The majority of herbaceous ornamental plants are commercially produced in greenhouses so that they are in flower when marketed to consumers in the spring. Most garden plants are produced in two distinct phases: a young plant stage, in which seedlings and cuttings are established, and a finish plant stage, in which the young plants are transplanted into a larger container and grown until marketable (Ball, 1998). During the young plant stage, seedling plugs are typically produced during the winter and early spring (from January through March in the northern hemisphere) when the photosynthetic daily light integral (DLI) can be a limiting production factor. For example, without supplemental lighting (SL), young plants are typically grown under a mean DLI of 5 to 10 mol·m−2·d−1 from October until March at latitudes of 35 to 45 °N (Korczynski et al., 2002). A desirable minimum mean DLI is 10 to 12 mol·m−2·d−1 at plant canopy level (Runkle, 2007; Verberkt et al., 2004).
DLI influences crop timing and plant quality in many floricultural crops (Erwin et al., 2004; Pramuk and Runkle, 2005a, 2005b). Some species grown under a high DLI (e.g., greater than 15 mol·m−2·d−1) flower earlier because they develop fewer nodes before flower initiation. In addition, plant temperature can be higher than the air temperature under high light intensities and SL (Faust and Heins, 1997), which can accelerate crop development.
For shade-avoiding species, plant biomass typically increases in proportion to the mean DLI (Warrington and Norton, 1991). In addition, a high DLI usually promotes flowering and improves plant quality attributes (e.g., flower number and branching) in many floriculture crops. For example, increasing DLI hastened flowering in vinca [Catharanthus roseus (L.) G. Don.; Pietsch et al., 1995], petunia (Petunia ×hybrida Vilm.-Andr.; Kaczperski et al., 1991), and eight other bedding plants (Faust et al., 2005). Vinca grown under the average DLI of 18 mol·m−2·d−1 had a flower diameter of 3.9 cm compared with 3.3 cm for plants grown under 9 mol·m−2·d−1. Chrysanthemum flower bud number increased from 3.0 to 10.2 as DLI increased from 9.8 to 16.9 mol·m−2·d−1 under an 8-h photoperiod (Warrington and Norton, 1991). Petunia grown under a higher DLI had more and larger flowers (Kaczperski et al., 1991). In addition, an elevated DLI inhibited stem and peduncle extension growth in pansy (Viola ×wittrockiana Gams.) (Niu et al., 2000).
Increasing DLI can promote the growth rate of herbaceous ornamentals by increasing photosynthesis (Nemali and van Iersel, 2004). A higher mean DLI increases dry matter and flower bud number at first flowering in rose yarrow (Achillea ×millefolium L.), gaura (Gaura lindheimeri Engelm. & Gray), lisianthus [Eustoma grandiflorum (Raf.) Shinn], red salvia (Salvia splendens Sell ex Roem. & Schult.), and French marigold (Tagetes patula L.) (Fausey et al., 2005; Islam et al., 2005; Moccaldi and Runkle, 2007). When the ambient DLI is low, the only way to increase the DLI is to provide SL. However, SL during the entire production period is usually not commercially feasible because significant investments in lighting equipment and operation are required (Graper et al., 1990).
One of the most cost-effective uses of SL is during the production of young plants, when plants are grown at a high density (Graper et al., 1990). Increasing DLI during propagation can increase transplant quality and hasten subsequent flowering in several herbaceous ornamentals. For example, as mean DLI during the seedling stage increased from 4.1 to 14.2 mol·m−2·d−1, seedling shoot dry mass per internode (a measure of plant quality) increased linearly by 64%, 47%, 64%, and 68% and flowering occurred 10, 12, 4, and 12 d earlier in celosia (Celosia argentea var. plumosa L.), impatiens (Impatiens walleriana Hook.), French marigold, and pansy, respectively (Pramuk and Runkle, 2005b). Similarly, an increase in propagation DLI, from 1.2 to 7.5 mol·m−2·d−1, increased the root and shoot dry mass of petunia ‘Tiny Tunia Violet Ice’ by 737% and 106%, respectively (Lopez and Runkle, 2008). In addition, subsequent time to flower decreased from 50 to 29 d as the propagation mean DLI increased from 1.4 to 10.7 mol·m−2·d−1. The objective of this study was to determine the impact of increasing the DLI for different durations and production periods during the seedling stage on transplant quality and subsequent growth and development of petunia and pansy.
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