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Two warm-season bedding plant species, zinnia (Zinnia elegans) and vinca (Catharanthus roseus), were used to determine if phosphorus (P) supply should be adjusted with light supply, and if deficiency and/or oversupply symptoms were apparent at different P rates when growth rates were altered by light levels. An additional goal was to determine the influence of P and light on overall P uptake efficiencies and water use efficiencies. Plants were grown in a greenhouse with or without shade over portions of the bench and supplied 0.1, 0.2, 0.5, 1, 2, or 4 mM P along with complete nutrient solution as needed with no leaching fraction. Optimum plant growth and flower development rate occurred at a P supply of 0.5 mM regardless of the light supply. Plant growth was greatly reduced by P supply below 0.5 mM regardless of shade conditions. Tissue P concentration was not influenced by light, but overall P content (mg P per plant) was higher when plants were grown without shading as a result of larger plants in higher light environments. The appearance or severity of deficiency symptoms also was not influenced by light. Water use efficiency was maximized when growth was not limited by P supply (at or above 0.5 mM). One hundred percent recovery of applied P was obtained at the 0.5 mM P supply in vinca, whereas recovery was less at the same P supply in zinnia. These results indicate no benefit for plant growth and flowering to P supply above 0.5 mM and illustrate how P content is demand-driven. However, there was no induction or delay of nutrient stress symptoms as a result of different plant growth rates in the different light differences environments. Use of shade curtains or whitewash are common strategies to reduce plant and air temperature in greenhouses (Aldrich and Bartok, 1994). These approaches typically reduce temperatures by 1 to 4 C, depending on the conditions, but photosynthetic light is also reduced by 30% to 75% (Aberkani et al., 2010; Al-Arifi, 2006). The reduced radiation results in lower photosynthesis (Taiz and Zeiger, 1991), water use (Nobel, 1991), nutrient demand (Adams, 1980; Chapin et al., 1995), fewer or smaller flowers (Oh et al., 2009), and increases in height along with thinner leaves (Pramuk and Runkle, 2005; Stanton et al., 2010). The influence of light on growth, flowering, and crop quality is well known and documented. Photosynthetically, a reduction of a certain percentage of light is often accompanied by an approximately equal reduction in whole plant photosynthesis (Frantz and Bugbee, 2005; Nemali and van Iersel, 2004). Shading [or a reduction in daily light integral (DLI)] also leads to decreases in flowering rate (Blanchard et al., 2011a, 2011b) and potentially alters plant or fruit quality, although the timeframe of those delays relative to shade application can vary depending on the crop (Gent, 2007). Reduction of incoming radiation and corresponding cooler conditions reduces water use in plants as well. In fact, Kim et al. (2011) found that DLI was the most significant correlation with water use. Additionally, Garland et al. (2012) found that DLI was strongly correlated to water use in plants, attributable in large part to larger plants and more leaf area in treatments receiving greater DLI. However, water use efficiency (WUE; amount of growth per unit of water; mg·L) tended to decline in higher DLIs. In floricultural crops, P supply can influence flowering and plant height. Whitcher et al. (2005) observed increases in plant height and flower number with both new guinea impatiens (Impatiens hawkeri) and vinca as P supply and foliar concentrations increased. Optimum rates, based on flower number, were 1 and 1.25 mM P for new guinea impatiens and vinca, respectively. The enhancement of flowering with P has resulted in a number of commercial products; so-called ‘‘bloom enhancer’’ fertilizers are sold and marketed with the promise of greater flowering as a result of much higher P concentrations and a nitrogen:P ratio of 1:2.5 to 1:5 (e.g., Bloom Booster, Scotts Miracle-Gro Co., Marysville, OH; Jack’s Classic Blossom Booster, JR Peters Inc., Allentown, PA; Dr. Earth Bud and Bloom Booster, Dr. Earth Co., Winters, CA). The belief in the product claims and aggressive marketing campaigns ensures their widespread use in commercial facilities. In a comprehensive series of experiments, the effects of P on height were evaluated for tomato (Solanum lycopersicum), marigold (Tagetes erecta), petunia (Petunia 3hybrida), impatiens (Impatiens walleriana), and gomphrena (Gomphrena globosa) (Nelson et al., 2002, 2012). For these crops, height generally increased with tissue P concentration up to 10 g·kg (1% dry weight) and occasionally continued to increase with high P concentrations. With few exceptions, P sufficiency for bedding plants is reported to be well below 10 g·kg and is often as low as 2.5 g·kg (Gibson et al., 2007; Mills and Jones, 1996). This indicates that although the plant does not require greater amounts of P, supraoptimal levels of P can cause responses that are undesirable in a production environment. So although flower number may be enhanced with extra P, height can be simultaneously increased leading to additional use of plant growth regulators to decrease stretch. Recommendations for P supply to plants are based on appearance of deficiency symptoms (Gibson et al., 2007), delays in growth and development, or generalized surveys of healthy plants in commercial production systems (Mills and Jones, 1996). Research-grade greenhouse conditions are typically used (e.g., Jeong et al., 2009), which may or may not be accurate representations of commercial production conditions. That is, in practice, growers will need to use shade curtain management for temperature control and therefore expose their plants to less-than-optimal light environments. Under those conditions, will the recommendations for nutrient supply hold true or will nutrient supply need to be altered to match nutrient demand by the plant? If the supply is altered, will plant height and flowering response be influenced under less-than-ideal growth conditions? In cases in which fertigation (the combination of fertilization and irrigation through the use of water-soluble fertilizer) is used, will water and nutrient use efficiency be impacted as DLI is altered as a result of shading to help cool a greenhouse? Growing two warm-season floriculture crops in a greenhouse with and without shading, varying concentrations of P were used to test general nutrient recommendations in commercially realistic conditions. The goals of the research were to determine: 1) if P supply should be adjusted or lowered as light decreases; 2) if deficiency and/or oversupply symptoms would be apparent at different P rates when growth rates decreased as a result of different light levels; and 3) the influence of P and light on overall P uptake efficiency and WUE. As a result of this research, appropriate rates of P supply could be Received for publication 8 Nov. 2012. Accepted for publication 19 Mar. 2013. I thank Alycia Pittenger, Russell Friedrich, and Doug Sturtz for their assistance throughout the work. Current address: DuPont Pioneer, 7305 NW 62 Avenue, P.O. Box 287, Johnston, IA 50131. To whom reprint requests should be addressed; e-mail jonathanfrantz319@gmail.com. 594 HORTSCIENCE VOL. 48(5) MAY 2013 identified and estimates of underor oversupply could be made. Materials and Methods Seeds of vinca ‘Cooler Grape’ were sown 7 July 2009 in seedling trays (Series 5005; Smithers-Oasis, Kent, OH), hydrated with tap water, and placed in a dark growth chamber set to a constant 22 C until germination. After germination, the growth chamber was controlled to 25/21 C day/night with a 16-h photoperiod. Seedlings were under a DLI of 8.6 mol·m·d during this stage. Seeds of zinnia ‘Oklahoma White’ were sown in a similar manner 2 weeks later but received light from sowing until transplant. Two weeks after sowing zinnia and 4 weeks after sowing vinca, seedlings were transplanted into 10-cm plastic containers filled with soilless media composed of 70% peat and 30% perlite. At mixing, substrate was amended with 3 g·L dolomitic lime and 0.59 g·L micronutrient charge (Micromax; Scott’s Co., Marysville, OH; with 12% iron, 2.5% manganese, 1.0% zinc, 0.5% zinc, 0.1% boron, and 0.05% molybdenum by weight). Planted seedlings were placed in a glassroofed, pad-and-fan-cooled greenhouse set to 28 C day and 23 C night. Humidity ranged from 60% to 85% daily average relative humidity for the duration of the study (9 weeks for zinnia and 6 weeks for vinca). Each pot was placed into a saucer to prevent leachate loss during irrigations. Electrical conductivity of the substrate was not measured. The greenhouse was divided into three complete blocks. Each block consisted of a bench with one randomly allocated end containing a frame with a single layer of window screen (40 squares/cm) that reduced the photosynthetically active radiation (PAR) by 47% and the other end of the same bench left unshaded. In the shaded and unshaded sections of each block, a quantum sensor (Model QSO-S; Apogee Instruments, Logan, UT) was connected to a data logger (CR10X; Campbell’s Scientific, Logan, UT) and recorded each of the previous 10-minute average photosynthetic photon flux and used to calculate DLI in mol·m·d. Plants were fertigated (irrigation with fertilizer combined) after transplanting with one of six nutrient solutions containing complete nutrients but variable amounts of P. Phosphorus treatments were (in mM) 0.1, 0.2, 0.5, 1.0, 2.0, and 4.0 supplied as KH2PO4 (Table 1). P treatments received the same amounts of potassium through the addition of K2SO4; sulfur supply varied concomitantly with P supply. Plants were checked once daily to determine watering needs; each container was judged, by picking it up, if water should be added that day. Up to 200 mL per fertigation was provided, slowly to each c

Uptake Efficiency of Phosphorus in Different Light Environments by Zinnia (Zinnia elegans) and Vinca (Catharanthus roseus)