The effects of gibberellin A3, photoperiod, and age on vegetative growth and flowering in Cosmos bipinnatus var. Sensation

Abstract

Plants of Cosmos bipinnatus Cav. 'Sensation' (a quantitative short-day plant) were grown continuously under conditions favorable or unfavorable for flowering, and some plants in each group were treated with gibberellic acid (GA3). Morphological and anatomical observations revealed that GA3 promoted floral initiation under non-inductive long days but not as efficiently as inductive short days, as judged by the rate of apical transition to the reproductive state. GA3 did not influence the total number of plants that flowered but did increase height growth. Plants treated with GA3 had a normal percentage of initiation, but a much lower developmental rate. Macroscopic recognition of flowering was not possible until the inflorescence stage of the apex.Floral apices of Cosmos are formed by the transition of previously vegetative apices, the apex passing into an intermediate stage typical of most plants held under non-inductive conditions. Transitional, prefloral, and inflorescence stages follow. The formation of an intermediate stage may account for many of the conflicting results observed in physiological studies since there is a great variation in response rate depending on age of plant and stage of the apex at the start of the experiment. Anatomical observations support the findings that although GA3 promotes floral initiation in Cosmos when applied under non-inductive conditions, floral development may not continue, thus resulting in an apparent lack of response. This may also be true of many other plants considered "non-responsive" to GA3 under non-inductive conditions.