UV-B辐射方向对白三叶克隆整合的影响

Abstract

增强UV-B辐射会对植物生长和生理生化过程产生有害效应。克隆植物中，相连的克隆分株对经常共享资源和激素，然而鲜有关于异质性UV-B辐射下UV-B辐射方向对克隆整合的影响及克隆植物形态结构变化的报道。模拟同质（克隆分株片段均处于自然背景辐射）和异质（克隆分株一端处于自然背景辐射，另一端处于补加的UV-B辐射）UV-B辐射，以克隆植物白三叶为材料，进行连接和隔断处理，研究UV-B辐射方向对克隆整合强度变化、叶片形态结构特化及生理可塑性的影响。结果表明：异质性UV-B辐射下，¹⁵N同位素标记端保留的¹⁵N百分比高于同质UV-B辐射处理，转移到无标记相连端的¹⁵N含量则降低，紫外辐射处理和同位素标记是否处于同一分株端对结果无显著性影响，说明克隆植物白三叶生理整合存在但整合强度降低，辐射方向与克隆整合强度无关；隔断处理组气孔长度增加，栅栏组织增厚，但连接处理组却无此变化，表明生理整合在白三叶叶片形态结构特化中发挥作用。UV-B辐射下，最小荧光、电子传递速率及光化学淬灭系数降低但非光化学淬灭系数升高，而生理整合却使结果相反；叶绿素和紫外吸收物可在异质性UV-B辐射相连的两端运输分享。以上均表明异质UV-B辐射环境中，UV-B辐射胁迫端克隆分株通过生理整合从非胁迫端获益，并以此提高胁迫环境中克隆植物对资源的利用效率。;Clonal plants are widely distributed in all types of ecosystems, and dominate in many of them. Vascular connections (e.g. stolons or rhizomes) of clonal plants can transport resources such as carbohydrates, water and nutrient between interconnected ramets. This feature has been long considered to be adaptive in all types of ecosystems, particularly in environments where resources or stressful factors are patchily distributed in both time and space. Many studies have shown that the stressed ramets of clonal fragments can be supported by resource translocation from the connected unstressed ramets, for example, clonal integration can alleviate local stress caused by shading, drought, salinity, serpentine soils, mechanical stimulation and sand burial. Ultraviolet-B (UV-B) radiation (280-315 nm) can result in deleterious effects on many plant growth processes because it is strongly absorbed by many macromolecules and causes their conformational changes, especially proteins and nucleic acids. Environmental UV-B radiation is highly variable in spatial and temporal distributions. It is controlled by a great deal of factors, such as changes in the solar elevation with latitude, time of day, season and clouds. Little is still known about the response of physiological integration in clonal plants to heterogeneous ultraviolet-B radiation. In this study, pairs of connected and severed ramets of the stoloniferous herb <em>Trifolium repens </em>were grown under the homogeneity (both of ramets received only natural background radiation) and heterogeneity of UV-B radiation (one of the ramet received only natural background radiation and the other was exposed to supplemental UV-B radiation). Changes in intensity of nutrient integration were followed with ¹⁵N-isotope labeling of the xylem water transport. Stomata density, length, the thickness of wax, cuticle epidermal cell and palisade tissue were observed to study leaf anatomic characters. The effects of heterogeneous UV-B radiation on chlorophyll fluorescence of a clonal plant <em>T. repens</em> were evaluated. In order to assess the patterns of physiological integration contents of chlorophyll and UV-B absorbing compounds were determined. When ramets were connected and exposed to heterogeneous UV-B radiation, the percentage of ¹⁵N left in labeled ramets that suffered from enhanced UV-B radiation was higher and their transfer to unlabeled ramets lower. The intensity of physiological integration of <em>T. repens </em>for resources decreased under heterogeneous ultraviolet-B radiation in favor of the stressed ramets. Severing groups under UV-B radiation had more closed stomas (according to stomatal length) and the thickness of palisade tissue, results were reversed in connecting groups, indicating physiological integration plays a role in anatomic characters. Additive UV-B radiation resulted in a notable decrease of the minimal fluorescence of dark-adapted state (<em>F</em>_o), the electron transport rate (<em>ETR</em>) and photochemical quenching coefficient (<em>q</em>_P) and an increase of non-photochemical quenching (<em>NPQ</em>) under heterogeneous UV-B radiation, while physiological connection reverse the results. Chlorophyll and UV-B absorbing compounds were shared between connected ramets under heterogeneous UV-B radiation. Ultraviolet-B stressed ramets benefited from unstressed ramets by physiological integration, improving resource efficiency on clonal plants in less favorable sites.