Growth of tomatoes under Hybrid LED and HPS lighting

The use of LEDs can be promising for greenhouse horticulture, but before it can be put into practice on a large scale more knowledge must be acquired on effects of LED lighting on crops. Furthermore, the growers will have to learn to grow their crops under LEDs and the efficiency of LEDs must increase even more. In order to gain more insight into the influence of LEDs on crop growth and production, an experiment was performed in the Wageningen UR greenhouses with a small Santa type tomato (‘Sunstream’) from October 2009 to June 2010. Four lighting treatments were applied, with each treatment in a separate greenhouse compartment: top lighting with HPS (1) or LED (2), and hybrid lighting with HPS above the crop in combination with LED lighting above the crop (3) or in between the canopy (interlighting) (4). The light intensity from the lamps in all treatments was maintained at 170 µmol m-2 s-1. The light was 50/50 divided between HPS and LED in the hybrid treatments. The climate in each treatment was adapted to the needs of the crop in each lighting system. The various lighting systems resulted in different greenhouse climates, in which more heating was required in the LED treatment and the least heating in the hybrid with interlighting. A strong crop developed under LED alone, and to maintain a proper crop balance the fruit load was altered by maintaining an extra tomato fruit per truss and increasing the stem density relative to that under HPS. The leaves of tomato grown under HPS were thinner and aged more rapidly in the winter than in the other treatments. Leaves lower in the canopy under LED alone or hybrid treatments had a higher photosynthesis capacity in the winter than leaves developed under HPS lighting. Differences in production were small, although the production under all LEDs was lower. There were only small differences in fruit quality. The amount of energy required per kilogram tomato was highest in the LED treatment and hybrid with top LED lighting. This was primarily due to the fact that a higher air temperature was necessary and these LEDs were cooled and the cost of cooling added to the use of energy. The consequences and future perspectives of the different types of supplementary lighting for crop growth and production as well as for crop management practices will be discussed.