缺锌苹果树根系中锌的累积机制及其调控

Fruit trees are susceptible to rosette disease induced by zinc deficiency, and this is one of the major nutritional diseases that limited the yield and quality of apple trees in some of the main producting areas. Our previous studies showed that zinc supplement in soil has a weak effect on zinc deficiency, while zinc supplement through branches before the germination or zinc spraying on leaves before the late autumn can effectively relieve zinc-deficient symptoms. Further, we confirmed that inefficient transportation and utilization of zinc accumulated in secondary root or sequestrated in root apoplasts are the main reason for zinc-deficiency in aerial parts. However, the specific mechanism is still not clear yet. In this study, adult apple trees at full productive age and young trees both grown on zinc-deficient soils are selected as materials; the synchrotron radiation X-ray fluorescence (SRXRF) and isotopic tracer technique are used. In order to define the distribution and accumulation characters of zinc in roots of zinc-deficient apple trees, we will demonstrate the cellular compartmentation and molecular forms of zinc in root cells. In addition, the organic acid secretion, apoplast PH and electric charge distribution on cell wall of root cells will also be measured and the relationship among these factors will generated. Our study would reveal the physiological mechanism of zinc accumulation in roots of zinc-deficient apple trees and provide a basis for the remobilization of accumulated zinc. This work could also provide a theoretical support for the further reuse of accumulated zinc in roots as well as a better guide to prevent the occurence of zinc deficiency in field.

果树缺锌易导致小叶病发生，缺锌小叶病已成为苹果主产区限制苹果产量和品质提高的主要营养病害之一。前期研究表明，土壤补锌对矫治苹果小叶病效果不显著，而春季萌芽前枝干补锌或落叶前根外喷锌均可有效地缓解缺锌症状；锌在次生根中累积和在细胞壁等质外体中区隔化分布而不能有效运转利用，是导致苹果树缺锌发病的重要原因。但是锌在病树根中大量累积而不能有效运转的机制尚不清楚。为此，本项目采用同步辐射X射线荧光和同位素示踪等技术，分别以苹果盛果期大树和盆栽幼树为试材，研究缺锌树体根系中锌的微区分布规律和主要分子形态，明确缺锌小叶病苹果树根系锌分配累积特性；探讨缺锌苹果树根系有机酸分泌、质外体pH、细胞壁负电荷分布等与锌累积的关系，揭示缺锌条件下苹果根系锌累积的生理机制、探究累积锌再活化的调控基础，为通过养分调控促进缺锌小叶病树根系累积锌再利用、更好地指导生产上预防苹果缺锌小叶病的发生，提供科学依据。

本项目以苹果盛果期大树、两年生砂培红富士/平邑甜茶苹果幼树和水培平邑甜茶砧木幼苗为试材，研究缺锌胁迫下锌在根系中累积的可能机制。组织切片原位染色结果表明在缺锌胁迫后期，尤其是果实膨大期(9月)，缺锌病树根系内锌含量明显高于正常树。锌在盆栽幼树各器官的分配累积特性结果表明：随着缺锌胁迫时间的延长，低锌处理盆栽幼树地上部器官锌含量显著降低、根系锌含量显著增加，而正常处理地上部器官和根系锌含量变化不显著；缺锌胁迫促进了盆栽幼树根系有机酸的分泌，其中草酸为最主要的有机酸；缺锌胁迫后期，四种根系内有机酸总含量显著降低，其中柠檬酸含量降低至正常处理的50%-30%；质外体pH原位分析结果表明，缺锌胁迫下质外体pH升高。缺锌胁迫初期促进了水培幼苗根系的生长，而随着缺锌胁迫时间的延长，根系生长受到抑制，幼苗根际pH显著降低。由此提出缺锌胁迫下锌在根系累积的可能机制：缺锌胁迫促进根系有机酸和质子的分泌，降低根际pH，提高了根际环境中锌离子的有效性，促进根系对锌离子的吸收；根系内有机酸含量减少，降低了锌离子在木质部的装载运输能力，抑制了锌离子向地上部的转运。