Age and Flower Initiation
For the majority of herbaceous photoperiodic species, there is clearly no period of total inability to flower, but the sensitivity to daylength increases with increasing age of the plants. Some workers believe that flower initiation cannot occur before a definite ratio of immature- to mature-leaf area is reached. Evidence concerning this possibility has been found in several species, e.g., the SDP soybean, the LDP Scrofularia arguta, and the DNP tomato, in which removal of young expanding leaves hastens flower formation in unfavorable conditions. Insensitivity of the cotyledons and first-formed leaves to favorable daylength conditions is believed by most flowering physiologists to be the primary reason for juvenility in photoperiodic species. In trees, such as Citrus and Larix, juvenile scions do not flower more rapidly when grafted onto mature plants bearing flowering shoots, suggesting that the meristems of juvenile scions are incapable of responding to stimuli from mature leaves, i.e., they remain juvenile.