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−1) 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 ×hybrida), 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 (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−1 and is often as low as 2.5 g·kg−1 (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 identified and estimates of under- or oversupply could be made.
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