Quantifying and Understanding Factors Affecting Tissue Nitrate Accumulation in Organic Celery

Progress report for GS21-239

Project Type: Graduate Student
Funds awarded in 2021: $16,497.00
Projected End Date: 08/31/2023
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Dr. Xin Zhao
University of Florida
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Project Information


Organic agriculture emphasizes the improvement of soil organic matter and stresses the foundation of maintaining healthy soil structure and diverse biological activity. With a fundamentally unique approach to soil fertility, the avoidance of many synthetic inputs leaves the organic grower much more dependent on soil biological processes that govern nutrient pools and transformations. Differences in fertility management and the challenges associated with navigating site- and season-specific variations in nutrient release and availability may result in different concentrations of tissue nitrate in vegetable crops. Previous studies have indicated that nitrate concentration is lower in organic foods than their conventional counterparts in many cases. Celery is known as a nitrate accumulator, and celery powder has been utilized as an alternative to traditional food preservatives. Regardless of the controversy associated with nitrates and human health and the restriction on maximum nitrate levels of certain leafy greens in Europe, the USDA National Organic Program currently places conventional celery powder on the National List for non-organically produced agricultural products allowed as ingredients in processed products labeled as organic. Concerns have recently been expressed questioning the use of conventional celery powder in the production of organic processed meats, although a lack of consistent, research-based recommendations for organic growers to meet the needs of the processing industry remains a significant barrier. To date, little research-based information is available regarding varietal differences in celery nitrate levels and the impact of soil fertility management on nitrate accumulation in celery.

In addition to the inherent variation of tissue nitrate concentration between species and within organic and conventional counterparts, there are also other factors that can influence the accumulation of nitrate. Intraspecific variation among different cultivars is common and has been observed in species including celeriac and lettuce. Diurnal fluctuation in nitrate concentration has been observed in several species, likely due to the influence of light quality and intensity on the activity of nitrate reductase. Given the lack of information on nitrate accumulation dynamics in organically grown celery, this project focuses on the examination of cultivar, harvest time, and nutrient management factors affecting nitrate concentrations in celery under organic production. Findings from this project will help improve our understanding of celery nitrate accumulation in relation to soil nitrogen (N) availability and crop productivity.


Project Objectives:

Objective 1: Assess commercially available celery cultivars in terms of their yield performance and capacity to accumulate tissue nitrate throughout the growing season under organic production.

Objective 2: Determine the effect of early morning and afternoon harvesting on the accumulation of tissue nitrate in organic celery.

Objective 3: Compare different rates of total seasonal N application using various preplant and liquid organic fertilizer types to quantify effects of fertilizer source and rate on crop yield, soil N availability, and tissue nitrate concentration.


Materials and methods:

      Field trials addressing all objectives have and will continue to be conducted on certified organic land at the University of Florida Plant Science Research and Education Unit in Citra, FL.

Objective 1: Assess commercially available celery cultivars in terms of their yield performance and capacity to accumulate tissue nitrate throughout the growing season under organic production.

In Fall 2021, a selection of 10 commercially available celery cultivars (‘Balada’, ‘Conquistador’, ‘Command’, ‘Kelvin F1’, ‘Merengo’, ‘Pink Plume’, ‘Tall Utah’, ‘Tango’, ‘TZ 6200 F1’, and ‘Victoria’) for organic production was seeded in a research greenhouse at the University of Florida campus in Gainesville and fertigated using 2N-1.3P-0.8K Neptune’s Harvest fish and seaweed fertilizer (Gloucester, MA) with N applied at the concentration of 200 mg L-1. Eight weeks after emergence (22 Nov. 2021), plants were transplanted onto plastic-mulched raised beds in double rows (1.2 m between-bed spacing and 30 cm plant spacing) irrigated with drip irrigation. A preplant application of 10N-0.9P-6.6K Nature Safe organic fertilizer (Darling Ingredients Inc., Irving, TX) was applied at 35% (78.8 kg ha-1 N) of the total seasonal rate of 225 kg ha-1 N with weekly fertigation using 5N-0.4P-0.8K Aqua Power liquid fish fertilizer and 0N-0P-41.3K Big K (JH Biotech Inc., Ventura, CA). No supplemental phosphorus (P) was targeted due to high levels of soil P, and potassium (K2O) was applied at a total rate of 179 kg ha-1 based on soil testing. Drip irrigation was scheduled based on crop ET. The experiment was arranged in a split-plot design with four replications. Cultivar and harvest dates on a 21-day interval served as the whole and sub-plot factors, respectively. Total fresh weight was measured on six celery plants from each experimental unit and reduced to four plants as biomass increased throughout the season. Sub-samples from each experimental unit were divided into leaf and petiole tissue to assess the biomass portioning and identify differences in nitrate accumulation across different tissue types. Leaf and petiole tissue samples were dried separately to a constant weight at 60 °C, and the dry samples will be sent to Waters Agricultural Laboratories (Camilla, GA) for total-N analysis. Nitrate-N will be analyzed on a dry-weight basis, with analysis currently underway. Additional sub-samples from each experimental unit were frozen at -20 °C immediately after harvest. Samples will be thawed, macerated using a blender, and centrifuged at 10,000 g for 10 min. Supernatant will be analyzed for nitrate-N concentration using the nitration of salicylic acid microplate assay. Briefly, salicylic acid in concentrated sulfuric acid is reacted with nitrate-containing samples, and sodium hydroxide halts the reaction after 20 min. Samples are then analyzed at 410 nm and compared to a standard curve developed using KNO3 standards. Soluble solids content (SSC) will also be measured from the supernatant using a handheld refractometer. At final harvest, relative chlorophyll content was measured from recently mature leaves in each experimental unit using a SPAD-502Plus Chlorophyll Meter (Spectrum Technologies, Aurora, IL), and crown diameter was measured using a digital caliper at the cut surface immediately above the soil line. Plant height was assessed by measuring from the bed top to the highest leaflet tip. Leaf number was recorded by counting leaves longer than 12 cm. Celery biomass, nitrate-N concentration, and total-N accumulation curves for the production season will also be developed.


Objective 2: Determine the effect of early morning and afternoon harvesting on the accumulation of tissue nitrate in organic celery.

In the same field trial described in Objective 1, celery cultivars were harvested in the morning (30 min after sunrise) and in the afternoon (30 min after solar noon) on each sampling date, following the protocols outlined in Objective 1. Harvest time was considered a sub-subplot factor, added to the experimental design of Objective 1 so that interactions between cultivar and sampling time can be analyzed for each harvest date.


Objective 3: Compare different rates of total seasonal N application using various preplant and liquid organic fertilizer types to quantify effects of fertilizer source and rate on crop yield, soil N availability, and tissue nitrate concentration.

Another organic celery trial was initiated in Fall 2021 to compare different seasonal N rates and organic preplant fertilizer sources. Celery transplants (‘Tango’) were grown as described in Objective 1 and transplanted onto similar plastic-mulched beds. The experiment was arranged in a split-plot design, with total seasonal N rate and preplant fertilizer type serving as whole and sub-plot factors, respectively. Total seasonal N rates included 140 (N140), 224 (N224), and 308 (N308) kg ha-1. Preplant fertilizer was applied at 35% of the total seasonal rates, with the remaining N applied using 5N-0.4P-0.8K Aqua Power liquid fish fertilizer. A zero-N control (N0) and a preplant-only control (N78C) was included, matching the preplant contribution of N224 but omitting in-season fertigation. Supplemental 0N-0P-41.3K was applied as necessary to achieve a total rate of 179 kg ha-1 K2O. The two types of preplant fertilizers included 10N-0.9P-6.6K Nature Safe and 3N-1.3P-2.5K Everlizer (Organic Growing Solutions, Live Oak, FL). Nature Safe is composed of processed bone, blood, and feathermeal and Everlizer is a heat-processed poultry litter product. Preplant fertilizers were incorporated using a rotary tiller to a depth of 15 cm, and soil NO3-N availability and flux were monitored on a weekly basis beginning immediately after preplant fertilization for six weeks. Soil testing analyzed concentrations of NO3-N, and the anion-exchange membrane method was used to assess soil NO3-N flux. Samples were taken on 69 and 101 days after transplanting (DAT) to assess yield tissue nitrate concentration and SSC. At both sampling dates, SPAD, crown diameter, and plant height were assessed, and leaf number was counted at final harvest following protocols outlined in Objective 1.


Data Analysis

To assess the results of the organic celery cultivar trial, a generalized linear mixed model was used within the GLIMMIX procedure of SAS (Version 9.4, SAS Institute, Cary, NC) for response variables of SPAD, crown diameter, plant height, and leaf number. Additional analyses are underway for fresh and dry weight, water content of leaf and petiole tissue, total-N, tissue nitrate, and SSC. A similar model was used to analyze data from the fertilization management study, with additional response variables for soil NO3-N concentration and flux.

Research results and discussion:

In the organic celery cultivar trial, cultivar did not significantly affect crown diameter or plant height at final harvest. Significant differences in SPAD were observed at final harvest across cultivars, with ‘Pink Plume’ showing significantly higher SPAD values than ‘Conquistador’, ‘Kelvin F1’, ‘Victoria’, or ‘Tango’. Cultivar significantly affected leaf number at final harvest, with ‘Pink Plume’ having significantly more leaves than ‘TZ6200F1’ or ‘Merengo’. The analyses of SSC and tissue nitrate concentration are underway to further understand the relationships between plant growth parameters and tissue nitrate concentration across cultivars and harvest times.

In the fertilization management study, Nature Safe as a preplant fertilizer significantly increased soil NO3-N flux for the first 3 weeks after preplant fertilization (WAPF) across all fertilizer rates. Flux peaked at 3 WAPF, where Nature Safe showed a 2.5-fold increase in flux relative to Everlizer. Total seasonal rate had a significant effect at 3-6 WAPF with N308 consistently leading to the highest NO3-N flux. N308 exhibited significantly higher NO3-N fluxes than N140 and N78C at 3-5 WAPF, while N140 and N78C had similar levels at 1-6 WAPF. Soil testing showed varied effects of rate and source on soil NO3-N concentration, with significant increases seen from baseline soil testing within 1 WAPF in all treatments. At 5-6 WAPF, soil NO3-N concentration in N308 was significantly higher than N140 or N78C.

At 69 DAT, preplant fertilizer source had a significant effect on celery yield, with Everlizer significantly increasing biomass across all total seasonal rates relative to Nature Safe. These results were likely associated with lower nitrate fluxes seen in the first few weeks after preplant fertilization with Everlizer, suggesting early-season N use-efficiency may have an impact on mid- to late-season yields in celery. A significant two-way interaction between preplant fertilizer source and total seasonal N rate was observed at 101 DAT. Everlizer resulted in significantly improved yields at each fertilizer rate, with N70C < N140 < N224 < N308, while Nature Safe showed similar yields at N140 and N224, and N308 failed to significantly increase yield compared with N224. Everlizer significantly improved yields relative to Nature Safe in N70C and N308, although no significant differences between preplant fertilizers were detected at other rates in the final harvest. The response of organic celery to fertilizer source and rate will be further evaluated in another trial in the Spring 2022 season in order to more clearly elucidate the release, availability, and uptake of soil-N as a result of using these fertilizer sources across different total seasonal rates.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

We plan to present research findings at the American Society for Horticultural Science 2022 Annual Conference as well as at the Florida State Horticultural Society 2022 Annual Conference. We plan on developing extension materials evaluating and comparing several common celery cultivars for Florida production conditions. In addition, we plan to create virtual tours during Summer 2022 and host a field day in the Fall of 2022. We also hope to prepare two research manuscripts for publication in peer-reviewed scientific journals upon completion of this project.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.