At a basic level, the quantity of light has an effect on the crop, not only bringing it to sales readiness earlier or later but also determining whether it will have robust leaves that are more resistant to pest and disease damage, he said. "But that means more units and higher running costs, so what is the optimum light intensity?"
More importantly from the point of view of LED lighting though, he said: "The colour of the light affects the plant's photosynthetic efficiency, its morphology and biochemistry. The plant has separate responses to infrared, red, blue and ultraviolet light. You can ruin a plant with the wrong light."
In tomatoes, for example, higher levels of blue light keep the plants compact, while experiments growing lettuce under 25 different light "recipes" yielded very different morphology across the spectrum from 100 per cent red to 100 per cent blue, said Davis. "But for a compact plant you need both red and blue."
Far-red light (between red and infrared in the spectrum) "makes plants stretch out again" while also slightly increasing mass, he added. "With cucumbers, for example, you need a tall plant for high-wire growing," said Davis, adding that while far-red counteracts the red light, green light appears to counteract blue. "We now have quite a robust model of how they will react."
Even the leaf colour of crops such as lettuce can be strongly influenced by light quality, he explained, by affecting the relative quantities of anthocyanin and chlorophyll. But he cautioned: "There is not much data on this yet."
Similarly, flavour can be manipulated, for example to produce a pungent or sweet basil plant, said Davis. "There is an opportunity to produce consistent high-value plants of the same quality year round." He added of this lab-based work: "We will take this into glasshouses to see whether we can retain the responses."
STC's other main recent research thrust in protected horticulture has been to compare lighting regimes for tomatoes in a specially adapted glasshouse. This comprises four compartments, one employing LED topand inter-lighting under diffuse glass, another the same under standard glass, one combining high-pressure sodium (HPS) top-lighting and LED inter-lighting, and one with HPS only.
"Diffuse light gets deeper into the crop and casts less shadow, but for us it didn't get quite the same yield, which may have been a question of temperature," said Davis. "Diffusion takes out heat from the sunlight so you have to add more."
All treatments had the same total energy consumption throughout the year. LED running costs were 83 per cent of those of HPS, but temperature was raised to compensate for their lower heat output. "Electricity costs a lot more than heating, so it's better to use more heat than more expensive-to-run lighting," said Davis.
"Screens would reduce the heat use so give bigger savings." One point in LEDs' favour is that blind taste tests rated fruit grown in this way as sweeter, he pointed out.