黄龙病对柑橘叶片同化物积累和矿质养分运输的影响

Effects of Candidatus Liberibacter asiaticus on assimilate accumulation and mineral nutrient transport in citrus leaves

【目的】探究感染黄龙病柑橘叶片中光合产物积累、矿质元素运转与病菌侵染的内在联系,为通过营养调节防控黄龙病提供理论基础。【方法】柑橘样品采集于位于广西阳朔的中国柑橘黄龙病研究中心,选取柑橘园中长势一致的健康、染病柑橘植株各6株,取中部有6~8个叶片的枝条为试材,分析叶片病原菌数量、叶片温度,枝条同化物运输及分配。采用X射线荧光光谱(XRF)离子显影、定量电感耦合等离子体质谱(ICPMS)法测定叶片养分分布及含量。【结果】韧皮部杆菌(C. Las)侵染柑橘叶片后,主要定殖于叶片中的筛管、伴胞附近(即叶中脉),叶中脉中病菌扩增阈值循环数(cycle threshold,Ct值)显著下降。与健康柑橘叶片相比,染病叶片斑驳黄化,叶绿素含量显著降低,叶片温度显著升高;病叶叶肉、叶中脉、韧皮部中淀粉、可溶性总糖、果糖显著高于健康叶片。营养元素在叶片各部位的聚集发生显著变化,Ca、Fe在叶肉明显聚集,K在叶中脉明显聚集,Zn、Cu、Mn在叶肉明显减少。叶肉中Ca与Cu、Cu与Zn呈显著正相关,叶中脉中Ca与Mn、Cu呈显著负相关,线性混合效应模型中病原菌浓度对K、Ca、Fe、Cu、Mn、Zn影响显著,其中对K、Cu、Mn为正效应,对Ca、Fe为负效应(P<0.05)。【结论】黄龙病病原菌主要定植在叶中脉,导致碳水化合物和Ca、Fe运输受阻,叶肉细胞中叶绿素含量下降,叶肉、叶中脉中淀粉、可溶性总糖含量显著升高。矿质元素间通过协同或拮抗关系平衡柑橘叶片的营养条件,病原菌数量与叶肉中K、Cu、Mn含量为正相关,与Ca、Fe含量为负相关,因此,注意协调Ca、Fe营养可作为抵抗病原菌生长的策略。

【Objectives】Huanglongbing is one of the most devastating citrus bacterial diseases worldwide,mainly caused by Candidatus Liberibacter asiaticus(C.Las)infection.We investigated the relationship between leaf mineral elements and the development of Huanglongbing to provide a theoretical basis for revealing the pathogenesis of the disease.【Methods】The surveyed citrus orchard was located in the Huanglongbing Disease Research Center,Yangshuo,Guangxi Province.Six healthy and diseased plants were selected,and the twigs in the middle part of the citrus trees with 6 to 8 leaves were chosen for leaf sampling.Leaf pathogen quantity,leaf temperature,branch assimilate transport and distribution were analyzed.X-ray fluorescence spectroscopy(XRF)method and ICP-MS method were used to measure to determine the nutrient contents and distribution in the leaves.【Results】The C.Las mainly colonized in the vicinity of sieve tubes and companion cells in the leaves(i.e.,midrib).The pathogen's cycle threshold(Ct value)in the midrib(P<0.05)decreased.Compared with healthy citrus leaves,the infected leaves were mottled yellow,with reduced chlorophyll content and increased leaf temperature.The starch,total soluble sugar and fructose(P<0.05)increased in the mesophyll and midribs,while the sucrose(P<0.05)increased in phloem.The accumulation of nutrient elements in different parts of the leaves significantly changed between the healthy and ill leaves.Ca and Fe accumulated in mesophyll,K in midrib,and Zn,Cu,and Mn were lacking in mesophyll.There was a(P<0.05)positive correlation between Cu and Zn in the mesophyll and a negative correlation between Ca and Mn and Cu in midrib.In the linear mixed effect model,the pathogen concentration had a(P<0.05)effect on K,Ca,Fe,Cu,Mn,and Zn,showing a positive effect on K,Cu,and Mn and a negative effect on Ca and Fe.【Conclusions】The pathogen of Huanglongbing mainly colonized the midrib of leaves,blocked the transport of starch and total soluble sugar from mesophyll,midrib and phloem to xylem of the leaves.The mineral elements balance the nutrition through synergistic or antagonistic relationships to resist the growth of pathogens.The number of pathogens is positively correlated with K,Cu,and Mn content and negatively correlated with Ca and Fe in mesophyll.Therefore,Ca and Fe nutrition is essential for dealing with the Huanglong disease of citrus.