Calendula (Calendula officinalis L.), or pot marigold, is an herbaceous annual or short-lived perennial (Mohammad and Kashani, 2012) that is widely naturalized and grown for ornamental and medicinal purposes throughout temperate zones. The species has been cultivated since antiquity for its purported general medicinal qualities. Recent work investigating calendula’s medicinal properties have reported antimicrobial (Mohammad and Kashani, 2012), antiviral (Kalvatchev et al., 1997), antitumor (Matic et al., 2013), and anti-inflammatory (Preethi and Kuttan, 2009; Preethi et al., 2009) effects. Pharmaceutically active compounds in calendula are commonly derived from the “essential oils” distilled from fully open flowers (Gazim et al., 2008; Khalid and Teixeira da Silva, 2010; Okoh et al., 2008). Essential oils, or “volatile oils” are complex mixtures of plant secondary compounds, whose quantity and quality may be affected by cultivar and growing environment (Jha et al., 2011; Khalid and Teixeira da Silva, 2010; Król, 2011; Ozturk et al., 2004).
Calendula varieties grown for ornamental and medicinal purposes both contain essential oils; however, the systems in which they grow may differ considerably. Ornamental cultivars are typically grown on a relatively small scale, often under irrigated and high nutrient input conditions (RezaeiNejad and KhosraviShakib, 2013). In contrast, calendula used for medicinal or oilseed purposes are grown on a relatively large scale, and may be in systems that are water and/or nutrient limited (Forcella et al., 2012).
Understanding how to optimize medicinal plant production under reduced input regimes (e.g., water and fertility) is key to the sustainable and profitable production of calendula and other medicinal plants in a variety of contexts. Calendula grown for essential oil is often produced in semiarid and arid regions where agricultural water is limited (e.g., Khalid and Teixeira da Silva, 2010; Maleki et al., 2014; Metwally et al., 2013). Production of medicinal plants has been suggested as an economic development activity in developing countries (Khalid and Teixeira da Silva, 2010; Srivastava et al., 1996), where access to conventional inputs may be limited (Srivastava et al., 1996). Sustainability-oriented production systems may be structured with similar constraints. Low external input farming systems seek to minimize purchased off-farm inputs, such as fertilizers and pesticides, and to increase the internal production and cycling of on-farm inputs (Parr et al., 1990). Similarly, organic farming systems often use fertility sources of biological origin (e.g., cover crops, crop residues, composts, and manures) and are based on minimal use of off-farm inputs (USDA NOSB, 1995). In all of these situations, nutrient availability and water limitation present management challenges and may have complex effects on essential oil production in medicinal plants.
Ensuring an adequate N supply is a key challenge to crop production in these systems (Parr et al., 1990), because the bulk of the N must be mineralized from the fertility source by microbial decomposition before plant uptake. Consequently, N availability may be difficult to predict (Gaskell, 2006; Gaskell and Smith, 2007; Hartz et al., 2010), leading to deficiencies that can reduce plant growth (Chand et al., 2011; Nourimand et al., 2012; Siddiqui et al., 2011). Although N stress is generally detrimental to plant growth, the effects on plant secondary compounds are somewhat more complex. Nitrogen deficiency may enhance synthesis of secondary chemicals including antioxidants of medicinal plants (Chand et al., 2011; Nourimand et al., 2012; Siddiqui et al., 2011). However, N fertilization has also been positively correlated with enhanced secondary compound synthesis and the ability to mitigate oil yield losses caused by drought (Rahmani et al., 2011).
Similarly, drought stress may reduce plant primary productivity, but the effects on secondary compounds are less straightforward. Drought can reduce plant biomass and simply increase the concentrations of secondary metabolites due to reduced fresh biomass in water-restricted plants (de Abreu and Mazzafera, 2005; Liu et al., 2011; Nogues et al., 1998; Selmar and Kleinwaechter, 2013). However, Selmar and Kleinwaechter (2013) opine that this is an oversimplification, and that drought stress may cause a plant to redirect carbon from growth to increased secondary compound production (Selmar, 2008; Selmar and Kleinwaechter, 2013). This may protect the plant from damage induced under stress conditions. For example, the medicinal plants Rosmarinus officinalis L. and Salvia officinalis L. produce more essential oils at higher concentrations when grown under drought stress conditions (Selmar and Kleinwaechter, 2013). The essential oils are effective antioxidants and thus protect the plant from reactive oxygen species that result when water-stressed plants are unable to dissipate surplus absorbed solar energy (Bozin et al., 2007; Selmar and Kleinwaechter, 2013).
The effects of both nutrient supply and water availability on medicinal plant growth and secondary compound production are important to understand singularly. However, in systems using biological sources of fertility, nutrient mineralization is mediated by soil microbial activity, which is highly influenced by moisture and temperature (Drinkwater and Snapp, 2007). As such, plant growth and secondary compound production in medicinal plants grown in these systems is a function of the interaction between the nutrient source and climatic factors. There has been little work examining the interaction of these factors on resource allocation in medicinal plant production, save a few notable exceptions (e.g., Jha et al., 2011; Król, 2011). Thus, improved understanding of how the interactions between plant growth, development, and secondary compound production are affected by nutrient dynamics and drought stress is important for the sustainable production of medicinal crops, such as calendula.
This work determined the influences of fertility source and water stress on N availability, plant growth, and essential oil production of four calendula cultivars in a greenhouse setting. Cultivars included herbal and ornamental varieties. Each cultivar was produced using four fertility treatments, including conventional, high-input organic, low external input organic, and no input treatments. Drought stress was induced in each combination of treatments.
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