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.

    碳納米管不同尋常的用途：肥料

    在19世紀初期以前，人們使用糞、堆肥以及灰燼作為肥料已有很長的歷史了。但是，人們一直不明白這些肥料是如何起作用的，到19世紀化學科學發展起來以后，人們才弄清楚這些肥料為植物體提供了氮、磷、鉀這些植物成長所需要的營養元素。現在，類似的情況可能正在發生，只不過這一次是與一種完全不同的肥料有關。在種子發芽之前，如果把種子用碳納米管處理之后，這些后來發芽的種子幼苗生長得更快、更大，但是其中的原因目前還沒有完全弄清楚。

    碳納米管恰如其名字所揭示的一樣，是由碳原子構成的一種圓柱體。人們認為碳納米管會有許多意想不到的用途，尤其是在電子學領域，但是碳納米管和其它的納米粒子（之所以稱之為納米粒子，是因為測量它們的大小是以納米為單位，一納米是10億分之一米）同時也讓人們擔心，因為如果碳納米管和其它的納米粒子變得無處不在的話，人們害怕它們會干擾細胞的正常工作，從而會對生物有害，其中也包括對人不利。

    對于植物來說，過去十年里的少數幾項研究表明，有些納米粒子確實能突破圍繞在植物細胞周圍的堅硬的細胞壁。不過，來自阿肯色大學小石城分校的（University of Arkansas at Little Rock） Mariya Khodakovskaya 和 Alex Biris認為這說不準也是一個好事。因此他們想到納米粒子有可能透過圍繞在不發芽種子周圍的堅硬外殼。

    他們在這方面的研究之所以有興趣是因為這些植物種子周圍的外殼的作用有好有壞。這些外殼能保護種子---以及正在發芽的幼苗---不失掉水分以及免于其它的物理上的破壞。不過在種子最終遇到適合生長的土壤時候，這些外殼減慢了種子吸收營養的速度。對于野生種子來說，這種作用是有意義的，但是對于莊稼用的種子而言，這種作用就沒有必要了。而且對于即使是最好的肥料來說，如果種子在初期不能發芽成長，它們也無能為力。

    但是如果Khodakovskaya 和Biris博士的研究結果正確無誤的話，那么納米管或許可以解決上述問題。研究人員們推理認為，如果碳納米管的確能穿透一般的細胞壁的話，那么它們或許也能穿透更加堅固的種子外殼。這樣的話，水以及溶解在水中的營養物質都能進入到種子內部，從而提高種子在初始階段的快速成長。

    上述想法得到了研究證實。Khodakovskaya 和Biris博士以及他們的同事們對西紅柿種子做了實驗，他們讓西紅柿種子在標準的植物生長環境中發芽，然后對比在摻有納米管的生長環境下和沒有摻納米管的生長環境下的結果。他們的研究結果發表在美國化學學會（ACS）雜志《納米》（Nano）上。研究顯示在有碳納米管存在的環境中，種子在3天內就開始發芽了。在沒有碳納米管存在的環境中，種子6天后后才開始發芽。而且，發芽早的種子對隨后的生長也有利。在第二十七天的實驗中，研究者們測量了這些植物的干莖長度、根系長度以及整個植物的重量。對比兩種條件下的植物生長情況，它們的根系長度是一樣的，但是受到碳納米管處理過的植物的平均干莖長度為6厘米，而沒有受到碳納米管處理過的植物的平均干莖長度為3厘米。兩種條件下生長的植物的重量差別更大。受到碳納米管處理過的植物，其平均重量超過了150毫克，而沒有收到處理的植物的平均重量只有60毫克。

    種子的這種提早生長是否僅僅歸因于碳納米管對種子外殼的穿透、或者是一種更為復雜的現象，目前仍然不得而知。可以肯定的是，在發芽的前幾天里，受碳納米管處理過的種子要比沒有處理過的種子多吸收了將近50%的水（因此也多吸收了近50%營養）.不過，當研究者們在電子顯微鏡下觀察種子幼苗的組織的時候，他們發現碳納米管實際上進入到了活的細胞中。他們推測，這不僅僅是讓更多的水進入到種子中的問題，也關系到讓碳納米管本身進入到細胞里面的問題。也許碳納米管扮演著和天然蛋白質通道類似的角色，這些通道控制著水進出細胞。然而，這就和在19世紀之前人們對傳統肥料的認識一樣，沒有人確切知道它們是如何工作的。

    Khodakovskaya 和 Dr Biris博士觀察到種子在早期有這種突飛猛進的生長，但是人們同樣也不知道這種"快長"是否導致植物早熟或者作物產量更高。這還需要進一步的實驗來證實。關鍵是目前還不知道這些碳納米管是否會進入成熟植物的果實中。因為Khodakovskaya 和 Dr Biris博士的實驗表明，碳納米管確實能對生物體有顯著的影響，如果能知道碳納米管是否會進入成熟植物的果實中的話，這將是個了不起的發現。

    Vocabulary:

    Carbon Nanotube:碳納米管

    Unusual:不同尋常的

    Fertiliser:肥料

    Manure:糞

    Compost:堆肥

    Nitrogen:（化學元素）氮

    Phosphorous:（化學元素）磷

    Potassium:（化學元素）鉀

    Expose: 暴露；顯露

    Germinate: 發芽

    Seedling: 幼苗；秧苗

    Sprout: 抽芽；抽條；發芽

    Tiny: 微小的

    Cylinder: 圓柱體

    Billionth: 十億分之一

    Ubiquitous: 似乎無處不在的；十分普遍的

    Breach: 在…上打開缺口

    Rigid: 堅硬的

    Penetrate: 穿透；透過

    Unsprouted: 沒有發芽的

    Desiccation: 干燥；失水

    Pampered: 寵壞的

    Cultivar: 品種

    Initial: 初始的；起步的

    Pierce: 刺透；穿透

    Dissolved: 溶解的

    Accelerate: 加速

    Speculate: 推測

    Analogue: 類似物質

    Protein: 蛋白質

    Spurt: 迸發

    Maturity: 成熟

    Organism:有機體；生物 （尤指微生物）