One trial compared growing midi cucumbers on a high-wire format in Poland under three lighting regimes: high-pressure sodium (HPS) toplighting and LED interlighting; both LED topand interlighting; and only HPS toplighting, each in a 60 sq m compartment at 3.14 plants per square metre.
These were first planted in December 2014. Over the four months of the trial, the all-LED plot yielded 43.1kg of fruit, compared to 40.6kg for the hybrid format and 34.7kg for the HPS-only control - indicating an uplift of 24 per cent for the all-LED format.
A further crop begun in December 2015 then tested the proposal that higher planting densities, hence higher yields, would be possible under LED lighting. Indeed at 3.66 plants per square metre, the all-LED format yielded 54.3kg in four months, and the hybrid 47.5kg, while at the lower density the yield under HPS also improved to 39.6kg, but this still left the all-LED system 37 per cent ahead.
"With 100 per cent LED the fruit development is also faster while the water use is nearly 14 per cent lower," Van Baar added. "But adding LED interlight to an existing HPS system can also give you more grams per mol of light."
A separate trial at a Dutch commercial grower in a glasshouse too low for overhead lighting assessed what benefits if any are brought by double LED interlighting booms between high-wire long cucumber aisles.
"With 18 hours of lighting there was significantly more production," said Van Barr, adding that this had the side-effect of raising temperature over much of the trial period. The interlighting appeared to promote earlier production, gave fewer signs of Mycosphaerella infection and yielded fruit with five-to-seven days longer shelf life.
Trials are also ongoing to ascertain the value of LED lighting in pepper production, he said. "Interlighting appears to strengthen the pepper plant and boost production by seven-to-10 per cent, but it takes two months for the crop to grow high enough to fully benefit." LED toplighting remains another area for further work, he added. "It should make winter pepper production possible - you can't do it with just interlighting."
He estimated that around two per cent of the Netherlands' glasshouse area now uses some form of LED lighting, but pointed out: "You need a specialist to work out the most efficient light distribution for your crop."
CGA technical officer Derek Hargreaves noted: "HPS is effective but not profitable - not due to the lighting, but to the price of cucumbers."
LED lighting further complicates the already complex interactions of factors governing the behaviour of pests, beneficial insects and the growing environment, Stockbridge Technology Centre (STC) director of entomology and sustainable agriculture Dr David George told the conference. These include visual, chemical and physical cues, he said. "Plants alter their chemistry in response to pests, to limit damage and attract biocontrol agents, and the nature of this response can be affected by the type of light."
Enclosed LED systems such as the LED4Crops facility at STC "require back-to-basics thinking" to optimise pest monitoring, choice of beneficial insects and pollination, he said. "Supplemental lighting in glasshouses and polytunnels could present similar opportunities and challenges."
His STC colleague Dr Phillip Davis, who manages LED4Crops, explained the growing understanding of individual crops' lighting needs. "Light colour influences leaf pigmentation and morphology," he said. "Flowering is inhibited by red light but promoted by far-red light. Under blue light you can get lettuces that are almost black, and so high in anthocyanins. Blue also helps to open stomata, though there is a trade-off between biomass and quality. Plants are very sensitive to UV-B light as it's very damaging."
With all this in mind "we can start to design a lighting regime that gives you the kind of plant you want, including by supplementing sunlight in a glasshouse", he said. "Selecting light spectra can improve quality and efficiency, and plants will benefit from different spectra at different stages of growth."
He added: "We are hoping to look next at the interaction of light and pathogens - but informally we seem to be enhancing plant immunity with LEDs."