Seeding the seeds

Science & Technology
Nanobiotechnology

Seeding the seeds
Nov 5th 2009
From The Economist print edition

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, is it 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.