Timing growth and development of Campanula by daily light integral and supplemental light level in a cost-efficient light control system

Two campanula species Campanula portenschlagiana (‘Blue Get Mee’) and Campanula cochlearifolia (‘Blue Wonder’) were grown in a cost-efficient light control system and the effect of supplemental light level and daily light integral (DLI) on growth and development was quantified. The alternative light control system (DynaLight desktop) automatically defines the most cost-efficient use of supplemental light based on predefined setpoints for daily photosynthesis integral (DPI), forecasted solar irradiance and the market price on electricity. It saves energy in high-cost periods of electricity, but creates irregular light periods which may disturb circadian rhythms and thereby affect plant growth and flower development. Plants were grown in four treatments controlled by DynaLight desktop with two setpoints for DPI (300 and 600 mmol m−2 leaf day−1) and two levels of supplemental lighting (48 and 84 μmol m−2 s−1). We found that differences in supplemental light levels, daily light hours or DLI had no effect on leaf area expansion and stem elongation, but there was a linear relation between dry matter accumulation and cumulative light integral (CLI) in both species, and a linear relation between the number of flowers and buds and CLI in ‘Blue Get Mee’. The results demonstrate that DLI was the main limiting factor for prediction of growth and development when two campanula species were grown in a cost-efficient light control system where the number of daily light hours was often below the critical day length of 14 h. However the light hours was always distributed in two or more light periods during the day, which thereby prevented dark periods longer than the critical night length of 9 h for campanulas.

► We studied growth of Campanula in a cost-efficient light control system.
► We examined the effect of light level and daily light integral (DLI).
► DLI was the main limitation for prediction of growth and development.
► A linear relation between growth and the cumulative light integral was found.
► The irregular distribution of light periods had no effect on growth and development.