FEW breakthroughs in arable farming can claim to bring bigger yields and tastier, healthier food without manipulating a single gene or adding a single chemical. It may sound bizarre, but all it takes is some sheets of coloured plastic spread under the growing plants. Light reflected from the plastic does the rest.
The story of this momentous discovery begins in the 1940s with a small boy growing up on a farm in Iowa. Kids ask some funny questions, but what do you say to a child who wants to know why pigweed plants are easier to pull up when they are growing close together than when they are alone? Young Michael Kasperbauer noticed that when the plants were crowded together they had fewer roots, but no one could tell him why. So Kasperbauer decided to find out for himself.
Over the decades his quest has led him from an esoteric investigation to a practical discovery that has the potential to change agriculture for good. By answering his own question, Kasperbauer has found a way to trick plants into increasing their yields of edible leaves, fruits and vegetables鈥攗sing coloured plastic. The same technique can ward off pests, make plants produce more anticancer chemicals and even improve their flavour.
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In 1961, Kasperbauer got his first academic post with a research team at the US Agricultural Research Service (ARS) in Beltsville, Maryland. Just before he arrived, the researchers there had made a discovery which was to prove crucial to his subsequent work. They identified a pigment called phytochrome that switches between two different forms when bombarded with light of particular wavelengths. Red light with wavelengths around 660 nanometres converts the pigment to Pfr鈥攖he active form which stimulates growth responses鈥攚hile far-red light, which has a slightly longer wavelength, turns Pfr back to the inactive form, Pr.
Light work
We now know that there are five genes coding for a family of phytochromes that help plants respond to light (鈥淪ensitive flower鈥�, 快猫短视频, 26 September, p 24). How they work is not yet fully understood, and back in the early days the picture was even more confusing. For example, although the Pr form of the pigment is biologically inactive, the Beltsville researchers found that its accumulation did seem to lead to growth under certain circumstances. When they exposed seedlings to far-red light at the end of the day鈥攃onverting Pfr to Pr鈥攖heir stems lengthened. But given a few minutes of red light after the far-red dose, the plants鈥� stems remained short. The results were contradictory, but one message was clear鈥攍ight of different colours affects growth in very specific ways.
These were exciting times and the findings fuelled Kasperbauer鈥檚 curiosity. He hadn鈥檛 been at Beltsville long before he was asking another funny question. Would phytochrome respond to light coming up from beneath the plant onto the lower surface of the leaf? 鈥淭he initial response was that, well, the Sun is usually above the plants,鈥� he remembers. 鈥淲hen the Sun goes below the plants, we鈥檒l look at it.鈥� Before long, however, the team had set up some experiments. And sure enough, red and far-red light from below had exactly the same effects on plant growth as from above.
It was the first clue to solving the pigweed puzzle, but its significance wasn鈥檛 apparent until Kasperbauer moved to the ARS鈥檚 Coastal Plains Soil, Water, and Plant Research Center in Florence, South Carolina, and started doing plant spacing experiments. He wasn鈥檛 surprised to find that plants placed close together had fewer roots. They also had taller stems and there seemed to be a trade-off between the two, as if the crowded plants could sense competition and were channelling their energies into reaching towards the sunlight. The Beltsville work had centred on growth, light and control, and here again were these familiar elements. It got Kasperbauer thinking about phytochrome, and then he wondered whether the light reflected up from the leaves of neighbouring plants might be the key.
In 1967, he dragged a primitive portable spectrophotometer into the field, which revealed that neighbouring plants do reflect high levels of the invisible far-red light onto nearby leaves, increasing the far-red to red ratio. Kasperbauer was convinced that plants were using this type of light as a signal to invest more energy in growing taller. What鈥檚 more, his next experiments showed that it didn鈥檛 matter what was reflecting light up onto a plant鈥檚 leaves 鈥攃oloured soil, straw mulch and dead plant residues left over from last year鈥檚 crops all had an effect. Soya bean seedlings grown over brick red soil, for example, grew taller and had fewer roots than plants grown over white soil where the ratio of far-red to red light reflected is lower.
Runaway colour
The problem with these experiments was that the straw mulch and coloured soils got washed away by rain or blown away by wind, so Kasperbauer and his Florence colleague Patrick Hunt started to use red paint on the black plastic sheets or 鈥渕ulch鈥� used by farmers to seal in moisture, warm the soil and stifle weed growth. Finding the perfect reds to increase the ratio of far-red to red reflected light was the difficult bit, but once achieved the plants consistently allocated more growth to shoots and fruits. The responses to competition were clear鈥攇reater height to maximise a plant鈥檚 photosynthetic potential and more seeds to increase the chances of dispersal for the next generation. 鈥淭he plant can鈥檛 get up and move,鈥� says Kasperbauer. 鈥淚t either adapts or dies.鈥�
Until then, Kasperbauer had been working on a non-commercial scale. In 1986, at the suggestion of horticulturist Dennis Decoteau鈥攏ow at the University of Pennsylvania, Philadelphia鈥攈e tried the red plastic sheeting underneath a tomato crop in the field. The result, that year and the next, was an impressive 15 to 20 per cent higher yield than plants grown over traditional black or clear plastic. Experiments using cotton plants were equally successful. It was time to take out a patent. The product hit the US market in 1996, and last year sales increased eightfold, although total sales are still very modest. At present, those benefiting most are small-scale producers of valuable crops, such as tomatoes, who want to get their produce to the market as early in the season as possible. But the distribution company is working on ways to produce mulches more cheaply so that more farmers can afford them.
Meanwhile, Kasperbauer and his team had been experimenting with orange, blue, green and white sheets in an attempt to improve the yield of root crops such as turnips and carrots. Their aim now was to minimise the amount of far-red light reflected off the ground, in the hope that this would trick plants into increasing root growth. The increased yield was not as great as with tomatoes, but there was a surprising bonus.
While working on turnips, some students in the lab from nearby Clemson University, South Carolina, asked Kasperbauer if the coloured plastics changed the taste of the vegetables. 鈥淢y response was to say, `Get the knife, we鈥檒l find out.'鈥� They discovered that turnips grown under blue plastic had a good, sharp flavour whereas roots grown under green sheets were more bland. 鈥淓veryone could taste the difference, except for one guy who was a heavy smoker,鈥� remembers Kasperbauer.
What was going on? To find out, the researchers sent samples to George Antonious of Kentucky State University in Frankfort. Antonious analysed the vegetables for compounds called glucosinolates that give turnips and horseradish their characteristic 鈥渂ite鈥�. Roots grown under blue plastic contained higher concentrations of glucosinolates and vitamin C. Those grown under green had raised sugar concentrations. Kasperbauer believes that light reflected up from the blue sheeting must influence an enzyme involved in the metabolic pathway that converts glucose to glucosinolates, resulting in the altered root flavour. The latest study shows that coloured sheeting can also make strawberries sweeter. This time the researchers compared results from fruits grown over ordinary black plastic with those grown over a bright red layer, which reflected high levels of far-red light. 鈥淭he yield was more with the strawberries, and the berries were bigger,鈥� says Kasperbauer. 鈥淲e also felt that some days we could taste the difference between red and black.鈥� Unpublished chemical analysis reveals that the red mulch changes the ratio of sugar to acids.
Good flavour is important, but Antonious and Kasperbauer are even more excited by the prospect of using colour to change the nutrient content of fruits and vegetables. Glucosinolates, for example, do more than just give turnips bite鈥攖hey may also reduce the risk of certain cancers. And the researchers are now looking at carotenoids鈥攕uch as betacarotene in carrots鈥攚hich help protect the body from destructive chemicals called free radicals. For three years the team has been growing carrots under sheeting of various colours. 鈥淲e have huge differences between the colours in the sugar content and the betacarotene component,鈥� says Antonious. But he is not giving anything away until the results are published later this year.
Antonious plans to analyse the carrots and turnips for important minerals such as iron and calcium. He will also look at how the different colours affect the amount of two dozen nutrients in salad greens and dried or frozen samples of vegetables grown over the past few years. In the future, says Antonious, farmers may produce health-promoting crops without having to resort to genetically engineered varieties, simply by using a brightly coloured plastic on their fields.
Coloured plastic might also protect the health of the plants themselves, and reduce the need for pesticides. 鈥淲e know for sure鈥攂ut this is in its infancy鈥攖hat there is a positive response to controlling pests,鈥� says Michael Orzolek of Pennsylvania State University in University Park. Aphids and the plant viruses that they transmit, for example, seem to be attracted to yellow and repelled by silver. In general, yellow attracts pests, says Orzolek, while red and blue are their least favourite colours鈥攁s luck would have it, the same colours that Kasperbauer has had most success with in modifying plant growth and flavour.
It is not clear how colour cues influence pest behaviour, but the researchers have discovered one indirect effect. Plants grown over red plastics seem to be affected less by nematodes simply because they have fewer roots for the worms to attack. Kasperbauer and Bruce Fortnum, an entomologist from Clemson University, sterilised soil to remove all nematodes, then planted tomatoes over black and red plastic. They then inoculated the roots with different numbers of nematodes ranging from none to 200 000 eggs per plant. Plants inoculated with 200 000 eggs and grown over black sheeting produced only 3.5 kilograms of tomatoes, while plants inoculated with the same number of eggs and grown over red plastic produced almost 8 kilograms.
With such dramatic results, Kasperbauer and his colleagues believe this simple technology has huge potential. Already, bright red fields are becoming more common. Soon commercial root crops could be grown in a sea of blue plastic. The curiosity of an Iowa boy looks set to change the face of the countryside, replacing brown earth with a rainbow of colours.
