Geraniums (Pelargonium spp.) are considered one of the most valuable potted plants produced in the United States with an estimated wholesale value of over $100 million in 2010 for both seed and vegetatively produced plants (U.S. Department of Agriculture, 2010). Geraniums are typically grown under greenhouse conditions during winter and spring to respond to the high demand of spring commercial markets. Nitrogen use efficiency is of great concern in floriculture production under greenhouse conditions. Nitrogen fertilization requirements and plant tissue analysis standards for geraniums are available (Biamonte et al., 1993; Dole and Wilkins, 2005; Kofranek and Lunt, 1969; Krug et al., 2010; Mills and Jones, 1997; Price et al., 1997). However, monitoring N status based on leaf sampling and foliar analysis is time-consuming and expensive. Because N is a major nutrient affecting plant chlorophyll content (Moorby and Besford, 1983), measuring chlorophyll concentration can be a useful index to assess the growth status and leaf N level of a plant (Seemann et al., 1987). Ground-based remote sensing of plant chlorophyll content and concentration offers the possibility of rapidly estimating crop N status and plant quality. For several horticultural crops, leaf N and chlorophyll concentration were found to be strongly correlated using a SPAD chlorophyll meter (Neilsen et al., 1995; Shaahan et al., 1999; Vos and Bom, 1993; Wang et al., 2004; Westerveld et al., 2003; Zanin and Sambo, 2006). In addition, the NDVI index is commonly used to differentiate plant properties such as chlorophyll, biomass, and plant nutrition (Raun et al., 1998). Use of an NDVI index is widely accepted in large-scale agronomic field production and some horticultural crops (Baghzouz et al., 2007; Bell et al., 2004; Carrillo, 2006; Clay et al., 2006; Eitel et al., 2008; El-Shikha et al., 2007; Peñuelas et al., 1994; Xiong et al., 2007) but has limited reports for greenhouse settings.
Based on assessing variability of reflectance measurements on plant growth and physiological conditions, using a statistical approach to extract useful information could be used to improve crop performance (Parihar et al., 2003). Discriminant analysis, a multivariate statistical classification model technique, is a powerful tool for selecting the predictor variables that allow the discrimination between different group levels and for classifying cases into different groups with a better than chance accuracy (Fernandez, 2010). It has been used successfully to separate N and water stresses in Helianthus annuus L. (Peñuelas et al., 1994), evaluate N status in Triticum aestivum L. (Filella et al., 1995), assess water and N stresses in Zea mays L. (Karimi et al., 2005a, 2005b; Strachan et al., 2002), and evaluate N and potassium deficiencies in Olea europaea L. orchards (Gómez-Casero et al., 2007). To our knowledge, there is no report about the detection of N status in potted geraniums to predict N fertilization needs using discriminant analysis.
In a previous experiment, Wang et al. (2012) found that NDVI and SPAD values can be used to reflect N status in geranium, and that cultivar and N rate can significantly affect flower quantity and quality. The objective of this study was to test the reliability of three NDVI measurements and SPAD values as indicators of geranium N status. Through assigning the aforementioned NDVI and SPAD values to predefined N levels based on leaf N content, we also conducted discriminant analysis with the purpose of integrating information as a tool to classify different N statuses.
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