by Carlquist(1977)to express the conductive safety and efficiency within xylem part. Tab. 1 shows that the vessel frequency is larger, vessel element length is shorter and vessel diameter is much narrower in H. rhamnoides. thus lead to a smaller vulnerability index and mesomorphy index, indicating then a slightly higher safety and conductive efficiency. Additionally, other features, such as exclusively simple perforation plate and helical thickening, are general trends for species confined to dry sites.

Tab.1 Quantitative anatomical features of two speciesin Elaeagnaceae
3.3 Horizontal variation in vessel element length and fibre length

Vessel element length and fibre length within and between trees are showed on Fig. 1. Vessel element length approaches a decreasing value in the first four rings for both species, and vessel element length drops somewhat in H. rhamnoides and remains small increase in E. angustifoli. from the fourth ring. The most apparent characteristic of fibre length is its fluctuation from the pith outwards. There are significantly differences in vessel element length and fibre length within species and between species(Tab. 2).

Fig.1 Variations in vessel element length and fibre length of two species of Elaeagnaceae
Tab.2 Analysis of variance of fibre length and vessel element length of two species
4 Discussions 4.1 Ecological wood anatomy perspectives and significant functions

Wood or secondary xylem provides a complex tissue for water transport, mechnical strength, and for metabolic processes such as storage and mobilization of reserve carbohydrates and lipids. The species occuring in habitats subject to high water stress, their anatomical features of both xylem and leaf show adaptations correlated to environmental extremes. Leaf surface reduction with thick cuticles, photosynthesizing by green stems and cutinization of the outer walls in leaf epidermis enable plants to withstand dry climatic periods(Lindorf, 1997). Ecological and evolutionary trends in vessel diameter, perforation plate type, vessel frequency, vessel member length, total vessel length, and fibre type have all been discussed in terms of their input to the safety and efficiency of water transport(Baas, 1986). The tendencies are for vessel members to become shorter and narrower as the aridity increases to prevent collapse of vessels under high negative pressures and vessels towards grouping in arid environments(Baas, 1986). Vessels mainly solitary or few vessel grouping, narrow and numerous could lead to greater conductive safety because it renders the inactivation of any vessel less harmful by enabling the water transport to be transferred to an adjacent vessel.

Both efficiency or maximal conductivity and safety are strongly related to vessel diameter and vessel frequency. Increased vessel diameter increases efficiency of water conduction dramatically, but at the same time it decreases safety. However, ring-porosity and presence of different vessel size classes in general are of importance for the combined efficiency and safety of xylem sap transport at different times in or throughout the growing seasons(Baas et al.1987). The gradually decreased vessel diameter from earlywood to latewood allows for optimal transport efficiency by wide vessels and provides great conductive safety through the narrow latewood vessels(Woodcock, 1994). Vessel elements tend to be shorter and narrower and more frequent in H. rhamnoide. than in E. angustifoli. (Tab. 1) that are mainly solitary or few vessel grouping, narrow and numerous could lead to greater conductive safety.

The value of indices of vulnerability and mesomorphy could reflect a range of ecological factors(Baas, 1982; Carlquist, 1988). Lower values of V and M (less than 1 and 50 respectively) show species to be xeromorphic. Tab. 1 shows that H. rhamnoide. is more xeromorphic than E. angustifolia.

Apart from these quantitative characters, the qualitative characters show ecological correlations. Some narrow vessels in these species have coarse helical thickenings, which increase cell wall strength to withstand high pressures or enlarge wall surface to promote water bonding to the surface(Carlquist, 1975). In this study, all species show helical thickenings, together with vessel groups and tracheids, which are associated with greater conductive safety in arid environments.

4.2 Horizontal variations in vessel element length and fibre length

Variations in cell length and volume have been discussed for long time due to their marked effect on product quality and the utilization of wood. Particularly, fibre length is considered as one of the more important indicators of wood quality, which related to the mechanical strength and longitudinal shrinkage of wood. It is intensively studied within individual annual rings, from the base to the top of tree, among different rings within one tree, among species, and even on different sides of tree in relation to the sun and temperature.

Within-tree pattern of each species in each environment is different. Commonly, previous studies on wood fibre length in the axial variation agreed with the tendency that fibre length increase more often to a point well up the bole and then decrease(Ridoutt et al., 1993). Other researchers have reported a constant length or a decrease in fibre length with height. Most patterns of fibre length in horizontal direction increasing with age in rings near the pith, following by a more gradual increase until a maximum reached have been reported for both hardwood and softwood(Stringer et al., 1987), but there are other trends, such as, the cell length is constant, or considerable fluctuations or decrease with age. In present study, fibre length and vessel element length of selected two species are shown unremarkable fluctuations or nearly constant. This trend is different from general conclusions that Furukawa et al. has obtained from the study on 71 species hardwoods.

Vessel element length and fibre length is dictated by fusiform cambial initial cell length (Carquist, 1988). Additionally, fibre dimensions are determined by the dimensions of the cambial fusiform cells from which they are derived from and by process that occur during cell differentiation(Ridoutt et al., 1993). In species with non-storied cambium, the increase of fibre length is explained on the basis of the increase in the length of cambial initials with increasing cambial age. In this study, most species have regular or irregular storied structure, and nearly constant length could result similarly from a retarded production from the cambial initials and lower degree of intrusive growth. The difference in range of fibre length is attribute to differences in age of wood and to the different species.

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