No physical quantity regulates and stimulates the development of plants as strong as light. Light is electromagnetic radiation characterized by its quality (different wave lengths) and intensity. Plants are able to measure both parameters and to react to them. Quite a variety of light-induced and light-dependent reactions exist, which means that plants do accordingly have a number of different light receptors (photoreceptors, sensor pigments). The importance and the mechanism of photosynthesis have already been discussed, the absorption spectra of chlorophyll and the accessory pigments have been shown and compared to the action spectrum of photosynthesis. The aim of photosynthesis is the yield of energy: a flow of photons is transformed into a flow of electrons. In the phenomenons discussed in the following, will we focus especially on the control of energy-consuming processes: phototaxis, for example, is a light-induced movement of organisms consisting of a single or just a few cells. It occurs usually towards the light (positive phototaxis). A movement in the opposite direction is called negative phototaxis.
Phototropism is plant growth towards a light source, typical for multicellular plants though many model experiments were performed with a chlorophyll-free, non-plant egg cell: the sporangiophore of the fungi Phycomyces.
Photomorphogenesis is the light-induced control of plant growth and differentiation. Certain wave lengths function as a signal causing the generation of an information within the cell that is used for the selective activation of certain genes.
Photoperiodism
Most species of the Central European flora are day-neutral, i.e. day length has no influence on their flower formation.
A further topic that covers numerous phenomenons is the endogenous rhythm. It is a priori independent of light, though the length of its periods may be determined by light.
Finally a short notice about flower colours: since insects and other pollinators distinguish reflected light (colours), have plants that developed different signals an advantage.
The action spectrum of a light-induced reaction allows to draw conclusions about the chemical nature of the underlying photoreceptor. It has been discussed for quite some time whether carotenoids or flavins are more important in the absorption of blue light. It seems as if they are of equal importance and that they co-operate in many cases. Just like in photosynthesis does it seem as if the low-molecular chromophore is associated with proteins and maybe also with the membrane. We will therefore have to consider the aspects position and orientation of receptor molecules, too.
Most photomorphogenous processes of terrestrial plants are controlled by bright red and an alternation between red (lambda = 660 nm) and far-red (lambda = 730 nm) light. The accompanying receptor is phytochrome, a protein-chromophore-complex existing in at least two different states that can be transferred from one state to the other upon light exposure.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de