Complex responses shown to light quality

Experiments at Stockbridge Technology Centre's (STC) LEDS4CROPS have shown the complexity of crops' responses to light quality and therefore the complexity of managing this, the facility's manager Dr Phillip Davis told an AHDB Horticulture lighting event last month.

LEDs: inter-lighting systems compared to top lighting in trials carried out at Wageningen University & Research
LEDs: inter-lighting systems compared to top lighting in trials carried out at Wageningen University & Research

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.

Before commenting please read our rules for commenting on articles.

If you see a comment you find offensive, you can flag it as inappropriate. In the top right-hand corner of an individual comment, you will see 'flag as inappropriate'. Clicking this prompts us to review the comment. For further information see our rules for commenting on articles.

comments powered by Disqus

Read These Next

Dutch vegetable open days

Dutch vegetable open days

Brassicas, squashes, salads, roots and alliums were all on show as growers, advisers, agents and buyers visited the main seed breeders' sites, Gavin McEwan reports.

A growing choice - the industry assesses alternatives to peat

A growing choice - the industry assesses alternatives to peat

Industry efforts to reduce peat continue as coir suppliers invest in continuous supply and growers take part in trials to assess alternatives, Gavin McEwan reports.

National Fruit Show 2016 - Business post-Brexit to be a key talking point

National Fruit Show 2016 - Business post-Brexit to be a key talking point

The post-Brexit political and regulatory landscape will be the subject of much attention from growers at this year's event.