Flowering and growth of Calluna vulgaris (L.) Hull cultivars
This thesis reports a study of the flowering response of heather, Calluna vulgaris (L.) Hull and of experimental procedures designed to modify the time of onset of that process, with particular reference to year-round production of the flowering plant, in line with recent expansion in the European and British pot plant industries. Microscopic apical examinations were employed throughout to identify and monitor development of floral primordia. Research concentrated on flowering responses to photoperiodic stimuli, provided by various lighting regimes, including fluorescent and tungsten sources, differing in spectral composition and irradiance intensities. Calluna clearly required long day lighting for both floral initiation and development. This response was shown to be influenced by temperature conditions, with higher temperatures promoting flowering. Various changing-temperature regimes were also investigated. Reduced-temperature storage of plants with floral buds was successfully employed as a method of delaying flowering time. Influences of cultivar and seasonal flowering type were also investigated. Results indicated that seasonal flowering sequences were maintained under artificial lighting conditions; so that later flowering types required longer periods of lighting. Evidence suggested a cultivar-dependent sensitivity to irradiance intensity. This information was incorporated in an evaluation of costs incurred during photoperiodic induction, and from this an economic protocol for production of flowering Calluna was compiled; commercial viability being an integral part of the overall study. The minimum plant size capable of floral induction was investigated and related to different modes of propagation; in both macro- and micro-propagated plants flowering occurred soon after root development. Seedlings, however, had to pass through a juvenile phase. The limited micropropagation study revealed that adventitious shoot production involved transitory tissue rejuvenation.