About twenty years ago the question arose as to whether lamps with an emission spectrum adapted to the characteristics of the plant were to be recommended for plant irradiation.
So far this question has not been answered conclusively (Helson, 1965; Ruf, 1966; Schoser, 1966; Bickford, 1967; Ruge, 1967; Thomas & Dunn, 1967a, b).
It has been stated that lamps emitting radiation in the blue (400–500 nm), in the red (600–700 nm) and in the far-red part of the spectrum (700–800 nm) would be the most suitable.
This statement is based on action spectra of photoprocesses occurring in plants and on absorption spectra of the corresponding pigments.
It will be clear, however, that one cannot compare experimental results obtained under very strict laboratory conditions with plant irradiation in glasshouses or in growth rooms.
Two photoprocesses can be distinguished: 1. photosynthesis, i.e. the formation of carbohydrates, and 2. photomorphogenesis, i.e. the formative effect of light on internode elongation, leaf development, flowering of daylength sensitive plants, etc. (Meijer, 1968). The action spectra of these processes show a minimum of activity in the green part of the spectrum, as a consequence of a minimum of absorption of green light by the corresponding pigments.
This does not mean that green light is not utilised by plants.
In photosynthesis green light - as far as it is absorbed - is even more efficient than blue light.
And plants do absorb green light (Gaastra, 1959, fig. 9).
The pronounced elongation of plants exclusively grown in red light is not the result of a promotive effect of red light on stem lengthening but due to a lack of blue light (Meijer, 1959). In a glasshouse sturdy plants can be obtained with supplementary red light as daylight supplies sufficient blue radiation.
The elongating effect of far-red light depends strongly on the intensity of the visible radiation.
The higher the light intensity, the less effective far-red radiation is (unpublished results).
The effect of light quality on plant growth was studied by growing gherkin seedlings in light of four different types of fluorescent lamps.
High pressure gas discharge lamps were used in experiments on supplementary lighting in glasshouses with tomato and cucumber.