Responses of Spinach to Salinity and Nutrient Deficiency in Growth, Physiology, and Nutritional Value

By Chenping Xu, B. Mou. 2016

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Salinity and nutrient-depleted soil are major constraints to crop production, especially for vegetable crops. The effects of salinity and nutrient deficiency on spinach (Spinacia oleracea L.) were evaluated in sand cultures under greenhouse conditions. Plants were watered every day with Hoagland nutrition solution, deprived of nitrogen (N), phosphorous (P), or potassium (K) for nutrient deficiency, either with or without 20/10 mM sodium chloride (NaCl)/ calcium chloride (CaCl2) for salinity treatment. Salinity significantly decreased shoot fresh weight (FW) and dry weight (DW), leaf relative water content (RWC), and specific leaf area (SLA) relative to controls after 4 weeks of treatment and increased chlorophyll content, maximum photochemical efficiency (Fv/Fm), and photochemical yield [Y(II)]. Nitrogen deficiency greatly reduced shoot FW and DW, SLA, and chlorophyll content, regardless of salt treatment. Y(II) and Fv/Fm were reduced by N deficiency and salinity treatment. Phosphorous and K deficiencies, similarly, decreased shoot FW and DW irrespective of salinity treatment and increased chlorophyll content without salt stress. Phosphorous deficiency increased Y(II) under control and Fv/Fm under both control and salt treatment. Salinity and nutrient deficiency also affected the nutritional value of spinach. Salt stress increased carotenoid and flavonoid contents, and reducing power in full-strength Hoagland solution, and decreased leaf ferrous ion chelating ability (FICA). Nutrient deficiency increased reducing power regardless of salinity treatment. Nitrogen deficiency increased anthocyanin and total phenolic contents, decreased carotenoids and flavonoids regardless of salinity treatment, and increased antioxidant capacity under no-salt conditions. Phosphorous deficiency increased carotenoid and flavonoid contents under no-salt condition and enhanced total phenolic content and reduced FICA and amino acid content under salt stress. Potassium deficiency increased total phenolic, carotenoid, and flavonoid contents and antioxidant capacity under non-salt condition, but decreased FICA regardless of salinity treatment. These results suggest that spinach nutritional value could be improved with only moderately or slightly reduced yield through cultural practices that impose either low fertilizer levels or slight salt stress. High-salinity conditions in agricultural soil and irrigation water is one of the most serious challenges faced by agricultural crops in the world. It is estimated that salt-affected soils impact nearly 10% of the land surface and 50% of irrigated land in the world (Ruan et al., 2010). The response of plants to salinity is complex and involves changes in morphology, physiology, and metabolism. Salinity effects on plants include cellular water deficit, ion toxicity, nutrient deficiencies, and oxidative stress, leading to growth inhibition, molecular damage, and even plant death (Orcutt and Nilsen, 2000). The global annual losses in agricultural production from salt-affected land are in excess of $12 billion and rising (Flowers et al., 2010; Qadir et al., 2008). Another important limiting factor for crop production is soil nutrient deficiency. Nitrogen is considered the most common deficient nutrient, followed by P and K (Halvin et al., 2005). Nitrogen is a major constituent of proteins, nucleotides, chlorophyll, and many other metabolites and cellular components (Lawlor, 2002), and thus N deficiency affects enzyme activities and metabolite content, hastens senescence, causes nucleic acids and proteins to break down, and therefore inhibits plant growth (Crafts-Brandner et al., 1998; Lemâıtre et al., 2008; Wingler et al., 2006). Phosphorus is a macronutrient of crucial importance in signaling, photosynthesis, and other metabolism (Marschner, 1995). Potassium is an essential nutrient that affects most of the biochemical and physiological processes that influence plant growth and metabolism (Wang et al., 2013). Spinach is an important leafy green vegetable that contains large quantities of bioactive compounds and nutrients that are not common to most other vegetables, such as r-coumaric acid derivatives that exhibit strong antioxidant activity and glucuronic acid derivatives of flavonoids (Bergman et al., 2001; Edenharder et al., 2001; Pandjaitan et al., 2005). It was identified as a moderately salt-sensitive vegetable (Shannon and Grieve, 1999). Salt stress reduced spinach germination, root elongation, seedling growth, chlorophyll content and photosynthesis, and increased membrane permeability (Delfine et al., 1998; Downton et al., 1985; Kaya et al., 2002; Robinson et al., 1983). Langdale et al. (1971) reported that the tolerance threshold for spinach is 2.0 dS m, but irrigation with saline water of 4.0 dS m on sandy soils did not result in yield reduction (Pasternak and De Malach, 1994). Studies in solution cultures have shown that no significant growth reduction occurs up to 8 dS m (Osawa, 1963). There is limited information Received for publication 17 Aug. 2015. Accepted for publication 20 Oct. 2015. This research was supported by a grant from the California Department of Food and Agriculture Specialty Crop Block Grant Program (SCB13047). The technical assistance of Phi Diep and critical review by James McCreight and Renee Eriksen are greatly appreciated. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Corresponding author. E-mail: chenping.xu@ars.usda.gov. 12 J. AMER. SOC. HORT. SCI. 141(1):12–21. 2016. about the effects of micronutrient deficiency on spinach growth and physiology. With regard to nutritional quality, previous studies indicated that mild environmental stress or low fertilization could be exploited to enhance nutritional values of vegetable crops. Low N fertilization (Galieni et al., 2015; Stagnari et al., 2015) or environmental stresses, such as salt, heat, chilling, and high light intensity (Oh et al., 2009; P erezL opez et al., 2013), enhanced phenolic content and antioxidant capacity of lettuce (Lactuca sativa L.). Similarly, fruit nutritional value of tomato (Solanum lycopersicum L.) could be improved by low N supply (B enard et al., 2009) or salt stress (Borghesi et al., 2011). However, literature on spinach nutritional values affected by abiotic stresses is very scarce. The current study aims to assess the effects of salinity and single nutrient (N, P, or K) deficiency on spinach growth, physiology, and nutritional value. Materials and Methods PLANT MATERIALS AND TREATMENTS. Two trials, each with four replications, were conducted from 3 Feb. to 27 Mar. 2015 and 24 Feb. to 17 Apr. 2015, in a greenhouse located in Salinas, CA (lat. 36 40#40$N, long. 121 39#20$W). The average temperature ranged from 12 to 31 C and relative humidity ranged from 30% to 80%. The greenhouse was supplemented with light of a 12-h photoperiod (Sun System 3; Sunlight Supply, Vancouver, WA). The spinach cultivar Crocodile was used in this experiment because the preliminary study indicated that it has middle salt tolerance. For each trial, 2 weeks after seeded in rock wool cells (Grodan Group, Roermond, Netherlands), uniform spinach seedlings were transplanted into plastic pots (2.3 L) filled with sand. Plants were thinned to one plant per pot and watered every day with fullstrength Hoagland nutrition solution [electrical conductivity (EC) = 2.5 dS m] (Hoagland and Arnon, 1950). For each trial, salinity and nutrient treatments were applied 10 d after transplanting. Salinity stress was induced by adding 10/5 mM NaCl/ CaCl2 into full-strength Hoagland nutrition solution (EC = 4.6 dS m) for 2 d, and then by adding 20/10mM NaCl/CaCl2 (final EC = 6.5 dS m). To induce mild salt stress, 20/10 mM NaCl/CaCl2 was used since preliminary studies indicated that adding 40/20 mM NaCl/CaCl2 into full-strength Hoagland nutrition solution (EC = 10.2 dS m) for 4 weeks caused 50% reduction in shoot FW. Exposure of plants to increasing salt concentration allows gradual acclimation of plants to salinity conditions to avoid sudden death of plants at high salt concentration. For N, P, or K deficiency, the sources of N, P, and K in Hoagland’s solution were replaced with alternative chemicals. The N sources, calcium nitrate (5 mM) and potassium nitrate (5 mM), were replaced with CaCl2 (5 mM) and potassium sulfate (2.5 mM), respectively. The P source, potassium dihydrogen orthophosphate (1 mM), was replaced by potassium sulfate (0.5 mM). The K sources, potassium dihydrogen orthophosphate (1 mM) and potassium nitrate (5 mM), were replaced with sodium dihydrogen phosphate (1 mM) and sodium nitrate (5 mM), respectively. The plants were watered every day for 4 weeks with excess amount of respective solutions to keep the N, P, K, and salt contents in sand relatively consistent. GROWTH AND PHYSIOLOGY MEASUREMENTS. For each trial, 4 weeks after starting the treatment, leaf Fv/Fm and Y(II) were measured with a fluorometer (MINI-PAM-II; Heinz Walz, Effeltrich, Germany) on the first and third leaves from bottom of each plant. Leaf Fv/Fm was measured after leaves were adapted in darkness for 30 min. Leaf discs were collected using a cork borer from the four largest leaves of each plant to measure RWC, SLA, chlorophyll content, and nutritional values. Specific leaf area was calculated Fig. 1. Effect of salinity and nutrient deficiency on spinach shoot (A) fresh weight and (B) dry weight 4 weeks after treatment. The values are means of eight replicates ±SE. Different letters on top of bars indicate significant difference at P # 0.05 according to Student’s t test. J. AMER. SOC. HORT. SCI. 141(1):12–21. 2016. 13 as SLA = area/DW, where DW is leaf dry weight after drying at 65 C for 3 d (Evans, 1972). Leaf RWCwas calculated as RW

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