THE INFLUENCE OF THE TYPE OF FLUORESCENT LAMPS ON THE GROWTH OF YOUNG VEGETABLE PLANTS
Normally the types of lamp commonly used for lighting are also used for supplementary artificial irradiation of plants. The influence of light and/or temperature on the growth of the young plants varies due to the - at times considerable - deviations of the spectra of the different lamps. However, light and temperature are essential factors for the plants and must be retained in a favourable mutual relationship so that there is optimal utilization of the light by the plants. Compared to their full radiation, the lamps emit a low percentage of visible light whereas the infrared percentage can vary between low and moderately high. Hence, every lamp has to be examined for its optimal range of use, and which type possesses the most favourable spectrum for the growth of plants must be established.
Young vegetable plants, especially cucumbers and tomatoes, are usually raised by the aid of low- and high-pressure mercury lamps which, if compared to incandescent lamps, yield more light. The incandescent lamp must, however, be mentioned. Its installation is much cheaper, and in some cases it can produce quite serviceable plants, especially if the light/temperature-relationship is well adjusted (figures 1 and 2). The high-pressure mercury lamp (HPL or HQL) yields a little less light but more heat than the low-pressure or fluorescent lamp. This is why the fluorescent lamp, at an equal power consumption, causes a higher production of dry matter in the plants during the dark winter months, whereas the HQL or HPL lamps yield better results from the middle of February onwards. At this time with natural radiation increasing, the temperature factor has more influence, a fact which becomes especially noticeable in the growth of young cucumber plants. Figure 3 illustrates the production of dry matter in cucumber and tomato plants at 2 temperatures after 32 days of additional irradiation (200 W/m2 net) between January and May. Because of their greater heat emission, the distance between the surface of the growing medium and HQL and incandescent lamps was 1 m; for the TLF lamps the distance was 0.50 m, due to their lower heat production, and in order to achieve a fuller light utilization. In January, with temperatures of 25°C during the day and 18°C at night,