| 因此，顶端分生组织可能形成一个无毒区。茎尖培养正是利用这一概念来获得无毒苗。
为了揭示茎尖经超低温脱毒的机理，对超低温处理后成活细胞在茎尖的分布和超低温处理前病毒在茎尖的分布做了大量研究。结果表明，茎尖经超低温处理后，绝大部分细胞被冻死，仅有分生组织顶端几层细胞和第一、第二叶原基的部分细胞能成活。这些成活的细胞，在超低温处理前，细胞胞核大、核仁明显，胞质中的液泡小，高尔基体、线粒体、内质网和原质体明显。胞核与胞质比大。对甘薯羽状斑驳病毒(SPFMV)和甘薯褪绿矮缩病毒(SPCSV)在茎尖分生组织中的定位研究表明,不管是单一感染还是混合感染，SPFMV仅能侵染第四叶原基及其以外的组织；而SPCSV则只能侵染第五叶原基及其以外的组织。上述结果表明，带毒的茎尖经超低温处理后，带毒的细胞基本不能成活，而成活的细胞大多是不带毒的，由此而发育的完整植株则可能是无毒的。
值得注意的是，上述解释仅适合于不能侵染茎尖分生组织的病毒种类，如洋李痘包病毒、香蕉条纹病毒和葡萄A病毒等；而对那些能侵染茎尖分生组织的病毒(如RBDV)是不适合的。对这类病毒而言，单一的茎尖超低温处理难以脱毒，热处理结合茎尖超低温处理才可能脱毒。
3超低温脱毒与传统脱毒技术的比较
3.1脱毒率高
大量的研究表明，茎尖超低温处理的脱毒率远远高于传统的茎尖培养。以GVA为例，茎尖超低温处理的脱毒率为97%，而茎尖培养仅能获得12%的脱毒率。
3.2茎尖超低温脱毒率不受茎尖大小的影响
茎尖大小是影响茎尖培养脱毒率的关键因素。茎尖培养时，多用0.2-0.4mm茎尖,取材难。与之比较，茎尖超低温脱毒率不受茎尖大小的影响。以SPFMV和SPCSV为例，用超低温处理0.5-1.5mm的茎尖均能获得100%的脱毒率；而茎尖培养时，0.5-1.0mm的茎尖能完全脱毒，当茎尖增大到1.5mm时，对SPFMV的脱毒率仅为7%。
3.3茎尖超低温脱毒率不受超低温方法的影响
大量的实验结果表明：一种超低温处理方法可能仅适合于某一(或某些)基因型，而不能适合另一(或另一些)基因型。到目前为此，针对某一(或某些)基因型已建立了多种茎尖超低温保存方法。不同基因型多种超低温保存方法的建立，为不同基因型的超低温脱毒提供了技术条件。已有的研究表明，不同茎尖超低温处理方法对脱毒率没有影响。
3.4茎尖超低温脱毒的周期与茎尖培养相似
脱毒苗生产周期的长短会直接影响无毒苗的生产成本。已有的研究表明，茎尖超低温脱毒所需时间与茎尖培养相似，而明显短于热处理及热处理结合茎尖培养。
3.5茎尖超低温脱毒表现出基因型特异性
大量实验结果表明，超低温处理表现出基因型特异性，主要影响超低温处理茎尖的成活率与再生率。这是限制超低温脱毒广泛应用的关键因素。然而，传统的脱毒技术，如茎尖培养，同样存在基因型特异性。
3.6茎尖超低温处理可同时实现脱毒与种质资源长期保存的双重目标
将茎尖置于液氮中长期处理，可实现资源的长期保存；而将茎尖置于液氮中行短暂处理，可实现脱毒，获得无毒苗。因此，茎尖超低温处理可同时实现脱毒与种质资源长期保存的双重目标]。
4结论与展望
到目前为此，茎尖超低温疗法已成功地用于脱除包括李(Prunus)、香蕉(Musa)、葡萄(Vitisvinifera)、草莓(Fragariaglandiglora)和树莓(Rubusidaeus)等果树的6种病毒和柑桔黄龙病。与传统脱毒技术相比，茎尖超低温脱毒具有多种优点，已被确认为一种新的脱毒技术。目前，尚缺乏对超低温脱毒苗的遗传稳定性鉴定与田间表现观察的研究，同时，研究的种类较少。应加强该技术在其它果树植物上的研究，并逐步在生产中应用。
致谢
本研究得到陕西省“13115”项目和西北农林科技大学校长基金的资助，一并致谢。
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