Effect of day and night temperature alternations and of plant growth regulators on stem elongation and flowering of the long-day plant Campanula isophylla Moretti

Abstract

Stem elongation in Campanula isophylla Moretti was influenced more by the difference (DIF) between day temperature (DT) and night temperature (NT) than by average daily temperature (ADT) or absolute DT and NT from 12 to 24°C. Plant height at anthesis increased 230% as DIF increased from −12 to +12°C. When DT and NT were constant (CT), plant height was reduced 24.9% as CT increased from 12 to 24°C. DIF only affected internode length as the number of leaves on the stem was not influenced by DIF. A daminozide spray reduced plant height by 51, 33 and 37% for 12, 0 and −12°C DIF treatments, respectively. Daminozide overcame positive DIF-induced stem elongation, while gibberellic acid (GA 3) could overcome inhibition of stem growth in negative DIF-grown plants. Morphogenetic responses of DIF were mimicked by GA 3 application. Mean relative growth rate (RGR) reached a maximum level at an ADT of ∼ 19–20°C. Although the final dry weight at marketing stage was highest at 12°C NT, as averaged over all DT, highest plant quality was found at an ADT of 18°C when the DT and NT were delivered as −3 to −6°C DIF. The number of days to marketing stage was reduced from 120 to 68 days for cultivar ‘Alba’ and from 110 to 63 days for cultivar ‘Blå’ when CT or ADT was increased from 12 to 24°C. Rate of flower initiation and flower development were mainly a function of ADT. However, under high natural irradiance in summer, negative DIF resulted in 5–9 days earlier flowering compared with a positive DIF. GA 3 application had a small influence on flowering. Daminozide spray (3.12 × 10 −2M) delayed visible bud from 0 to 4 days and anthesis from 9 to 16 days for various temperature treatments and cultivars. Plant morphogenesis seemed to be controlled in a different way to flower initiation and flower development in C. isophylla.