Effect of deficit irrigation on nitrogen accumulation and capsaicinoid content in Capsicum plants using the isotope 15N
Introduction
Chilies are susceptible to environmental changes or stresses such as drought, nutrient deficiency, temperature or light stress (Gurung et al., 2011, Macias-Bobadilla et al., 2020, Pagamas and Nawata, 2008). For optimal growth, they require higher temperatures (18–26 °C), which can only be achieved in the temperate zone in greenhouses (Abdel et al., 2016). In greenhouse crop production, there is a constant need for irrigation, which leads to high water consumption (2.7–4.5 l per plant for chilies) (Ahemd et al., 2016). Water shortage is becoming a serious problem in the last quarter of the century, affecting many areas around the world (Kummu et al., 2016). In line with the increasing water scarcity in certain areas, many studies have been conducted to determine how to reduce and optimize water consumption (Kummu et al., 2016, Tundisi, 2008). In agriculture, one of the largest water consumers, solutions could lie in deficit irrigation (Fereres and Soriano, 2006). Studies have shown that deficit irrigation at the right intensity can improve the chemical quality of vegetables, with minimal or no yield loss compared to optimally irrigated crops, resulting in lower water consumption (Costa et al., 2007, Keabetswe et al., 2019). Deficit irrigation can be treated as low intensity drought stress (Kögler and Söffker, 2017). Drought is the most important stressor in agriculture and can significantly reduce yield and quality (Macias-Bobadilla et al., 2020). It also affects quality and studies have shown that low drought stress can improve fruit and vegetable quality (Stagnari et al., 2016). Drought reduces the uptake of nutrients such as nitrogen (N), which reduces plant growth and development (Ciríaco da Silva et al., 2010).
The uptake and accumulation of N in plants is influenced by precipitation, previous crop, soil properties, crop N demand, field management practices, and N form (Tao et al., 2018). N translocation in plants is associated with source/sink interactions. The major sinks in plants during vegetative growth are leaves and stems, and during the reproductive phase, seeds become an important sink (Tegeder and Masclaux-Daubresse, 2018). Using non-radioactive nitrogen isotopes (15N), we can track N transport and accumulation in plants from source to sink (Soong et al., 2014). We can also determine N fertilizer utilization and other parameters such as the percentage of isotope 15N from fertilizer in the plant (% Ndff), N in the plant (N yield), how much N comes from the plant from fertilizer (fertilizer N yield) (FAO/IAEA, 2001).
In our study, we tested two chili cultivars Capsicum annuum L. var 'Chili- AS Rot' and Capsicum chinense Jacq. var 'Naga Morich' under two irrigation regimes, the first was at field capacity of peat substrate and the second was deficit irrigation. Using the stable isotope 15N, we were able to determine in detail how the N was distributed in the plant among roots, leaves, stems and fruits under normal irrigation and deficit irrigation. We also investigated whether there is a relationship between the accumulation of N in the placenta, seeds, or pericarp of fruits and their capsaicinoid content, since capsaicinoids contain N in their molecule. This study answers several questions about the translocation of N and the use of N fertilizer in chilies under drought stress, with the use of the isotope 15N, which has rarely been detected in chili plants.
Section snippets
Experiment design
In Kamnik, Slovenia (46°13'33.88 "N; 14°36'39 "E), a randomized pot experiment was conducted with two chili cultivars. The first cultivar was the less hot (30.000 Scoville Heat Units) Capsicum annuum L. var 'Chili- AS Rot' and the second was the very hot (1.000.000 Scoville Heat Units) Capsicum chinense Jacq. var 'Naga Morich' (both purchased from Austrosaat). Each cultivar was represented by 20 plants (10 plants per treatment) sown in February. The seedlings were transplanted into ϕ 9 cm
Irrigation data
Irrigation data are presented in Table 1. The cultivar 'Chili-AS Rot' received 396.3 mm water at field capacity of peat substrate and 197.44 mm water at deficit irrigation. The average soil water content and soil water tension under field capacity irrigation were 23.5% and −23.9 kPa, respectively, and 12.3% and −51.2 kPa, respectively, under deficit irrigation. The cultivar 'Naga Morich' received less water through irrigation than the cultivar 'Chili-AS Rot' with 332.3 mm under field capacity
Discussion
In both cultivars, leaves had the highest total N content, followed by seeds or placenta, depending on the cultivar. Seeds are an important N sink during the reproductive phase (Tegeder and Masclaux-Daubresse, 2018), which would explain higher N contents in seeds and placenta. During growth, on the other hand, roots, leaves and stems are strong N sinks and since our plants were harvested at the end of the growing season, this would explain why N levels in vegetative parts were not as high as
Conclusion
Using deficit irrigation and 15N isotope assays, we observed N transport in two chili cultivars. Our study highlighted a novelty in N transport in chili as we examined roots, stems, leaves and three different fruit parts. Both cultivars responded differently to N fertilization, with the cultivar 'Chili- AS Rot' having lower N fertilizer utilization under deficit irrigation than under field capacity irrigation, while 'Naga Morich' had better N fertilizer utilization under deficit irrigation. We
CRediT authorship contribution statement
T.Z. experimental process, writing, metabolite extractions, statistical analysis; A.S. reviewing of manuscript, conceptualization, methods; S. L. isotope methods and reviewing of manuscript; V.Z. conceptualization, methods, reviewing of manuscript, financial support.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is part of the Horticulture P4-0013-0481 and Agroecosystems P4–0085 programs supported by the Slovenian Research Agency and the Infrastructural Centre IC RRC-AG (IO-0022-0481-001). Isotope analyses were supported by the SRA program P1-0143. This work was partially financed by IAEA TCP SLO5004 and IAEA CRP D1.50.18.
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