酸枣叶片结构可塑性对自然梯度干旱生境的适应特征

叶片是植物体暴露于环境中面积最大的器官,其最易感知环境变化而发生形态和结构上的改变。为探究植株叶片结构对不同生境的适应机理,研究以生长在烟台-石家庄-宁夏-新疆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枣叶片的结构的可塑性对不同自然梯度干旱环境的适应特征。结果表明:酸枣叶表皮着生有表皮毛,表皮细胞外覆有角质层与蜡质。叶肉为全栅型,栅栏组织发达,海绵组织退化,叶肉中有晶体及大量的分泌细胞。从烟台至新疆随生境梯度干旱加剧,酸枣叶片叶面积逐渐变小,叶片厚度依次增加,叶表皮角质层加厚,且上角质层厚度大于下角质层厚度;叶片上下表皮细胞长径及短径先增后降,栅栏组织总厚度和密度依次增大、层数减少,各层栅栏组织细胞的长径逐渐增加。叶脉薄壁细胞相对厚度逐渐减小,导管管径增大,晶体(草酸钙晶体)数增多。在梯度干旱环境中酸枣植株通过减小叶面积、提高栅栏组织密度、增加叶片及角质层厚度降低蒸腾作用,减少水分散失;通过增大导管管径提高水分利用率;通过增加晶体数量提高叶片机械性能,改变细胞的渗透势、提高吸水和保水能力。上述叶片结构的变化是酸枣植株长期对不同自然梯度干旱生境的适应特征。由此可知,叶片形态结构中叶面积、叶片厚度、角质层及叶肉组织(栅栏组织)随环境变化的可塑性较大。

Leaf morphological plasticity of Ziziphus jujuba var. spinosa in response to natural drought gradient ecotopes

The leaf is the largest organ that plants expose to the environment; it is sensitive to environmental changes and its plastic morphology can adapt to different environments. This study explored the leaf morphological plasticity of Ziziphus jujuba var. spinosa in response to natural drought gradient ecotopes to elucidate the mechanism of leaf plasticity response to different environments caused by climate change. Leaves were sampled from Yantai and Shijiazhuang cities and Ningxia and Xinjiang provinces of China that formed a drought gradient environment according to their soil moisture, annual precipitation, and humidity coefficients. Epidermal hair was distributed on the leaf surface, and cuticle and waxiness covered the surface of the epidermis. Mesophyll was all palisade tissue because the spongy tissue was degraded, but beyond that, there were a large number of stomata and secretory cells distributed into mesophyll. From Yantai to Xinjiang, with drought stress increasing sharply, the leaf area gradually became significantly smaller while leaf thickness became significantly greater. The thickness of cuticle on the leaf epidermis was increased and the cuticle on the upper epidermis was thicker than that on the lower epidermis. The long and short diameters of both upper and lower epidermis cells were first increased and then decreased. Total thickness and density of palisade tissue and the long diameter of palisade cells in each pile were increased, but the number of piles was decreased. The thickness of parenchymal cells in leaf veins was decreased. The diameter of the vessel was not significantly increased; however, the number of crystals(calcium oxalate) was significantly increased. Within different natural drought gradient ecotopes, the response of Ziziphus jujuba var. spinosa was to reduce transpiration and water loss through decreased leaf area and increased palisade tissue density, leaf thickness, and cuticle thickness; to enhance water use efficiency by increasing the diameter of vessels; to increase leaf hardness by increasing the number of crystals; and to improve water uptake and retaining capacity by changing cell osmotic potential. All of these changes in the leaf morphology of Ziziphus jujuba var. spinosas were ecological adaptations necessary to tolerate different natural drought ecotopes over the long period. In conclusion, the leaf area, leaf thickness, cuticle, and mesophyll tissue showed significant plasticity in different ecotopes.