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近年来,全球气候的持续变化加剧了干旱对森林的威胁[1]。以往研究表明:水力安全阈值(导水率损失50%时的水势与最小水势差)与年均降水量无关,这意味着干旱引发的森林衰退存在全球趋同现象[2]。在严重干旱后的数年内,植物普遍存在生长缓慢和恢复不完全的遗留效应,植物生长强弱根据水力安全阈值而异[2-3],生长在湿润地区的植物受遗留效应的影响,恢复能力较弱,难以在未来更为频繁的干旱威胁下正常生长,从而导致湿润区生态系统的永久损伤,并进一步造成森林碳汇的普遍退化[4]。通过对湿润区不同水分环境植物的研究,有助于理解水分有效性对湿润区植物生存策略的影响。
水力失效是干旱期间植物生产力下降和死亡的主要原因[5]。水分供需矛盾的加剧迫使更多空气进入木质部管道,由此产生的栓塞阻碍了植物的水分运输,最终导致水力失效。植物的输水效率常通过植物茎的比导率(Ks)来衡量[6],湿润生境下的植物倾向于最大化输水效率而非增加木质部对栓塞的抵抗力以满足生长需求[7]。栓塞抗性(植物导水率损失50%的水势,P50)常用来表征植物应对干旱的能力[6]。研究表明:植物的栓塞抗性与干旱胁迫强度呈正相关[2, 8]。当水势降至P50以下时,木质部栓塞加速,水力运输功能明显受阻。通过对水力性状的研究,有助于描述不同植物水力策略的范围,进而深入理解植物的驱动因素[9]。
裸子植物和被子植物的木质部结构差异较大[10-11],管胞在运输和支撑方面发挥作用,导管则仅具有运输功能。依赖纤维提供木材强度[12-13],厚度跨度比及纹孔形态作为管壁的重要特征,与栓塞抗性密切相关[14]。输水效率由管腔面积及管道密度决定。根据Hagen-Poiseuill定律[15],木质部管腔面积分数的减少可以通过增大管道尺寸弥补。相比由管胞构成的裸子植物木质部,被子植物复杂的木质部结构可独立改变导管结构以优化运输,为机械强度或储存功能提供更大的木质部空间[16]。
环境水分有效性对植物的水力策略具有选择性,从而驱动植物群落的分布[17]。本研究通过比较7种裸子植物和7种被子植物在不同生境下的栓塞抗性、输水效率和解剖结构的性状差异,探究植物水力性状与木质部解剖结构的关系,以期为研究湿润区亚热带植物在不同水分条件下的水力适应策略提供参考。
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研究区位于浙江省杭州市临安区,该区地处中亚热带向北亚热带过渡区域,四季分明,区内年平均气温为8.8~14.8 ℃,年降水量为1 390~1 870 mm,无霜期为209~235 d,相对湿度为76%~81%。在研究区内选择自然和人工生境进行植物样本采集(表1)。自然生境位于西天目山景区,除自然降水外无额外灌溉。植被类型以常绿-落叶阔叶混交林为主,除研究树种外(表2),其他常见树种有北美香柏Thuja occidentalis、短尾柯Lithocarpus brevicaudatus、榧树Torreya grandis和榉树Zelkova serrata等。人工生境为浙江农林大学植物园,相比自然生境土壤水分有效性高,人工灌溉充分。除研究树种外(表2),其他常见树种有枫香Liquidambar formosana、桂花Osmanthus fragrans、冬青Ilex chinensis、垂柳Salix babylonica和竹柏Podocarpus nagi等。
表 1 自然和人工生境的基本特征
Table 1. Basic characteristics for study sites in natural and artificial habitats
生境 经纬度 海拔/m 坡向 坡度/(°) pH 生长季土壤含水量/% 自然生境 30°26′N, 119°73′E 400~450 西南 9~12 4.85±0.16 29.53±1.21 人工生境 30°15′N, 119°43′E 51~74 西南 15~20 5.23±0.12 35.98±1.22 说明:pH和生长季土壤含水量数值为平均值±标准误(n=3) 表 2 7种裸子植物和7种被子植物基本概况
Table 2. Basic overview of 7 species of gymnosperms and 7 species of angiospermae
植物 科 植物 生长习性 生活型 裸子植物 杉科 Taxodiaceae 柳杉 Cryptomeria japonica 落叶 乔木 松科 Pinaceae 金钱松 Pseudolarix amabilis 落叶 乔木 杉科 Taxodiaceae 落羽杉 Taxodium distichum 落叶 乔木 杉科 Taxodiaceae 杉木 Cunninghamia lanceolata 常绿 乔木 松科 Pinaceae 雪松 Cedrus deodara 常绿 乔木 柏科 Cupressaceae 日本扁柏 Chamaecyparis obtusa 常绿 乔木 杉科 Taxodiaceae 北美红杉 Sequoia sempervirens 常绿 乔木 被子植物 槭树科 Aceraceae 三角槭 Acer buergerianum 落叶 乔木 大戟科 Euphorbiaceae 重阳木 Bischofia polycarpa 落叶 乔木 胡桃科 Juglandacea 青钱柳 Cyclocarya paliurus 落叶 乔木 壳斗科 Fagaceae 青冈 Cyclobalanopsis glauca 常绿 乔木 木犀科 Oleaceae 女贞 Ligustrum lucidum 常绿 乔木 木兰科 Magnoliaceae 广玉兰 Magnolia grandiflora 常绿 乔木 樟科 Lauraceae 樟树 Cinnamomum bodinieri 常绿 乔木
Relationship between hydraulic properties and xylem anatomical structure of subtropical plants
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摘要:
目的 植物水力系统需要对不断变化的水分环境做出适应性调整,因此,量化植物水力性状与木质部解剖结构在不同水分状况下的关系,将有助于理解植物的适应策略。 方法 以亚热带7种裸子植物和7种被子植物为研究材料,对比分析不同生境中(自然生境和人工生境)植物的栓塞抗性(植物导水率损失50%的水势)、输水效率和解剖结构的性状差异,探究植物水力性状与木质部解剖结构的关系。 结果 ① 在不同生境下,自然生境植物的输水效率(Ks)较大,栓塞抗性(P50)较小,且植物解剖结构性状对不同生境植物水力性状具有一定指示性。② 相关性分析表明:被子植物的导管水力直径、导管密度与Ks、P50呈显著相关(P<0.05),但相关性在不同生境下呈相反趋势;裸子植物的管胞水力直径、管胞密度与Ks、P50之间的相关性在不同生境中具有相同趋势。 结论 在湿润区内,植物通过增加输水效率以适应相对干旱环境的生存策略可能较为普遍。木质部结构与功能的差异可能是同一生境下植物水分策略存在差异的原因。图6表2参43 Abstract:Objective Plant hydraulic system needs to adapt to the changing water environment. This study aims to quantify the relationship between plant hydraulic properties and xylem anatomical structure in different habitats, so as to understand the adaptation strategies of plants to water changes. Method Taking 7 species of gymnosperms and 7 species of angiosperms in subtropical zone as research materials, the differences of embolic resistance (water potential with 50% loss of plant hydraulic conductivity, P50), water transport efficiency (Ks) and anatomical structure of plants in different habitats (natural habitat and artificial habitat) were compared and analyzed, and the relationship between plant hydraulic properties and xylem anatomical structure was explored. Result (1) In different habitats, Ks of plants in natural habitat was larger, and P50 was smaller. The plant anatomical structure traits had a certain indication of plant hydraulic traits in different habitats. (2) Correlation analysis showed that the conduit hydraulic diameter and conduit density of angiosperms were significantly correlated with Ks and P50 (P<0.05), but the correlations showed the opposite trends in different habitats. The correlation of tracheid hydraulic diameter and tracheid density of gymnosperms with Ks and P50 showed the same trend in both habitats. Conclusion It may be common for plants to adapt to relatively arid environment by increasing water transport efficiency in humid areas. The differences in xylem structure and function may be the reason for the differences in plant water strategies in the same habitat. [Ch, 6 fig. 2 tab. 43 ref.] -
表 1 自然和人工生境的基本特征
Table 1. Basic characteristics for study sites in natural and artificial habitats
生境 经纬度 海拔/m 坡向 坡度/(°) pH 生长季土壤含水量/% 自然生境 30°26′N, 119°73′E 400~450 西南 9~12 4.85±0.16 29.53±1.21 人工生境 30°15′N, 119°43′E 51~74 西南 15~20 5.23±0.12 35.98±1.22 说明:pH和生长季土壤含水量数值为平均值±标准误(n=3) 表 2 7种裸子植物和7种被子植物基本概况
Table 2. Basic overview of 7 species of gymnosperms and 7 species of angiospermae
植物 科 植物 生长习性 生活型 裸子植物 杉科 Taxodiaceae 柳杉 Cryptomeria japonica 落叶 乔木 松科 Pinaceae 金钱松 Pseudolarix amabilis 落叶 乔木 杉科 Taxodiaceae 落羽杉 Taxodium distichum 落叶 乔木 杉科 Taxodiaceae 杉木 Cunninghamia lanceolata 常绿 乔木 松科 Pinaceae 雪松 Cedrus deodara 常绿 乔木 柏科 Cupressaceae 日本扁柏 Chamaecyparis obtusa 常绿 乔木 杉科 Taxodiaceae 北美红杉 Sequoia sempervirens 常绿 乔木 被子植物 槭树科 Aceraceae 三角槭 Acer buergerianum 落叶 乔木 大戟科 Euphorbiaceae 重阳木 Bischofia polycarpa 落叶 乔木 胡桃科 Juglandacea 青钱柳 Cyclocarya paliurus 落叶 乔木 壳斗科 Fagaceae 青冈 Cyclobalanopsis glauca 常绿 乔木 木犀科 Oleaceae 女贞 Ligustrum lucidum 常绿 乔木 木兰科 Magnoliaceae 广玉兰 Magnolia grandiflora 常绿 乔木 樟科 Lauraceae 樟树 Cinnamomum bodinieri 常绿 乔木 -
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