Four different light treatments in four greenhouses were used for the tomato cultivar Komeett. In addition to a reference crop with only HPS lighting, equal light intensities were realized by two hybrid systems combining HPS top light and LED interlighting, and a hybrid LED top light and LED interlighting. One of the crops under hybrid HPS and LED interlighting were performed in a greenhouse with diffuse glass. This crop was expected to benefit from more light penetration deeper into the crop, from both artificial interlighting as well as from diffuse light. And that is what happened: 89.1 kg per m2 is 6.6% more than in the reference crop. Under the same hybrid lighting, but without diffuse glass 1.6% more tomatoes were produced than in the reference crop. The combination LED top light and LED interlighting also produced a healthy and well-producing crop. Here, the absence of heat radiation from the HPS lamps which lowers the plant temperature, was compensated for by maintaining a slightly higher greenhouse air temperature. A model calculated the light requirement of the crop, and was then used to provide the right amount of light. Thus, only the required amount of light was given and that was favourable for use of electricity.
Diffuse light in the Winter is positiveThe fact that diffuse light increases the light sum and tomato production in the Winter has been demonstrated once more. More light was measured under diffuse glass than under standard glass, probably because condensation increased the light transmission of the glass. This was the case especially during the Winter, with 5% more production during the Winter as a result. Production is optimized by finding the proper balance between vegetative and generative growth. Temperature plays an important role as well. In this experiment, we learned that the crop can be better controlled, by maintaining an extra stem earlier in the season, in addition to some leaf picking in the top of crop. Both under LED interlighting and diffuse glass, it is advantageous not to pick leaves lower on the stem. After all, these leaves remain green for a longer time. The leaf quality under HPS lamps was clearly the lowest.
Source: Tom Dudek, Wageningern UR Greenhouse Horticulture