Cool rays for red roses

Plants are sensitive souls. Take the colour red, for example. When growth is concerned, they differentiate very precisely between red and far-red light, so called because it is at the far end of the visible spectrum, close to the infrared region. Both are present in sunlight in roughly the same proportion, but only red light actually makes plants grow.    

This sensitivity is the result of photoreceptors, also known as phytochromes, which control plant growth much in the manner of a sensor that is activated by red light with a wavelength of around 665 nanometres and deactivated by far-red light with a wavelength of around 735 nanometres. Plants grow when the phytochromes are exposed to red light, whereas exposure to far-red light halts the growth process. Little wonder then that the wrong type of light can soon make horticulturists see red.
   
Yet help is now on hand at least for plants that are grown under glass. This is because Merck has developed Solarflair, a special pigment for greenhouses that blocks far-red solar radiation without affecting the transmittance in the red region. The Solarflair pigment is a powder that is simply mixed with a bonding agent and sprayed onto the roof and walls of the greenhouse.   

The pigments achieve their effect according to the laws of physics. The individual pigments consist of tiny transparent flakes of mica covered with an extremely thin coating of titanium dioxide. Since each material has a different refractive index, the rays of light change direction as they pass through the boundary layer between the two. In fact certain rays of light change direction to such an extent that they no longer pass through the boundary layer and are instead reflected away. The colour of the light rays that are reflected in this manner depends on the precise composition of the pigments. Indeed the latter can be tailored in such a way that a large proportion of far-red light is reflected, whereas red light still passes through without hindrance.
   
In effect this kills two birds with one stone. Since the Solarflair pigments reflect rather than absorb the far-red radiation, its energy does not get converted into heat either on the surface of the greenhouse or inside. This makes it easier to control the greenhouse climate, particularly in the summer, and maintain the temperature at a level conducive to optimal growth. “Roses,” explains Silvia Rosenberger, technical consultant at Merck, “like it to be light up top and cool around their roots.”

In a joint project with the Dutch company Mardenkro B.V., a major manufacturer of greenhouse shading systems, Rosenberger has now been able to witness the economic benefits of this development at first hand. The two companies have developed the liquid shading agent “ReduHeat,” which is based on Solarflair. “The use of the Solarflair pigments gives ReduHeat a gain of up to 20 percent in transmitted light with a simultaneous temperature reduction of two degrees Celsius,” she explains. “This in turn boosts productivity by up to 20 percent or more.”

A further advantage of Reduheat comes into play during autumn, when the sun’s power begins to decline. Then the coating can be washed off and disposed of with very little trouble. “Solarflair,” Rosenberger emphasises, “does not contain any toxic substances.”