Nitrogen uptake by plants is generally in anionic (NO3 −), cationic (NH4 +), or neutral (CH4N2O) forms. Excessive addition of a single element may cause an imbalance of other nutrients (Marshner, 1995; Mengel and Kirkby, 2001; Mills and Jones, 1996) and may predispose plants to disease (Engelhard, 1989; Hoffland et al., 2000; Huber and Watson, 1974; Jarvis, 1992; Mansfield, 1980; Marshner, 1995). The probability of N toxicity is rare because unlike micronutrients, a toxic N concentration is several-hundred-fold higher than sufficient N concentrations. However, the practice of periodic fertigation (fertilizer mixed with irrigation water) of concentrations substantially higher than published recommendations is not uncommon under intensive greenhouse production systems. The N use efficiency is critical in determining N supply rate, which ultimately can reduce input costs while protecting the environment from N deposition (Good et al., 2004). In greenhouse production systems, especially under soilless culture, plant N uptake is solely dependant on external supply. Objective assessment of N use efficiency will provide vital information for the greenhouse industry, which leads to prudent management of resources.
Much of the literature on plant nutrition focuses on plant growth effects in the nutritional extremes of either toxicity or deficiency. There are fewer evaluations of the sometimes subtle changes in plant growth and appearance and the potential for increased plant pathogen susceptibility in commonly encountered N concentrations (Kent and Reed, 1996; Nelson et al., 1978; Smith et al., 1998).
Studies have linked N supply and disease (David et al., 2003; Engelhard, 1989; Huber and Watson, 1974; Mengel and Kirby, 2001). High N supply has resulted in increased disease susceptibility to Pseudomonas syringae van Hall pv. tomato (Okabe) Young, Dye & Wilkie and Oidium lycopersicum Cook & Mass. for tomato (Lycopersicum lycopersicum L.) (Hoffland et al., 2000) in contrast with resistance to B. cinerea with high N supply in tomato (Hoffland et al., 1999). This inconsistency could be the result of the type of pathogens, N form, source and concentration, type of substrate, irrigation frequency, and growing environment. Commercially blended fertilizers that contain high concentrations of NH4 + are lower in Ca (Nelson, 1996). High levels of Ca and K have significant influence on disease resistance (Elad and Volpin, 1993; Marshner, 1995; Mengel and Kirby, 2001). Calcium has a major physiological role in forming the cell membrane and cell wall, which protects against pathogens (Elad, 1988; Elad and Evensen, 1995; Volpin and Elad, 1991). High potassium also suppresses disease in plants (Mengel and Kirby, 2001; Talbot and Zeiger, 1996). These confounding findings could be a potential factor for conflicting reports of the influence of N in disease-related studies.
Botrytis cinerea is a ubiquitous pathogen that infects leaves, stems, and flowers. Symptoms of the disease appear as water-soaked lesion spots, which quickly coalesce affecting the entire tissue, causing major economic loss (Elad, 1988; Hausbeck and Moorman, 1996). Susceptibility to B. cinerea disease has been reported to decrease with NO3 − form but increase with NH4 + form (Huber and Watson, 1974). Conversely, it has also been reported that B. cinerea shows no growth benefit based on the form of N supplied to the plant (Townsend, 1957).
Begonia [Beg (Begonia × tuberhybrida Voss)] and new guinea inpatiens [NGI (Impatiens hawkeri Bull.)] are categorized as high-valued plants grown in pots and hanging baskets (U.S. Dept. Agr., 2005). These plants are succulent with tender tissues and are considered inefficient in terms of water use, requiring ample water supply but not water-logged conditions (Hartley, 1995). This creates a moist growing environment causing the plants to be susceptible to the ubiquitous fungal pathogen B. cinerea (Elad and Shtienberg, 1995; Hausbeck and Moorman, 1996). The nutritional requirement is moderate for these species (Nelson, 2005). All these factors contributed to the selection of these greenhouse plants as model species for biotic stress and nutritional interaction studies.
This study was conducted to investigate the growth of Beg and NGI, N uptake efficiency, shoot N use efficiency, and shoot N utilization efficiency and the potential susceptibility of these plants to biotic stress (disease: B. cinerea) to N concentrations that are commonly found in commercial greenhouses. This study provides insight about the nutritional economy of plants across a range of N supply.
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