Transplant shock is very common in globe artichoke [C. cardunculus (L.) var. scolymus L. (Fiori)] grown in semiarid regions of the United States, such as southwest Texas. High air temperatures and drought stress after transplanting can delay root and shoot growth and significantly reduce marketable yield (Leskovar and Xu, 2013). Therefore, effective nutrition and irrigation management strategies must be used during artichoke growth and development in the nursery period.
Containerized transplants have been widely used to improve stand establishment, control plant spacing, produce uniform plants, provide crop earliness, and increase total marketable yield (Leskovar and Xu, 2013; Russo, 2005). Several pretransplant conditioning methods have been used in the nursery to improve transplant quality, which include growth regulators such as abscisic acid (ABA), antitranspirants, water deficit stress, media inoculation with mycorrhizal fungi, and temperature control (Campanelli et al., 2014; Garner and Björkman, 1996; Shinohara and Leskovar, 2014). Exogenous ABA induced stomatal closure and enhanced drought tolerance; however, film-forming antitranspirants were not effective in mitigating drought stress in artichoke transplants (Shinohara and Leskovar, 2014). Moreover, mycorrhizal fungi (Glomus viscosum) improved stomatal conductance (gs) and chlorophyll content index (SPAD) of micropropagated globe artichoke (Campanelli et al., 2014).
About 50% of N applied as fertilizer is used by plants whereas the remaining 50% is emitted to the atmosphere (≈25%), leached or depleted to aquatic systems (≈20%), and stored in soils (≈5%) (Galloway et al., 2004). In containerized transplants, proper N level can markedly reduce N leaching from the growing media and improve transplant growth and development (Soundy et al., 2005). Fertigation techniques can also potentially impact transplant quality. For example, FR using FL irrigation can lead to salt accumulation in the upper layer of media in the cells of the tray (Liu et al., 2012) and reduce transplant quality. Since there is little runoff from the growing medium when FL or subirrigation is used, a recommended fertilizer guideline is to reduce fertilizer (20N–10P–20K) concentration in the irrigation water when FL system is used in petunia (Petunia ×hybrida) (Klock-Moore and Broschat, 2001). However, FR of ornamental pepper [Capsicum annuum (L.)] using OH and subirrigation revealed that subirrigated plants should not be fertilized with lower N than OH irrigated plants (Kang et al., 2004). Overhead irrigation can better control salt accumulation in the container than FL because the excess water can leach out salts and prevent buildup in the growing medium (Liu et al., 2012); however, nutrient deficiency and low water use efficiency are still concerns.
In the field, subsurface irrigation delivered through drip systems placed deep in the soil (e.g., 30-cm depth) could lead to water deficit in the top soil surface due to slow capillary movement of water (Rowe et al., 2014). When water fronts radiating from the surface and subsurface drip emitters do not merge, they can leave dry regions between emitters (Rowe et al., 2014). Conversely, OH irrigation delivers water by gravity, distributing more uniformly over the planted area (Rowe et al., 2014). However, using preconditioned transplants could provide an adaptive mechanism to withstand microclimate shock in the field (Franco and Leskovar, 2002). This is because transplant root growth in the field is related to management practices (e.g., N level supplied) during the transplant stage in the greenhouse (Liptay and Nicholls, 1993). The use of FL fertigation system in the nursery period allowed muskmelon [Cucumis melo (L.)] to adapt to hot and dry weather and showed a trend of increased yield when compared with OH fertigation (Franco and Leskovar, 2002). In fact, FL fertigation maintained a uniform lateral root development in jalapeno pepper transplants and is recommended for promoting transplant hardiness (Leskovar and Heineman, 1994).
Root growth and development is significantly affected by management strategies impacting water and nutrient levels, but the underlying mechanisms are poorly understood (Zhang et al., 1999), especially for containerized vegetable transplants. In Arabidopsis, nitrate and phosphorus (P) levels can alter root system architecture (Linkohr et al., 2002). Increasing nitrate concentration in growing media reduced primary root length, while it increased with increasing P supply (Linkohr et al., 2002). In addition, lateral root elongation was suppressed by high nitrate and P levels (Linkohr et al., 2002). Although reduced N fertilization is effective to control transplant height, this approach can significantly limit other growth characteristics such as stem diameter and root volume (Liu et al., 2012). For example, dry matter allocation to roots increased by 7.5% and 12% after 72 and 120 h of N deprivation in rice [Oryza sativa (L.)] transplants, but the hydraulic conductivity of the same N-deprived plants was reduced by 24% of that in the control after 48 h (Ishikawa-Sakurai et al., 2014).
The objectives of this study were to 1) identify the impact of N level (low vs. high) and FR technique (OH vs. FL) based on changes in root/shoot growth and leaf level physiological responses, and to 2) determine if the FR method and N level used in the nursery significantly modifies the early vegetative growth or yield when grown under surface, subsurface, and OH linear irrigation. Information from this study will be useful to better understand the importance of N and irrigation strategies on improving transplant quality and stand establishment of globe artichoke when transplanted in hot and drought-prone environments.
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