The efficacy of herbaceous grafts relies on the replacement of a scion root system with that of a rootstock with known disease resistance. The use of disease-resistant rootstocks in grafted tomato production has proven efficacious in managing numerous economically significant soilborne diseases (Kubota et al., 2008; Kunwar et al., 2015; Lee and Oda, 2002; Louws et al., 2010). In addition, certain rootstocks can improve tolerance to abiotic stress such as cold soils, salinity, drought, and flooding (Albacete et al., 2015; Colla et al., 2006; Djidonou et al., 2013; Estañ et al., 2005; He et al., 2009; Venema et al., 2008; Yetisir et al., 2006). These additional benefits afforded by certain rootstocks can allow a grower to custom tailor the scion–rootstock combination to their local production environment and disease pressure.
Tolerance to edaphic stress has been linked to RSM. Increased TRL has been attributed to improving nutrient uptake, especially phosphorus (P), when availability is low (Hill et al., 2006; Lambers et al., 2006). In drought conditions, increased TRL, particularly in the deeper soil profile, can improve water acquisition (Comas et al., 2013; Ho et al., 2005; Lopes and Reynolds, 2010; Schenk and Jackson, 2005; Wasson et al., 2012). A smaller root diameter may also aid water uptake in dryer conditions by reducing hydraulic resistance (Huang and Eissenstat, 2000; Passioura, 1988; Rieger and Litvin, 1999; Sharp et al., 1988; Steudle and Peterson, 1998). A reduction in root diameter has also been observed in response to low P concentrations (Hill et al., 2006; Zobel et al., 2007) and salinity (Lovelli et al., 2012).
SRL, defined as a proportion of TRL to root dry matter, is a metric used to describe the ratio of the morphological benefit to metabolic cost in root system development (Eissenstat, 1992). An increase in SRL, in relation to a reduction of root diameter, has been observed as a response to low P (Christy and Moorby, 1975; Hill et al., 2006; Lambers et al., 2006; Schroeder and Janos, 2005), drought (Huang and Eissenstat, 2000), and salinity (Lovelli et al., 2012).
Though both intrinsic RSM and the changes observed in response to abiotic stress and reduced resources have been well studied, limited research has been conducted to compare tomato root systems. Differences by cultivar have been observed in the RSM of processing tomato (Portas and Dordio, 1979; Zobel, 1975). For tomato rootstock root systems, one hydroponic study comparing two commercial rootstocks (‘Beaufort’ and ‘Heman’) indicated differences in root density but not of average root diameter (Oztekin et al., 2009). The physical aspects of solid substrates can greatly affect root systems grown in soil compared with those in hydroponics (Chapman et al., 2012). The spatial heterogeneity of nutrient and water content within solid substrates affects both root morphology and architecture (Desnos, 2008; Forde and Lorenzo, 2001; Lopez-Bucio et al., 2003). Hydroponic systems are designed to optimize growth by supplying roots with homogenous root zone resources. Consequently, translation of results from hydroponic studies, especially solution-based systems, to plants grown in the field or solid substrate should be done so with caution. To date, no research has been conducted comparing commercial tomato rootstock RSM in substrate-grown plants.
The goal of this study was to assess tomato rootstock RSM and development in solid substrates. This information may help characterize rootstocks for their potential increase in abiotic stress tolerance observed and aid in screening and/or breeding for stress tolerance in tomato rootstocks. The specific objectives of this study were to 1) compare RSM of 17 commercially available rootstocks and one commonly used tomato cultivar grown in a porous, solid substrate, at the seedling stage and 2) determine how RSM in these cultivars changes over time.
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