EFFECT OF SHORT-DAY TREATMENT ON FLOWERING TIME AS AFFECTED BY PLANT AGES AND ITS VARIETAL DIFFERENCES IN SOYBEAN (GLYCINE-MAX (L) MERR)

Soybean plants belonging to eight varieties originating from Japan and from low latitude regions of South America were exposed to short photoperiods at different ages for varietal comparison of the effect of short photoperiods on flowering in relation to plant age. Thirteen to 33 day-old plants were exposed to a 12.5h photoperiod. The number of days to flowering and number of trifoliate leaves on the main stem which increased during the period of the treatment decreased linearly with the plant age at the onset of the treatment, and significant differences were observed in the rate of the decrease with age among the varieties. Thirteen to 33 day-old plants which were defoliated except for the uppermost two leaves, as well as intact plants at the same age, were exposed to photoperiods of 12h and 12.5h. Although the number of days to flowering varied with the plant age and photoperiod, no significant differences were observed between the defoliated plants and intact ones. Seven day-old sedlings were grafted onto one of the branches which had developed from the axils of primary leaves of stock plants. The grafted plants were exposed to a 12h photoperiod after defoliation of either scions or stocks. The number of days to flowering of the scions was the same as or smaller than that of the stocks. Thus, it was suggested that the enhancement of the flower induction effects of short photoperiods with plant age was not due to the increase of the sensitivity of the growing points to the floral stimulus but to the increase of the response of the uppermost leaves to short phtotoperiods. Thirteen and 33 day-old plants were exposed to photoperiods ranging from 11 to 12.5h. Under a 12h or shorter photoperiod, plants at the same age and of the same variety came to flower at almost the same time, but the flowering was somewhat delayed under a 12.5h photoperiod. The differences in the number of days to flowering with the age was more conspicuous under a 12.5h photoperiod than a 12h or shorter photoperiod, and also in the varieties from low latitude regions than in the Japanese varieties. It is concluded that the low response of plants at early growth stages to short photoperiods which was observed in the varieties from low latitude regions is beneficial as it secures vegetative growth under the short day conditions prevailing in those regions.