Timing and Duration of Supplemental Lighting during the Seedling Stage Influence Quality and Flowering in Petunia and Pansy

Increasing the photosynthetic daily light integral (DLI) during the seedling stage promotes seedling growth andflowering in many bedding plants. Our objective was to determine the impact of increased DLI for different periods during the seedling stage on young plant quality and subsequent growth and development. Seeds of petunia (Petunia ·hybrida Vilm.-Andr. 'Madness Red') and pansy (Viola ·wittrockiana Gams. 'Delta Premium Yellow') were sown into 288-cell plug trays and placed under a 16-h photoperiod provided by sunlight plus 90 mmolm -2 s -1 (supplemental lighting (SL)) or 3 mmolm -2 s -1 (photoperiodic lighting (PL)) from high-pressure sodium lamps when the ambientgreenhouse photosyntheticphotonflux was less than400 mmolm -2 s -1 from0600 to 2200 HR. Plants were grown at 20 8C under PL or SL for the entire seedling stage or were exposed to SL for one-third or two-thirds of the seedling stage. Seedlings were then transplanted into 10-cm pots and grown until flowering with SL at 20 8C. Shoot dry mass of transplants increased linearly with increasing DLI provided to seedlings in petunia (y = -4.75 +1.86x,R 2 = 0.76) and pansy (y = -3.94 +3.47x,R 2 = 0.78) in which y = dry mass (g) and x = DLI (molm -2 d -1 ). SL during the last two-thirds or the entire plug stage increased shoot dry mass and the number of leaves in both species compared with SL during the earlier stage or PL. SL during the last two-thirds or the entire plug stage accelerated flowering, but plants had a lower shoot dry mass and flower bud number at firstflowering compared with that in SL during thefirst third or two-thirds or that in PL. Therefore, SL generally had greater effects on transplant quality and subsequent flowering when provided later in the plug stage than if provided earlier in production.