Action du CCC et du B 995 sur la mise à fleur d'une plante bisannuelle, l'Œnothera biennis

Summary

Oenothera biennis L. is a typical biennial plant with an absolute cold requirement for subsequent floral initiation under long-day conditions. Flowering of vernalized Oe. biennis is associated with transition from a rosette habit to the formation of a long flower-bearing stem. Vernalization in Oe. biennis consists of two consecutive stages: (1) preparation for flowering; (2) preparation of stem elongation. At the end of the second stage, the level of endogenous gibberellin-like substances is sufficient to allow the stem elongation which is requisite for subsequent floral development.

Applications of (2-chloroethyl)trimethylammonium chloride (CCC) via the roots, when made after the cold treatment, have no effect on internodal elongation and subsequent flowering of vernalized Oe. biennis. Relatively small amounts of CCC applied onto the apical bud of the rosette promote stem growth. However, 750 μg of CCC act as a growth inhibitor and delay the beginning of stem elongation but this retardant has no effect on the number of flower buds.

In contrast, N-dimethylaminosuccinamic acid (B 995), when applied after a cold treatment, delays the beginning of stem growth, this delay increasing with greater amounts of applied B 995. However, with relatively small amounts of B 995, the rate of later stem growth increases rapidly so that treated plants flower at the same time as controls.

If gibberellic acid (GA₃) is applied on the apical bud just at the same time as B 995, it reverses the effects of the growth retardant, even when the amount of B 995, applied by itself, is sufficient to inhibit entirely stem elongation and flowering.

B 995 is particularly effective if the preceding cold treatment was just sufficient for effective vernalization. If the cold treatment was extended beyond this time, a greater amount of retardant was required to obtain the same degree of stem growth inhibition.

B 995 probably acts by interference with the metabolism of endogenous gibberellin-like substances. The delay of floral initiation in vernalized Oe. biennis by B 995 is a consequence of the retarded internode elongation; if this retardation exceeds a certain limit, the plant is devernalized and exhibits a cold requirement identical with the original one.