Stem elongation and keeping quality in poinsettia ( Euphorbia pulcherrima Willd.) as affected by temperature and supplementary lighting

Abstract

Euphorbia pulcherrima cultivars ‘Lilo’ and ‘Starlight’ were grown under five different day/night temperature (DT/NT) regimes from the start of short days (SD) to flowering for two different starting periods, 1 October (replicate A) and 12 October (replicate B). The average daily temperature (ADT) was kept at 18–20°C. Plants were given supplementary lighting with high pressure sodium lamps (HPS) for 10 h day −1 at three photosynthetic photon flux densities (PPFD): 12, 37 and 73 mol m −2 s −1. At the marketing stage the plants were moved to the interior holding room for testing the keeping quality. Both plant height and plant diameter were lower when the plants were grown at DT < NT (negative DIF) compared with plants grown at DT > NT (positive DIF) or with constant temperature (zero DIF). The response to DIF was stronger in replicate A than B, and considerably greater in ‘Starlight’ than in ‘Lilo’. Negative DIF-grown plants resulted in delayed visible cyathia and flowering compared with zero or positive DIF. Plants grown at constant temperature flowered earliest. Plants reached the marketing stage about 1 week earlier with 73 compared to 12 mol m −2 s −1 supplementary lighting. DT<NT (negative DIF) increased the abscission of cyathia in ‘Lilo’ during the first 2 weeks in an interior climate. Increasing PPFD reduced abscission of cyathia in both cultivars. Negative DIF during the growth period resulted in more bract necrosis than zero or positive DIF in ‘Lilo’. No bract necrosis was observed in ‘Starlight’. DIF had no influence on leaf abscission.