Potted flowering plants for indoor or patio use are the second largest (17.2%) segment of the U.S. commercial floriculture industry after the bedding plant segment (USDA, 2010). In 2009, the reported wholesale value of potted flowering plants in the top 15 producing states was $632 million (USDA, 2010). Current market trends indicate that consumers are demanding new and unique potted flowering crops, including tropical flowering plants (Pizano, 2005). Although there is a growing market for tropical flowering plants, most northern U.S. greenhouse growers experience difficulties growing and inducing these crops to flower, mostly as a result of their energy-intensive environmental requirements (Davis and Andersen, 1989; Fausey and Cameron, 2005; Pizano, 2005). Therefore, extensive research needs to be conducted to successfully manipulate and schedule potential new floriculture crops to obtain rapid, uniform, and complete flowering (Davis and Andersen, 1989).
The exotic, unique, and unusual appearance of tropical flowering plants appeals to many flower consumers (Pizano, 2005). The yellow trumpet bush (Tecoma stans) is a tree in the Bignoniaceae family native to tropical and subtropical regions of Central and South America. It produces large funnel-shaped, bright yellow, fragrant flowers that compliment its glossy green, pinnately compound leaves. Tecoma ‘Mayan Gold’ was selected as a potential new annual patio flowering crop as a result of its compact nature, drought and heat tolerance, long-blooming characteristics, and few disease and pest problems (PanAmerican Seed, 2010).
Most potted flowering plants are vegetatively propagated by shoot-tip cuttings; however, a few are propagated from seed. The propagation success of seedlings requires an understanding of the effects of irradiance [photosynthetically active radiation (PAR) or photosynthetic photon flux (PPF)] and temperature to manipulate plant growth and development (Fausey and Cameron, 2005). Some desired characteristics of floriculture seedlings and plugs are increased biomass (shoot and root), compactness (reduced internode elongation and proportional height), and adequate leaf area to promote growth and development after propagation (Faust et al., 2005; Lopez and Runkle, 2008; Pramuk and Runkle, 2005; Styer and Koranski, 1997).
Photosynthetic DLI is an important environmental parameter closely related to growth and quality of greenhouse-grown crops. In the northern part of the United States during winter and early spring, when most propagation occurs, outdoor DLI ranges from 5 to 10 mol·m−2·d−1 (Korczynski et al., 2002). Furthermore, outdoor DLI levels can be reduced by up to 50% or more by shading from greenhouse glazing, thermal energy curtains, structures, obstructions, and hanging baskets, resulting in DLI as low as 1 to 5 mol·m−2·d−1, which can be further reduced during extended periods of cloudy weather (Lopez and Runkle, 2008).
DLI recommendations for most floriculture crops are to provide ≈4 to 6 mol·m−2·d−1 with a PPF between 100 and 200 μmol·m−2·s−1 during propagation under a 12-h photoperiod during the first stage of root initiation. Subsequently, commercial growers should increase DLI to ≈6 to 8 mol·m−2·d−1 (PPF between 200 and 400 μmol·m−2·s−1 under a 12-h photoperiod) to promote fully rooted cuttings and optimal shoot and root biomass accumulation (Dole and Hamrick, 2006; Lopez and Runkle, 2008). Pramuk and Runkle (2005) suggest that supplemental lighting can greatly increase quality and reduce subsequent time to flower when natural DLI levels are low (e.g., less than 8 mol·m−2·d−1). Thus, when natural irradiance falls below optimum levels, high-intensity discharge (HID) lighting should be used in commercial greenhouses to increase DLI.
As DLI increases, photosynthetic rates consequently increase (Donelly and Fisher, 2002; Holcomb and Berghage, 2001; Klopmeyer et al., 2003) contributing to faster rooting, biomass accumulation, compactness, subsequent flowering, and overall quality. For example, baby's breath (Gypsophila paniculata L. ‘Perfecta’) cuttings propagated under a DLI of 12 mol·m−2·d−1 rooted twice as fast as cuttings grown under a DLI of 7 mol·m−2·d−1 (Islam and Willumsen, 2001). Lopez and Runkle (2008) reported that rooting, biomass accumulation, and quality of New Guinea impatiens (Impatiens hawkeri Bull.) and petunia (Petunia ×hybrida hort. Vilm.-Andr.) cuttings increased as mean propagation DLI increased from 1.2 to 7.5 mol·m−2·d−1. For example, root number of petunia ‘Tiny Tunia Violet Ice’ after 16 d of propagation increased from 17 to 40, whereas cutting shoot height decreased from 6.3 to 4.5 cm, and root and shoot dry biomass increased by 737% and 106% for plants rooted under 1.2 and 7.5 mol·m−2·d−1, respectively. Similarly, shoot dry weight per internode for celosia ‘Gloria Mix’ (Celosia argentea var. plumose L.), seed impatiens ‘Accent Red’ (Impatiens wallerana Hook.), marigold ‘Bonanza Yellow’ (Tagetes patula L.), and pansy ‘Crystal Bowl Yellow’ (Viola ×wittrockiana Gams.) increased linearly as DLI increased from 4.1 to 14.2 mol·m−2·d−1. In addition, seedling became more compact as DLI increased in celosia, seed impatiens, and salvia (Salvia splendens Sell ex Roem. & Schult.) (Pramuk and Runkle, 2005).
To our knowledge, no studies have been published on the effects of DLI during Tecoma seed propagation. The objectives of this study were 1) to quantify how the mean DLI during propagation influences rooting, growth, and quality of Tecoma seedlings; and 2) to determine optimum DLI levels for propagation of high-quality seedlings.
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