Flowering Response in Relation to C and N Contents of Pharbitis nil Plants Cultured in Nitrogen-Poor Media

Flowering of seedlings of Pharbitis nil, strains Violet and Tendan, cultured in modified White's medium, was promoted by medium dilution, the critical dark period being shortened by about 15 min. Dilution of the N source alone was enough to cause the medium-dilution effect. Dilution of the culture medium during the day before and on the day of exposure to the dark-period (a total of two days) caused the maximum dilution effect. The C and N contents of the cotyledons and of the shoot apices changed rapidly in response to medium dilution. In 1/2-strength White's medium with 1/1,000 strength NO3− which was most effective for flower promotion, the C-N ratio was highest. In 1/2-strength modified White's medium, in which flowering was lowest with the longest critical dark period, the C-N ratio was lowest. Thus, there is a close relation between flowering response and the C-N ratio in cotyledons or shoot apices of Pharbitis nil.