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Three series of experiments were carried out in 1979 and 1980 to ascertain the hereditary behavior of inflorescence forms in Ambrosia artemisiifolia var. elatior and the effects of photoperiod, temperature and exogenous growth substances on the sex expression of inflorescences. 1. All the progenies in two generations from the mother plants with a monoecious standard form in which the racemes are staminate and the pistillate heads occur in leaf axils had almost the same form of inflorescence as the parents. From the mother plants with a female form in which the racemes are pistillate and with a monoecious mosaic form in which the racemes bear both staminate and pistillate heads, offsprings with these three forms of inflorescence appeared respectively with a different frequency. Although plants with the standard form were most numerous among the progenies from the female parents and the monoecious mosaic parents, the frequency in appearance of the individuals with the same form as the parents became higher in the second generation than in the first. 2. Strongly short day conditions with 20 cycles decreased conspicuously the ratio of staminate heads to pistillate heads, resulting in favour of femaleness. It was ascertained that female tendency was more promoted by 8 hours day-length than 6 hours, and in case when the short day treatment had been started at the stage of capitulum differentiation than when started at the stage of involucre formation. Though a low temperature (20-22℃) decreased the number of capitula, compared with the natural outdoor temperatures, and a high temperature (30-32℃) showed a reverse tendency, neither of them had an apparent effect on the conversion of sex expression. 3. Naphthaleneacetic acids sprayed to the whole plants with the concentration of 50 ppm and 100 ppm and gibberellin A3 with that of 100 ppm from the stage of capitulum differentiation or involucre formation showed a different effect on the sex expression of capitula in two experiments on the subject. Namely, both of the reagents promoted a female tendency in the experiment of 1979, but accelerated a reverse trend in 1980 with the exception that gibberellin supplied at the later stage caused the same tendency as in the previous year. It was suggested from the results mentioned above that more kinds of substances including auxins and gibberellins might be affected on the sex expression of inflorescences in Ambrosia artemisiifolia var. elatior.
1981-11-25

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石井滋規
奈良教育大

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