酸枣根系结构可塑性对自然梯度干旱生境的适应机制

根系是植物吸收水分和养分的主要器官,是直接接触土壤最先感受土壤逆境胁迫的部位。在干旱环境中,植物根系的结构特征必定发生改变以维持正常的生物机能而生存。目前,关于根系解剖结构的研究大多集中于根系的某一特定结构对单一逆境因子的响应。以生长在烟台-石家庄-银川-吐鲁番不同地域气候条件形成的自然梯度干旱环境中的酸枣为试验材料,应用植物显微技术研究酸根系结构的可塑性对不同自然梯度干旱环境的适应机制,结果表明:酸枣根的初生结构包括表皮、皮层和维管柱,表皮位于幼根的最外层,由单层体积较小、紧密排列的表皮细胞组成。皮层占根初生结构的大部分比例,由体积较大的多层薄壁细胞组成,薄壁细胞近似圆球形,数目众多,呈环形分布。维管柱位于最内层,细胞小而密集,由中柱鞘、初生木质部、初生韧皮部及薄壁细胞组成。随生境干旱加剧,酸枣根初生结构表皮细胞的厚度和宽度逐渐增加,皮层薄壁细胞的厚度和宽度、皮层薄壁细胞层数和皮层厚度均以宁夏银川样地的最大。酸枣根的次生结构包括周皮(木栓层、木栓形成层、栓内层)和次生维管组织(次生韧皮部、维管形成层和次生木质部)。从烟台至新疆吐鲁番随生境干旱加剧,酸枣植株根系周皮逐渐加厚、致密度提高。次生木质部中,导管的数量增加,管径增大。干旱环境中,酸枣植株根系结构上的变化一方面提高了吸水能力和输水效率,另一方面增强了保水能力,减少水分散失,这可能是其适应干旱逆境的机制之一。

Plasticity of root morphology of Ziziphus jujuba var. spinosa in response to natural drought gradient ecotopes

Water and nutrition are mainly uptaken by the root system, and the root system is directly grown in the soil and is sensitive to stress. In arid environments, the structure of the root system could be changed to maintain normal biology function and adapt to stress conditions. To date, most of the studies have focused on the structure or morphology of root system responses to single stress factors. However, less attention has been concentrated on the adaptive mechanism of the entire root structure to different ecotopes. Therefore, this study explored the root morphological plasticity of Ziziphus jujuba var. spinosa in response to natural drought gradient ecotopes. Root samples were selected from Yantai-Shijiazhuang-Yinchuan-Turpan of China. The four ecotopes formed a natural drought gradient environment according to their soil moisture, annual precipitation, and humidity coefficients. The purpose of this study was to elucidate the mechanism of root plasticity response to different environments caused by climate change. The results showed that root primary structure of Ziziphus jujuba var. spinosa included the epidermis, cortex, and vascular cylinder. The epidermis is on the surface of the young root, which is constituted by a single layer of epidermis cells that are small and arranged closely. The cortex takes the greatest proportion of the primary structure, and it is constituted by a larger quantity of parenchymal cells. The vascular cylinder is located in the innermost layer, and the cells are small and crowded together. It is composed of pericycle, primary xylem, primary phloem, and parenchymal cells. When drought aggravated, the thickness and width of the epidermis cells were increased. In addition, the thickness, width, and number of plies of parenchymal cells, and the thickness of the cortex were all largest at the Yinchuan ecotope. The root secondary structure of Ziziphus jujuba var. spinosa was divided into periderm (phellem layer, phellogen, phelloderm) and secondary vascular tissue (secondary phloem, vascular cambium, secondary xylem). As the drought intensified from Yantai to Turpan, the thickness and density of periderm was gradually increased. In addition, the diameter and quantity of vessels in secondary xylem were increased. These results illustrated that one of the adaptive mechanisms of plant to drought stress is the changes in the plasticity of root structure that enhance water uptake capacity and water transport efficiency. On the other hand, it improves water retaining capacity and decreases water desorption.