In 1959, Skierkowski [1] published the results of giving additional illumination to glasshouse tomatoes grown at Skierniewice. In these experiments which started between November 22 and 30, fluorescent tubes with a total light intensity of 4000–5000 lux were used. He studied: 1) duration of illumination of seedlings and transplants for 18, 30 or 62–69 days after germination; 2) applying additional light for eight hours in three different parts of the night; 3) the effect of three kinds of flourescent light (daylight, blue and white light); 4) arrangement of the tubes — either all above the plants, or half of the tubes between the potted plants. In all these treatments the growth of the plants was studied and fruit yields were recorded.

Continuing the experiments, Skierkowski [2, 3] paid special attention to the timing of the additional illumination. For 42 days, daily six hour periods of illumination (4200–4500 lux) were given as follows: 1) 4–10 p.m., 2) 6 p.m.–12 midnight, 3) 8 p.m.–2 a.m., 4) 10 p.m.–4 a.m., 5) 12 midnight–6 a.m., 6) 2–8 a.m. Control and illuminated plants were all kept at 14–15° C at night and 18–22° C during the day.

Plants grown with additional illumination, given either in the morning or at night in such a way that the length of day was extended, were somewhat better than those illuminated during the natural night. However, the differences between series 1, 2, 3, 4, 5 and 6 both fresh and dry weights, were relatively small and statistically nog significant. The plants illuminated from 8 p.m.–2 a.m., however, showed chlorotic spots on their leaves and tended to die; they were even smaller than the control plants which were not given additional illumination. Plants of all these six series were planted in a normal tomato glasshouse, and fruit yields recorded.

The results clearly demonstrate the importance of timing the additional illumination: 1) Good results are obtained by giving supplementary light immediately following the daylight hours, or in such a way that the dark period in the evening does nog exceed two hours. 2) The worst results are to be expected from a four hours' dark period in the evening. 3) When the dark period follows the daylight in the evening, it must be long enough, lasting at least six hours. 4) Continuity between daylight and artifical light is thus not necessary, provided the dark period following the daylight hours is long enough. 5) The length (or absence) of a dark period between the artificial light period and morning light has no marked effect on the growth of tomato plants, their earliness or their fruit yields.

Another experiment conducted in Skierniewice was concerned with the production of tomato plants with electrical illumination only (without sunlight, in a growth room with controlled temperature). Verkerk [4] in Holland was the first to mention such a possibility.

In this experiment Skierkowski used five tomato varieties under fluorescent lamps with an intensity of 4200–4500 lux. The seed was sown on March 10, and the illumination started on March 16, when the seedlings emerged. The experiment comprised three treatments, namely: 1) 17 hour's light at 18–20° C and 7 hours' darkness at 13–15° C. 2) Continuous light at 17–19° C. 3) Continuous light at 20–22° C. The plants were subjected to their respective treatments until April 26. At that time the plants showed the first flower cluster. The plants from all treatments showed the same size and appearance up to April 11; differences between treatments and varieties were however apparent on April 26. The plants of all varieties given 17

DOI: 10.17660/ActaHortic.1963.1.2