Effects of some Environmental Factors on Photoperiodic Induction of Beet and Dill

Abstract

1. If maintained on continuous light in the greenhouse, dill flowered in 40 days after sowing the seeds and beet in 31 days. Under conditions of relatively constant illumination at 1000-1400 foot-candles, dill required 40 days from time of planting to show some stem elongation; approximately 20 more days were required to bring about the same response under 500 foot-candles. Under similar conditions of constant illumination of high intensity, beet produced seed stalks 28 days after planting. On the other hand, beet grown under 500 foot-candles did not produce seed stalks. 2. With increasing amounts of illumination during the middle of the dark period, up to 6 hours, the average height of the dill plants was almost directly proportional to the amount of supplementary illumination received. 3. Under conditions of high light intensity, dill may be induced to flower by giving it one long photoperiod consisting of 20-21 hours of light and 4-3 hours of dark. Rapidity of stem elongation was about the same when the plants received one, two, or three long photoperiods. But if the plants received four long photo-periods, they elongated about as rapidly as those maintained on long day. 4. Dill plants previously treated with a high light intensity showed a response to continuous illumination of only 5 foot-candles if exposed to it for as little as 1 day. Those subjected to this treatment for 2 days showed some response, while those having 3-8 days of low light continuously were very slow in responding. 5. When dill was pre-treated with three short photoperiods each consisting of 9 hours of light of 4-12 foot-candles and 15 hours of darkness, and then subjected to intensities of 50-1000 foot-candles, there was a steady increase in average height with increase in intensity up to 300 foot-candles. The effect of higher intensities (500-1000 foot-candles) on rate of stem elongation was approximately the same as that produced by 200 foot-candles. 6. The effect of low temperature (4.5⚬ C.) on dill plants receiving cycles consisting of 20 hours of light and 4 hours of darkness was to suppress markedly the flowering response. Those plants receiving 2 days of cold treatment attained a greater height than did any of those receiving fewer or more days of such treatment. 7. A large proportion of young dill plants elongated their stems if the roots or stems were injured. Thus injury seemingly altered the metabolism sufficiently to bring about the flowering response. 8. Red light of wave lengths greater than 580 μ are capable of inducing the flowering response in dill when used as the sole source of illumination during the induction period. 9. Although red light of 50 foot-candles was approximately as effective in bringing about induction in dill as any intensity up to 500 foot-candles, it was not nearly so effective after 5 days of treatment as the higher intensities. 10. Annual beet did not flower if it received more than 10-11 hours of darkness in each 24. 11. Annual beet began to bolt after return to short day if it received 11 days of continuous light. The bolting of such plants, however, was no more rapid or pronounced than in those receiving cycles consisting of 21 hours of light and 3 hours of darkness. 12. Continuous illumination with 900-1000 foot-candles of light from white fluorescent Mazda lamps, under conditions of only slight temperature fluctuation, was conducive to flowering in beet. If the intensity did not rise above 700 foot-candles during the induction period, flowering did not occur. 13. Annual beet did not flower if subjected to the following cycles of light and dark: (a) 5 minutes of light and 5 minutes of dark (55 days); (b) 30 minutes of light and 30 minutes of dark (55 days); (c) 24 hours of light and 24 hours of dark (117 days); (d) one short photoperiod of 8 hours of light, 16 hours dark and one long photoperiod of 21 hours of light, 3 hours dark (117 days); (e) three short photoperiods of 8 hours of light, 16 hours dark and three long photoperiods of 21 hours of light, 3 hours dark (117 days).14. Exposure of a single leaf of beet to cycles of 20 hours of light and 4 hours of darkness did not result in the plant flowering.