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At current light levels, intermediate lighting has limited usefulness for tomatoes

So far, there have been very few tests to determine if intermediate lighting offers unequivocal advantages. Is it better than the same intensity of additional top lighting? Govert Trouwborst of Plant Lighting writes about this on 'Kas als Energiebron'. That is in the context of a study about tomatoes.

A model was used to calculate when (intensity of top lights and daylight) interim lighting can theoretically be useful at all. That is compared to the same intensity of extra top lighting. The study is part of the 'Fundamental knowledge development on LED lighting for practical application in greenhouses' project.

Pros and cons 
Intermediate lighting has advantages and disadvantages. Its 'gain' is mainly in reducing light losses. According to the model, intermediate illumination perception is ten percent higher than that of top lighting. It also increases photosynthetic efficiency. This is during the lighting season (Dec/Jan) from 200 µmol/m²/s lighting.

However, the model shows that there is less horizontal light distribution with lamps placed between the crops. That leads to leaf scorching (loss of light) and localized high light intensities (loss of photosynthesis efficiency). The plants' lower leaves do not age naturally either. That results in decreased dry matter redistribution to younger plants. Finally, lamps becoming dirty can cause light loss (~1%) too.

Increased yields
This model study shows that intermediate lighting distributed at 100/100 µmol/m²/s (top/intermediate) is not beneficial enough. Not when compared to 200 µmol/m²/s top illumination. If the total light level increases to 300 µmol/m²/s, crop photosynthesis increases by three percent at most. That is when the distribution is 250/50 µmol/m²/s (top/intermediate). At 200/100 µmol/m²/s (top/intermediate) distribution, it rises by five percent, relative to 300 µmol/m²/s top lights.

The potential three percent advantage is too limited to compensate for the disadvantages mentioned. However, there could be a two percent yield increase from the maximum five percent. That may well be interesting. But, it is difficult to prove statistically. If that additional production is a fact, you could justify the investment.

No, unless
For these reasons, Plant Lighting's answer for a conventional Dutch greenhouse is, "No, unless". The model study shows that intermediate lighting appears to offer insufficient added value compared to top lighting. That is for light levels up to around 200-250 µmol/m2/s PAR.

For light levels above 300 µmol/m2/s PAR, it can be considered. The market situation in the year's early months also plays a role. Presently, prices are highest in December and January. Should these become higher in March and April, it could make economic sense to continue lighting longer at a higher light level.

Read more about the ongoing study here (in Dutch).

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