Stevia rebaudiana Bertoni is a plant of the Compositae family inhabiting South America, and it produces a glucoside called “stevioside”in the leaf. Because this substance has sweetness 300 times that of sucrose, recently it has attracted attention as a possible new natural source of sweetening.
We have been working on this plant in Ishigaki to clarify the fundamental problems in selecting varieties adaptable to, and ascertaining optimum cropping seasons for subtropical areas. For these purposes, the photoperiodic response of flowering was examined under three controlled photoperiods (11, 12.5, 14 hours) and our natural day-length. The materials were of seed population introduced from Hokkaido Agricultural Experiment Station, and of early and late clones selected therefrom. From this work the following results were obtained:
1. Stevia rebaudiana Bertoni is apparently a short-day plant with a critical day-length of about 12 hours. Distinct retardation of flowering is shown under longer then 12 hr. day-length conditions.
2. The degree of retardation of flwering under long-day conditions varies widely among clones and has a close relationship with the flowering time. However, no variation is observed in the critical day-length or in the basic vegetative growth period among clones. From these results, earliness or lateness of flowering seems controlled primarily by the degree of retardation of flowering under long day influence.
3. Flowering under optimum day-length (short-day) begins at about 58 days after sowing, and with the ratoon crop it begins at about 38 days after topping. This means that the vegetative growth period in ratooning is shortened about 20 days from that in a seed-started crop.
4. The effect on flowering of short-day treatment differs according to the number of days of the treatment. Two-day treatment does not induce flowering, while treatment of over three days does. The longer treatments induce the earlier flowerings, and in the case of continuous treatment flowering is observed at about 28 days.
5. As reported by KUDO in rice study, flowering time under natural photoperiod of this plant is also roughly predictable by finding the intersection point of the photoperiodic reaponse curve with the curve of natural day-length in the given regions. By this method, the optimum cropping season in Okinawa would be as follows:
Sowing: late February to early March
Planting or topping : mid-March to early April
Harvestiong: June to July
6. In subtropical areas under conditions of high temperature and short photoperiod, medium to late flowering clones rather than early one would be more promising for high yield by maintaining greater vegetative growth.

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[in Japanese]

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