Plant growth control by light spectrum : Fact or fiction?

Plants are sessile organisms that have to cope with their environment as it is exposed to them in nature. To do so, they developed systems to sense environmental signals and to integrate these with endogenous developmental programs. As a result, they are well equipped to survive and flourish in very different environments, while it also ensures the timely production of offspring for survival in the long run. In horticulture, aims and environmental conditions for plant growth are often very different from nature. For growers, survival is associated with responding to market and societal demands, and with efficient and exact timed production of high quality plants and products at a minimum of costs (financial and environmental). Greenhouses and plant factories provide growers with means for extensive control of the growth climate, including the light environment. In nature, the light environment of plants is largely determined by the sun. Important characteristics of this natural light include intensity, length of the photoperiod, and the spectral composition, which are eventually modified by neighboring plants and or weather and provide the signals under which plants evolved their mechanisms to cope with their environmental conditions. Therefore, they have become genetically programmed to respond in accordance with these natural light signals. In horticulture, however, applied lighting can be very different from nature. Lengthening the photoperiod and increasing intensity with artificial lighting enabled the culture of plants at times and locations on earth, where this would never have been possible under the prevailing natural light climate. The introduction of LEDs as low-energy alternatives for conventional lamps, adds a new opportunity for control via the light spectrum, which is also not natural at all. Consequently, horticulture crops are now often exposed to light climates that they would never have experienced in nature. Applying lighting to plants beyond their natural comfort zone may also results in unexpected negative effects. This hampers the development of optimal lighting strategies in horticulture. In this paper, two examples of such negative effects on leaf photosynthesis are presented, aiming to induce a discussion on how genetic and physiological knowledge from natural systems may help to develop new lighting strategies for horticultural production in greenhouses and plant factories.