The incoming solar radiation covers a wide spectrum of wavelengths but only a relatively narrow range between 400–700 nm, roughly half of the solar irradiance, is used in photosynthesis. Within this range the photosynthetic photon flux density (PPFD) describes the number of photons which hit a leaf and are used for photosynthesis. Under natural conditions the number of photons can be substantially larger than 2000 μmol m-2 s-1. The efficiency with which plants utilize light for the production of biomass varies with time, light intensity, temperature and water availability and additionally it is affected by:
- the angle of incoming light (in relation to the leaf)
- the spectral composition of the radiation
- the intensity of diffuse or direct radiation
However, despite a rather complex interaction of plants with light, it has been shown in particular under field conditions, that biomass formed per integrated unit of intercepted light and conversion efficiency (LUE Light Use Efficiency, g MJ-1) is rather constant (Monteith 1977).
Monteith, J. L. (1977). "Climate and efficiency of crop production in Britain." Philosophical Transactions of the Royal Society of London.Series B, Biological Sciences 281: 277-294.
Light Quantity in controlled environments
In controlled environment of growth cabinets light is provided by fluorescent tubes, high intensity discharge (HID) lamps or, in a recent development by light emitting diodes (LEDs). Several points are essential here. First, the question of how homogenously the light intensity is distributed over the growth room needs to be considered. This is particularly the case when point sources such as HID lamps are used, where variation in light intensity of more than 50% at plant level may occur locally. Densely spaced fluorescent tubes or LEDs may provide better distributed light levels. The light intensity can drop by 20–50% close to the walls of a growth room. This reduction is partly mitigated by using clean reflective mirrors on walls. A white coating may also help, but is less effective. Light intensity generally increases the closer plants grow to the light sources (i.e. with increasing plant height). This is especially relevant for plants that become taller than 40 cm and vertical gradients are more prominent closer to the lamps and when plants are not sufficiently spaced and shade each other (Niinemets and Keenan 2012). Finally, light output also decreases with the age of the lamps. Measuring at the beginning and the end of the experiment and at least once every two weeks in longer experiments over various positions in the growth room is recommended to characterize light quantity.
In glasshouses, the intensity of the natural light varies with the geographical location. Glasshouse covers, frames and lamp fixtures hanging over the plants usually block 25–50% of the incoming radiation (Max et al. 2012). Still, PPFD inside can be over 1100 mmolm–2 s–1 on clear summer days. In winter or with cloudy weather intensities can go lower than 100 mmolm–2 s–1. Additional lamps can help to add extra quanta for plant growth and also extend the light period in the winter season to a defined duration. Although seemingly trivial, cleaning of the glasshouse cover from accumulated dirt helps to increase light availability as well. As mentioned before, peak irradiance is a poor characterization of a glasshouse light environment and should at least be supplemented with a value for the daily quantum input averaged per month or over the experimental period.
Max JFJ, Schurr U, Tantau HJ, Hofmann T, Ulbrich A (2012) Greenhouse cover technology. Horticultural Reviews 40, 40 259-396.
Niinemets, U. and T. Keenan (2012). "Measures of light in studies on light-driven plant plasticity in artificial environments." Frontiers in Plant Science 3.