Studies on the flowering behaviour of perennial Delphinium

Abstract

Delphinium seeds were sown early each month throughout the year and the seedlings were transplanted to an unheated greenhouse to study the seasonal change on the flowering behaviour of delphinium. The formation of flower buds was observed microscopically, whereas the effect of temperature on bolting and flowering was macroscopically studied m a phytotron. Furthermore, the bolting characteristics of several cultivars in a heated house under natural day-length were also studied. 1. Plower bud differentiation occurred under a wide range of temperatures almost year-round. When seedlings were planted in warm and hot seasons, the period from sowing to flower bud differentiation was shortened whereas, when they were planted during the cool or cold seasons, the period was prolonged. Therefore, the flower bud differentiation process is hastened by high temperatures. 2. Delphinium seedlings react sensitively to the higher temperature; when the temperatures were kept constant at or higher than 20 °C, seedlings bolted after the plants developed 5 or more leaves. However, plants kept at 15 °C remained in the juvenile phase for an extremely prolonged period add about half assumed the rosette form 3. When the seeds were sown in March-July and the seedlings were transplanted in summer, the plants differentiated flower buds at a lower node and produced poor spikes with few florets. Plants derived from seeds sown in August-February developed many leaves below the first floret, produced spikes of higher quality with more florets. Thus, the greater the ratio of leaves to floret, the more florets per spike were formed. 4. Low temperature did not act as vernalization inducing in the flower bud initiation, but caused the rosette to break or expanded the temperature range the plant could grow to lower side. Thus, low temperature acted to make growth activity higher. 5. Flower bud differentiation was always accompanied with bolting even under unheated conditions, so that the time of flower bud differentiation could be estimated easily. Therefore, early bolting plants resulting in a low cut flower quality can be eliminated at an earlier stage before transplanting. 6. When delphinium were grown in a heated house under natural day-length, many cultivars formed rosettes. Plants, which formed rosettes, were slow to bolt and produced malformed spikes.