Phenology of the Citrus flowers and their local flowering dates in north Izu region (1)

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The investigation of the Citrus flowers is important, not only for fruit-calture but also for phenological study. And if their operating and plant physiological conditions are cleared by some means, the differences of the flowering dates are able to use for the indicator of the climatic conditions. From this point of view the author studied the citrus flowers and came to the conclusions as follows:(1) The date of the first flowering, full blooming and falling of the flowers of Washington navel (C. sinensis OSBECK) come first of all, then Wase Unshu (an early ripe species of C. unshu MARC.), Natsu-daidai (C. natsudaidai HAYATA) and Unshu (C. unshu MARC.) follow in order. Washington navel are 5-6 days, Wase Unshu 2-5 days, and Natsudaidai 1-3 days earlier than Unshu in general.(2) The observed values in 8 years at Okitsu (in this time there are no changes of the operating conditions) lead us to the results as follows:(a) Correlation coefficient between the mean temperature of April or March and April, and the first flowering date of Unshu or Washington navel are about -0.9, as to Natsudaidai about -0.4, Wase Unshu -0.7--0.8, respectively. The experimental formula of Unshu is obtained.K=-8.52T+131.39K: first flowering date in May, T: mean temperature [(may. +min.)÷2]°C of April.(b) It is confirmed that the length of the terms from the first flowering date to full blooming date, and from full blooming date to falling flowers are both long every other year. For this reason, the first flowering date may be better than the other, as the citrus flowers are used to the climatic index.(3) Relations between the first flowering dates and operating conditions are analysed qualitatively: earlier at the more infertile soil, and earlier in the case of the slighter running.(4) The first flowering date also depend upon the age of the tree. The trees that are about 40 years old are the earliest.