Inhibition of stem growth and flower formation in Pharbitis nil with N, N-dimethylaminosuccinamic acid (B 995)

Summary

In the short-day plant Pharbitis nil, strain “Violet”, flower formation is inhibited by application of the growth retardant N,N-dimethylaminosuccinamic acid (B 995) via the roots for a period of 24 hours prior to one inductive long night. Terminal flower bud formation is suppressed by a B 995 concentration of 100 mg/l, but for complete suppression of all axillary flower buds 2000 mg/l is required. Inhibition of flower formation is also caused by B 995 application via plumules or cotyledons, even if made at the end of the inductive night. B 995 treatment always results in short, thick internodes and dark-green leaves.

Transport of ¹⁴C-labeled B 995 from cotyledons to plumules and roots takes place during a 16-hour dark period. However, very little label moves from a treated to an untreated cotyledon. Application of B 995 to one of the two cotyledons results in flower inhibition, although the untreated cotyledon produces sufficient flower hormone to induce optimal flower formation. It is concluded therefore that in the short-day plant Pharbitis B 995 does not affect flower hormone production, but rather inhibits floral initiation by interfering with the action of the hormone in the shoot apex.

Inhibition of flower formation by B 995 can be completely overcome by application of gibbrellin A₃ to the plumulus before the long nigh. A dose of 0.01 μg GA₃/apex is sufficient to re-establish flowering, but much more GA₃ is required to restore internode length equal to that of the control. Indole-3-acetic acid and naphthalene acetic acid are totally inactive in overcoming B 995 inhibition of flower formation and growth.

The growth rate of Pharbitis plants treated with B 995 and continuously grown in long-day conditions is initially low, but reaches the same level as in untreated plants approximately 25 days after treatment.

¹⁴C-labeled B 995 applied to cotyledons accumulates to a high degree in roots and in the basal part of the shoots. ¹⁴C-B 995 is metabolized very slowly and persists therefore in Pharbitis plants for prolonged periods of time.