The Effect of Photon Flux Density and Lighting Period on Growth, Flowering, Powdery Mildew and Water Relations of Miniature Roses

Abstract

Miniature roses ( Rosa sp .) were grown at 100 and 150 μmol m -2 ·s -1 photon flux densities (PFD) with 16, 20 and 24 h·day -1 lighting periods (LP) in a greenhouse compartment in midwinter at latitude 59 ° north. The study included 10 different treatments and six rose cultivars, altogether 900 plants. The 16 and 20 h LP were applied with or without a dark period of 8 and 4 h·day -1 , respectively, by timing the LP in relation to daylight that lasted for 7 - 8 h. Number of days until flowering decreased with an increase in PFD and in LP up to 24 day -1 and was unaffected by the timing of the 16 and 20 h·day -1 LP. Number of flowers and plant dry weight increased 20% to 30% by increasing the PFD. Plant dry weight increased by increasing the LP from 16 to 20 h·day -1 (about 25%), but no effect was found with a further increase to 24 h·day -1 . Mean growth rate until flowering increased 30% to 40% by increasing the PFD or by increasing the LP from 16 to 20 h day -1 , while little effect was found by a further increase to 24 h·day -1 . Increasing the photosynthetic active radiation (PAR) by increasing the LP from 16 to 20 h·day -1 increased the growth rate more than increasing the PFD did. Three of the cultivars were tested for water loss after the detachment of some leaves. Leaves that had developed without a dark period showed a considerably higher water loss than the treatments that included a dark period of 4 or 8 h·day -1 . The keeping quality at indoor conditions, however, was unaffected by the treatment due to sufficient watering. Powdery mildew developed significantly more on plants grown with a dark period of 8 h as compared with the other treatments. It was concluded that 20 h·day -1 LP including a dark period of 4 h·day -1 and a PFD of at least 150 μmol·m -2 ·s -1 should be applied to miniature roses during the winter months in order to effectively produce miniature pot roses with a high quality.