Tomatoes are warm-season crops that prefer growing temperatures of at least 20 °C during daytime and are sensitive to cold weather and frost [14]. They can be divided into categories depending on their growth habit, and the two main ones are determinate and indeterminate tomato varieties [13]. Determinate varieties are characterized by a shorter growth period, a more compact and bushy shape as well as a more uniform ripening time with fruits concentrated to the terminal end of the crop. The indeterminate varieties on the other hand have a longer growth period, are taller and shaped like a vine instead of a bush, with flower and fruit formation along the side of the stem and continuous ripening throughout the growing season. There are also semi-determinate tomato varieties that have combined characteristics from the two main types [13].
Since tomatoes are a very sun-loving crop, greenhouse tomatoes perform best under relatively high light intensities and long photoperiods of about 14-20 hours [5] [15]. Light quality is also a very important aspect for tomato development, and a full LED spectrum have been proven to significantly increase yields compared to narrowband red and blue spectra [11], as well as HPS lighting [18]. A relatively large proportion of red light in the broad spectrum promotes higher yields [16] and earlier ripening [17] and are balanced out by a blue component that for instance prevents excessive elongation of the plant [16].
A recent study investigated the energy demand for greenhouses in varying climates that replaced HPS with LED lighting through a model simulation. The result of the study, that included greenhouses from Chengdu, China in the south to Kiruna, Sweden in the north, showed that a 40% decrease in energy input for lighting can be obtained in most cases when transitioning to LEDs. Because of the loss of radiant heat from the HPS lights the LED greenhouses required more heating, especially in the winter. But when the total energy consumption was calculated, the report concluded that the predicted energy savings from a transition to LEDs still would be 10-25% [22].
