Carbon nanotubes find an unusual use as fertilisers

    Manure, compost and ash were used as fertilisers for centuries before the 1800s, but people did not understand how they worked until the science of chemistry was developed in the 19th century and it became clear that they supply plants with nitrogen, phosphorous and potassium. Today, something similar may be happening with a different sort of fertiliser altogether. For reasons that are not yet entirely clear, it looks as though exposing seeds to carbon nanotubes before they germinate makes the seedlings that subsequently sprout grow faster and larger.

    A carbon nanotube is, as its name suggests, a tiny cylinder of carbon atoms. Such tubes have been proposed for all sorts of fancy uses, particularly in electronics, but they and other nanoparticles （so called because their dimensions are measured in nanometres, or billionths of a metre） have also been objects of concern. The fear is that if they became ubiquitous, they might damage living creatures, people included, by interfering with the way cells work.

    In the case of plants, a few studies over the past decade have suggested that some nanoparticles can, indeed, breach the rigid walls that surround plant cells. Instead of viewing that as a threat, however, Mariya Khodakovskaya and Alex Biris of the University of Arkansas at Little Rock wondered if it might be an opportunity. They therefore considered the possibility of using nanoparticles to penetrate the tough coats that surround unsprouted seeds.

    The reason for their interest was that these coats are something of a mixed blessing. They are there to protect the seed-and the germinating seedling-from desiccation and physical harm. They also, however, slow the absorption of nutrients when a seed eventually finds soil that is good enough to grow in. That is sensible for a wild seed, but unnecessary for a pampered cultivar. Nor is the reduction in initial growth something that even the finest fertilisers can get around.

    But nanotubes, if Dr Khodakovskaya and Dr Biris are correct, may be able to. The researchers reasoned that if such tubes do penetrate conventional cell walls, they might also be able to pierce the even-tougher coat of a seed. That would let both water and dissolved nutrients in, and might promote rapid initial growth.

    And so it proved. The researchers and their colleagues did the experiment on tomato seeds, germinating them in standard plant-growth medium that had been doped with nanotubes and comparing the result with seeds grown in undoped medium. As they report in ACS Nano, the seeds exposed to the nanotubes started to germinate within three days. The untreated seeds took six.

    Moreover, this head start was reflected in subsequent growth. On the 27th day of the experiment, the researchers measured stem length, root-system length and the overall weight of the plants. The root systems were all the same, but the stems of the treated plants had an average length of 6cm, compared with 3.5cm for the untreated plants. The difference in weight was even greater. Treated plants weighed an average of more than 150 milligrams while untreated plants averaged 60 milligrams.

    Whether this accelerated early growth was due only to penetration of the seed coat, or was a more complex phenomenon, is still unclear. Certainly, during the early days of germination, the treated seeds were absorbing nearly 50% more water （and thus nutrients） than the untreated ones. When the researchers looked at the seedling tissues under an electron microscope, however, they could see the nanotubes had actually entered living cells. They speculate that it is not just a question of letting more water into the seed, but also into the cells themselves. Possibly, the nanotubes are acting as analogues of the natural protein channels that pump water in and out of cells. As with conventional fertilisers before the 19th century, though, no one knows exactly how they do work.

    Nor is it clear whether the early spurt of growth that Dr Khodakovskaya and Dr Biris have observed will translate into faster maturity or bigger crops. That remains to be seen in further experiments. And, crucially, it is not yet known if the nanotubes will find their way into the fruit of fully grown plants. Since this experiment shows that carbon nanotubes can, indeed, have significant effects on living organisms, that would be a good thing to find out.

    碳纳米管不同寻常的用途：肥料

    在19世纪初期以前，人们使用粪、堆肥以及灰烬作为肥料已有很长的历史了。但是，人们一直不明白这些肥料是如何起作用的，到19世纪化学科学发展起来以后，人们才弄清楚这些肥料为植物体提供了氮、磷、钾这些植物成长所需要的营养元素。现在，类似的情况可能正在发生，只不过这一次是与一种完全不同的肥料有关。在种子发芽之前，如果把种子用碳纳米管处理之后，这些后来发芽的种子幼苗生长得更快、更大，但是其中的原因目前还没有完全弄清楚。

    碳纳米管恰如其名字所揭示的一样，是由碳原子构成的一种圆柱体。人们认为碳纳米管会有许多意想不到的用途，尤其是在电子学领域，但是碳纳米管和其它的纳米粒子（之所以称之为纳米粒子，是因为测量它们的大小是以纳米为单位，一纳米是10亿分之一米）同时也让人们担心，因为如果碳纳米管和其它的纳米粒子变得无处不在的话，人们害怕它们会干扰细胞的正常工作，从而会对生物有害，其中也包括对人不利。

    对于植物来说，过去十年里的少数几项研究表明，有些纳米粒子确实能突破围绕在植物细胞周围的坚硬的细胞壁。不过，来自阿肯色大学小石城分校的（University of Arkansas at Little Rock） Mariya Khodakovskaya 和 Alex Biris认为这说不准也是一个好事。因此他们想到纳米粒子有可能透过围绕在不发芽种子周围的坚硬外壳。

    他们在这方面的研究之所以有兴趣是因为这些植物种子周围的外壳的作用有好有坏。这些外壳能保护种子---以及正在发芽的幼苗---不失掉水分以及免于其它的物理上的破坏。不过在种子最终遇到适合生长的土壤时候，这些外壳减慢了种子吸收营养的速度。对于野生种子来说，这种作用是有意义的，但是对于庄稼用的种子而言，这种作用就没有必要了。而且对于即使是最好的肥料来说，如果种子在初期不能发芽成长，它们也无能为力。

    但是如果Khodakovskaya 和Biris博士的研究结果正确无误的话，那么纳米管或许可以解决上述问题。研究人员们推理认为，如果碳纳米管的确能穿透一般的细胞壁的话，那么它们或许也能穿透更加坚固的种子外壳。这样的话，水以及溶解在水中的营养物质都能进入到种子内部，从而提高种子在初始阶段的快速成长。

    上述想法得到了研究证实。Khodakovskaya 和Biris博士以及他们的同事们对西红柿种子做了实验，他们让西红柿种子在标准的植物生长环境中发芽，然后对比在掺有纳米管的生长环境下和没有掺纳米管的生长环境下的结果。他们的研究结果发表在美国化学学会（ACS）杂志《纳米》（Nano）上。研究显示在有碳纳米管存在的环境中，种子在3天内就开始发芽了。在没有碳纳米管存在的环境中，种子6天后后才开始发芽。而且，发芽早的种子对随后的生长也有利。在第二十七天的实验中，研究者们测量了这些植物的干茎长度、根系长度以及整个植物的重量。对比两种条件下的植物生长情况，它们的根系长度是一样的，但是受到碳纳米管处理过的植物的平均干茎长度为6厘米，而没有受到碳纳米管处理过的植物的平均干茎长度为3厘米。两种条件下生长的植物的重量差别更大。受到碳纳米管处理过的植物，其平均重量超过了150毫克，而没有收到处理的植物的平均重量只有60毫克。

    种子的这种提早生长是否仅仅归因于碳纳米管对种子外壳的穿透、或者是一种更为复杂的现象，目前仍然不得而知。可以肯定的是，在发芽的前几天里，受碳纳米管处理过的种子要比没有处理过的种子多吸收了将近50%的水（因此也多吸收了近50%营养）.不过，当研究者们在电子显微镜下观察种子幼苗的组织的时候，他们发现碳纳米管实际上进入到了活的细胞中。他们推测，这不仅仅是让更多的水进入到种子中的问题，也关系到让碳纳米管本身进入到细胞里面的问题。也许碳纳米管扮演着和天然蛋白质通道类似的角色，这些通道控制着水进出细胞。然而，这就和在19世纪之前人们对传统肥料的认识一样，没有人确切知道它们是如何工作的。

    Khodakovskaya 和 Dr Biris博士观察到种子在早期有这种突飞猛进的生长，但是人们同样也不知道这种"快长"是否导致植物早熟或者作物产量更高。这还需要进一步的实验来证实。关键是目前还不知道这些碳纳米管是否会进入成熟植物的果实中。因为Khodakovskaya 和 Dr Biris博士的实验表明，碳纳米管确实能对生物体有显着的影响，如果能知道碳纳米管是否会进入成熟植物的果实中的话，这将是个了不起的发现。

    Vocabulary:

    Carbon Nanotube:碳纳米管

    Unusual:不同寻常的

    Fertiliser:肥料

    Manure:粪

    Compost:堆肥

    Nitrogen:（化学元素）氮

    Phosphorous:（化学元素）磷

    Potassium:（化学元素）钾

    Expose: 暴露；显露

    Germinate: 发芽

    Seedling: 幼苗；秧苗

    Sprout: 抽芽；抽条；发芽

    Tiny: 微小的

    Cylinder: 圆柱体

    Billionth: 十亿分之一

    Ubiquitous: 似乎无处不在的；十分普遍的

    Breach: 在…上打开缺口

    Rigid: 坚硬的

    Penetrate: 穿透；透过

    Unsprouted: 没有发芽的

    Desiccation: 干燥；失水

    Pampered: 宠坏的

    Cultivar: 品种

    Initial: 初始的；起步的

    Pierce: 刺透；穿透

    Dissolved: 溶解的

    Accelerate: 加速

    Speculate: 推测

    Analogue: 类似物质

    Protein: 蛋白质

    Spurt: 迸发

    Maturity: 成熟

    Organism:有机体；生物 （尤指微生物）