The Heteromorphic Leaves of <i>Broussonetia papyrifera</i> and its Ecological Adaptation to Environment

SHI Zi-jian; DU Hao-han; FANG Yu-xin; LIU Jin-chun

doi:10.13718/j.cnki.xsxb.2021.04.012

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The heteromorphic leaves of plants are the concentrated expression of environmental adaptation. Based on the field investigation and indoor analysis, the abnormal leaf and leaf function of Broussonetia papyrifera have been studied. The results show that B. papyrifera has complex heteromorphic leaves, with nine kinds of leaf shapes, including whole leaf and 1-8 lobes. In young plants, most of them are split leaves. In adult plants, most of them are whole leaf, and the ratio of even lobes is higher than that of odd lobes. With the increase of age, the specific leaf area decreased. In young plants, the specific leaf area of the whole leaf is larger than that of the split leaf, but in adult plants, the specific leaf area of the split leaf is larger than that of the whole leaf. Therefore, the functional properties of the leaves of B. papyrifera can reflect their response to light environment to a certain extent. However, whether it is related to other environmental factors such as water and nutrient needs further study.

Corresponding author: LIU Jin-chun

Received Date: 15/04/2020

Available Online: 20/04/2021

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Abstract: The heteromorphic leaves of plants are the concentrated expression of environmental adaptation. Based on the field investigation and indoor analysis, the abnormal leaf and leaf function of Broussonetia papyrifera have been studied. The results show that B. papyrifera has complex heteromorphic leaves, with nine kinds of leaf shapes, including whole leaf and 1-8 lobes. In young plants, most of them are split leaves. In adult plants, most of them are whole leaf, and the ratio of even lobes is higher than that of odd lobes. With the increase of age, the specific leaf area decreased. In young plants, the specific leaf area of the whole leaf is larger than that of the split leaf, but in adult plants, the specific leaf area of the split leaf is larger than that of the whole leaf. Therefore, the functional properties of the leaves of B. papyrifera can reflect their response to light environment to a certain extent. However, whether it is related to other environmental factors such as water and nutrient needs further study.

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许多植物种在发育、形态、生理、解剖结构或生殖性状等方面表现出可以支持功能调整的适应性塑性响应，并以此补偿环境胁迫对植物造成的有害影响. 异形叶性(heterophylly)就是植物自身基因调控与环境相结合，经过长期选择进化的产物，是一种对环境条件的适应^[1]. 如毛茛科植物毛茛有气生叶和沉水叶之分，眼子菜科钝脊眼子菜，也具有沉水叶和浮水叶两种叶型^[2]，这是水生植物对水生环境的一种适应. 陆生植物胡杨异形叶的差异也主要来自地区环境间的不同：在环境条件较好时，胡杨叶更多地表现为披针形叶片，随着环境的恶化，披针形叶光合碳同化能力及耐逆性较差，无法满足植株的需要，所以光合碳同化能力及耐逆性较强的锯齿阔卵形叶片逐渐替代了幼树上占主体地位的披针形叶片，成为胡杨成年树上分布最多的叶片类型^[3].

构树(Broussonetia papyrifera)是桑科构属乔木，除了极端低温(< -35 ℃)的区域之外，在我国乃至全球各地均有分布，自然分布的最高海拔可达3 500 m^[4]. 构树具有高抗逆性和较强的环境适应能力^[5]，有较为复杂的异形叶分化现象.

目前对构树的研究主要关注其提取物的用途，如提取出来的黄酮物质能抑制Hep G2细胞增殖并诱导其凋亡^[6-8]，达到治疗癌症的效果；黄酮类化合物对NO，iNOS和促炎细胞因子(TNF-α and IL-6)等有抑制作用^[9]；多酚类物质可以作为冠状病毒蛋白酶抑制剂参与治疗^[10]. 对构树的抗逆性研究主要集中在胁迫环境中构树的生理应答机制方面，如构树在盐胁迫下通过提高体内有机渗透调节物质来维持适当的细胞渗透势、增加抗氧化酶活性及非酶类抗氧化物含量来抑制膜脂的过氧化作用，从而增强自身的耐盐能力^[11]. 此外对于环境恢复具有一定的生态意义，在重金属条件下生长时对重金属有较强的转移能力，并且在其中生长良好，可以作为重金属污染厂区的生态防护树种^[12]；在土壤重金属复合污染的情况下，构树可以积累并富集重金属^[13]；对空气污染物如氯有较强的吸收作用^[14]. 也有少量文献研究其抗逆环境的基因，如抗铬基因^[15]、耐盐与抗干旱基因等^[16]. 对于异形叶在风中形态重构的仿生学方面也有所涉及^[17].

但对构树的异形叶性及其与环境的关系未有报道. 因此，本研究以构树为研究材料，试图构建构树的叶形库并从叶片功能性状角度探讨植物的叶性状分化特征及其对不同环境的适应策略.

1. 材料与方法

1.1. 研究地点

重庆市北碚区内某高校校园、重庆市北碚区体育公园和重庆市缙云山国家森林公园3个样点.

1.2. 研究方法

对选择的3个样点进行野外踏查，采集不同类型的叶片，进行拍照记录. 根据调查结果，通过查阅资料、专家咨询、团队讨论，按照裂缺的深度、裂缺的宽度、裂缺的对称度等建立分类标准，构建完整的构树叶形库. 其中，两裂与套叠多裂的区分，主要在于突出的尖的长度与明显程度，按照尖的长度与主脉长度1∶10进行区分，大于1∶10的称为两裂，小于1∶10的称为套叠多裂.

对样地内的每株构树测定株高(H)，如果H≤1.3 m，记录株高并认为是幼株；如果H＞1.3 m，记录1.3 m处胸径并认为是成株，同时对每株的叶片数、不同叶形的叶片数进行统计. 用数字化扫描仪(STD1600Epson USA)获得不同叶形的叶片图像，并使用win.Rhzo (Version 410B)根系分析系统软件(Regent Instmment Inc，Canada)对植物叶片表面积进行定量分析；使用DITP01 AL104电子天平测定叶片干质量. 计算比叶面积(SLA)，公式为SLA =S/DW，其中S为叶面积，DW为叶干质量.

1.3. 指标分析

利用Spss 20.0统计软件，用单因素方差分析对于幼株和成株植物的不同叶形数目、比叶面积等进行分析.

3. 结果与讨论

通过研究，我们发现构树具有复杂的异形叶性，因此，在构建异形叶库时应综合考虑缺裂数、缺裂的分布、缺裂的深度、缺裂的对称度等指标. 研究表明：构树具有全缘和1~8裂等9种叶形，在幼株中以缺裂叶为主，在成株中以全缘叶为主，且偶数裂叶的比例高于奇数裂叶.

比叶面积在一定程度上反应了叶片捕获光的能力和在强光下的自我保护能力，往往与植物的生长和生存对策有着紧密的联系，能够反映植物在不同生境下资源获取的能力. 本研究中，构树幼株的比叶面积显著高于成株，表明幼株生长环境中光照较弱，可通过增加比叶面积来提高光捕获能力. 同时，不管是在幼年期还是成年期，构树的比叶面积均随着年龄的增长呈现下降趋势，说明在成长过程中，构树能通过增高或者扩大冠幅捕获到足够的光照资源，因而降低了对比叶面积的需求.

通常，裂叶具有更高的比叶面积，如毛茛在水下成撕裂状，正是为了提高比叶面积从而提高光能的利用率. 但在我们的研究中，一方面，幼年构树中缺裂叶的比叶面积少于全缘叶，另一方面，幼年构树的裂叶数量远远大于全缘叶. 所以，构树并不是通过单片裂叶的比叶面积而是通过控制总叶片的比叶面积来提高对光能的利用率的. 构树成年后，构树的单叶裂叶的比叶面积大于全缘叶，但构树的全缘叶数量远远大于缺裂叶数量，因此，在成年构树中，光照并不是影响其生长的关键生态因素.

综上所述，构树具有复杂的异形叶性，其叶片的功能性状能够在一定程度上反应其对光照环境的响应，但是否与水分、养分等其他环境因素相关，还需要进一步的研究.

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The Heteromorphic Leaves of Broussonetia papyrifera and its Ecological Adaptation to Environment

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The Heteromorphic Leaves of Broussonetia papyrifera and its Ecological Adaptation to Environment

Corresponding author: LIU Jin-chun