5.1.1. Light

Light is one of the most important environmental variables influencing plant growth and development. Plants not only use light as a source of energy, but also extract valuable information about their environment from the light quality. Plants can perceive the light intensity, quality (colour), direction and photoperiod; as a result, they adjust their growth, composition and development accordingly. Failing to provide the plants with an adequate light environment can result in reduced yield, altered development and even death. In many cultivation systems light intensity and quality depend on the (geographical) location and weather conditions. In this case, to match the available light conditions, the right management practices like e.g.: species, sowing date and pruning are essential. In protected plant cultivation it is, however, possible to shade or provide additional light to plants. Here we list some considerations when able to influence light as a cultivation factor.

Light intensity should be high enough to properly drive photosynthesis but low enough to prevent photodamage and waste of unused light. As photosynthesis is a process driven by the number of absorbed photons between 400 and 700 nm, i.e. photosynthetically active range (PAR), the proper unit to use in plant sciences is the photosynthetically active photon flux density (PPFD). Typically, photosynthesis linearly increases with light intensity up to ~100 mmol.m^-2.s^-1 for a species like tomato (actual value heavily depends on species and growth conditions); at higher light intensities, photosynthesis saturates. The shape of a photosynthesis light response curve is similar to a rectangular hyperbola curve. Arabidopsis used for research and tomato plants under production conditions are grown under light intensities in the order of 150 and at least 350 mmol.m^-2.s^-1, respectively. Full sunlight has a light intensity of 2000 mmol.m^-2.s^-1.

Light spectral distribution (i.e. quality or colour) should match the natural light present in the environment were the particular plant species to cultivate evolved. Although artificial light sources matching sunlight are available, they are currently (2019) uncommon, expensive and might be unreliable (e.g. plasma lamps). Moreover, although plants are commonly grown under fluorescent tubes or high-pressure sodium lamps, they show altered development in compassion with plants grown under sunlight. Therefore, in greenhouses there is often a combination used of artificial light and sunlight. In the northern hemisphere there is, nevertheless a significant reduction of production in winter time. Indicating that light requirements are only partly met by the artificial lighting. In completely controlled growth environment (e.g. a climate room) light emitting diodes (LEDs) are the often used for practical reasons like low energy consumption, heat radiation to the plants, space usage, maintenance requirements and life expectancy. However, use of LED light for growing plants has its risks, especially when using monochromatic LED light. Some plants show adverse responses (e.g. chlorosis in cucumber) if the balance between blue and red light is not appropriate (Hogewoning et al. 2010). Similarly, failing to add far-red and or ultra violet light might also result in adverse responses (e.g. oedema in tomato).

Additional to the spectral distribution, the photoperiod has a profound effects in plants. For example, the duration of the day influences flowering in many species. In addition, the length of the period itself can have drastic consequences on plant physiology as plants are adapted to a 24 hour day period. Growing plants at longer, shorter or absent periods decreases plant fitness and can even be lethal for some species (Dodd et al. 2005; Velez-Ramirez et al. 2011).

Plants also perceive light direction, which allows them to properly orientate growth towards the light. Blue light is responsible for this process known as phototropism. When using artificial light in horticulture, problems might arise (e.g. leaf deformities in chrysanthemum) when direction and quantity of blue light are not properly balanced (Wim van Ieperen, personal communication).

Due to intensive research on plant-light relations, more and more information becomes available. Although not many studies on the effect of light in relation to plant composition has been done. In §5.5 we discuss the current knowledge on this topic.

This paragraph is copied from and based on Velez-Ramirez (2017)