Nitrate improves plant growth by promoting nitrogen assimilation and photosynthetic performance in citrus seedlings – BMC Plant Biology

Advancing Sustainable Agriculture through Citrus Nutrition Research

Alignment with Sustainable Development Goals (SDGs)

This research report outlines a methodological framework for investigating nitrogen (N) nutrition in citrus plants. The study’s objectives are directly aligned with several United Nations Sustainable Development Goals (SDGs), primarily focusing on enhancing agricultural productivity and environmental sustainability. By optimizing nitrogen use efficiency, this research contributes to:

• SDG 2 (Zero Hunger): Improving the growth and health of citrus, a significant food crop, thereby supporting efforts to enhance food security and promote sustainable agriculture.
• SDG 12 (Responsible Consumption and Production): Investigating how different forms of nitrogen affect plant metabolism to inform practices that reduce fertilizer waste, promoting sustainable management and efficient use of natural resources.
• SDG 15 (Life on Land): Providing foundational knowledge to mitigate the environmental impacts of excessive fertilizer application, such as soil degradation and nutrient runoff, which helps protect terrestrial ecosystems.

Methodological Framework

Experimental Materials and Cultivation

The study was conducted using two commercially relevant citrus varieties to ensure the findings are applicable to agricultural production, supporting SDG 2.

1. Plant Varieties: ‘Lugan’ (C. reticulata) and ‘Xuegan’ (C. sinensis).
2. Procurement: Seeds were sourced from healthy fruits purchased in Fuzhou City, Fujian Province, China.
3. Cultivation Timeline:
   • Sowing: March
   • Transplanting: May (at the four-to-five leaf stage)
   • Cultivation Period: May to October
4. Experimental Setup: Seedlings were grown in six-liter ceramic containers using sand cultivation. Each pot was an independent, controlled experimental unit, with two seedlings per pot pooled as a single biological replicate.

Nutrient Management and Treatment Protocol

The nutrient application protocol was designed to assess the efficiency of different nitrogen forms, a critical factor for developing sustainable fertilization strategies in line with SDG 12.

1. Nutrient Solution: A modified Hoagland’s solution was used.
2. Nitrogen Treatments:
   • Ammonium Nutrition (AN): Supplied as (NH₄)₂SO₄.
   • Nitrate Nutrition (NN): Supplied as Ca(NO₃)₂.
   • Both treatments provided a 4 mmol L⁻¹ N supply.
3. Controlled Variables:
   • Calcium (Ca²⁺) concentration was standardized to 5 mmol L⁻¹ using CaCl₂·2H₂O to eliminate confounding effects.
   • Dicyandiamide (DCD) was added to prevent nitrification.
   • Solution pH was maintained at 6.50 ± 0.05.
4. Irrigation Schedule:
   • Days 1-14: 1/4 strength nutrient solution every two days.
   • Days 15-45: 1/2 strength nutrient solution every two days.
   • Day 46 onwards: Full-strength nutrient solution until harvest.

Plant Sampling and Growth Analysis

After five months, plant growth and biomass were measured to quantify the impact of nutrient treatments on agricultural productivity.

1. Harvesting: Plants were harvested and separated into roots, stems, and leaves.
2. Sample Storage: Fresh white root tips and middle leaves were stored at −80 °C for subsequent analysis.
3. Growth Metrics:
   • Total plant biomass was determined.
   • Leaf and root growth characteristics were measured.
   • Root activity was quantified using tetrazolium chloride staining.

Physiological and Photosynthetic Efficiency Analysis

Photosynthetic efficiency and water use were assessed as key indicators of plant health and resource utilization, contributing to the knowledge base for SDG 12.

• Chlorophyll (Chl) Content: Determined spectrophotometrically from leaf extracts.
• Gas Exchange Parameters: Measured using a portable photosynthetic system under controlled conditions (light intensity: 1000 µmol m⁻² s⁻¹).
• Efficiency Calculations:
  • Water Use Efficiency (WUE)
  • Carboxylation Efficiency (CE)
  • Limitation of Stomatal (L_s)
• Chlorophyll a Fluorescence: Measured using a Handy Plant Efficiency Analyzer to assess photosystem health.

Ultrastructural and Biochemical Analyses

1. Transmission Electron Microscopy (TEM): Lower leaf and root samples were analyzed to observe cellular and subcellular structures, providing insight into plant stress responses at a micro-level.
2. Non-structural Carbohydrate (NSC), Free Amino Acid (FAA), and Soluble Protein: The concentrations of these key metabolic compounds were determined in dried and fresh samples using established colorimetric and spectrophotometric methods.

Nitrogen Metabolism and Accumulation

Direct measurement of nitrogen metabolism and uptake is fundamental to understanding how to improve fertilizer efficiency and reduce environmental impact, directly addressing the goals of SDG 12 and SDG 15.

• N Metabolism Enzyme Activity: The activities of key enzymes were measured:
  • Nitrate Reductase (NR)
  • Glutamine Synthetase (GS)
  • Glutamate Synthase (GOGAT)
  • Glutamate Dehydrogenase (GDH)
• NH₄⁺-N and NO₃⁻-N Concentrations: Determined in fresh root and leaf tissues using colorimetric methods.
• Total N Concentration, Accumulation, and Distribution: Dried samples were digested, and total N was measured using a continuous flow analyzer. N accumulation and distribution among plant organs were calculated.

Oxidative Stress Indicators

The activity of antioxidant enzymes and the content of malondialdehyde (MDA) were measured to assess the level of oxidative stress in the plants under different nitrogen treatments.

• Antioxidant Enzymes: Superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT).
• Lipid Peroxidation: Malondialdehyde (MDA) content was determined using the thiobarbituric acid (TBA) method.

Statistical Analysis

Data were analyzed to ensure the scientific validity of the results. All results were expressed as mean ± standard error (SE) with a minimum of four biological replicates. Statistical significance was determined using one-way and two-way ANOVA with an LSD test (P

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

   • SDG 2: Zero Hunger

     The article’s research on citrus plant nutrition is directly linked to improving agricultural productivity. By investigating how different forms of nitrogen affect plant growth, biomass, and photosynthetic efficiency, the study contributes to foundational knowledge for enhancing crop yields and ensuring food security.

   • SDG 12: Responsible Consumption and Production

     The study focuses on nitrogen uptake, metabolism, and use efficiency in plants. This is relevant to SDG 12 because nitrogen fertilizers are a significant natural resource input in agriculture. Research that leads to more efficient nitrogen use by crops can reduce the overall amount of fertilizer needed, promoting sustainable management of resources and reducing waste.

   • SDG 15: Life on Land

     The research has implications for protecting terrestrial ecosystems. Overuse of nitrogen fertilizers in agriculture leads to nutrient runoff, which pollutes water bodies and degrades soil quality. By exploring ways to optimize nitrogen delivery and uptake by plants, this research can help develop practices that minimize the negative environmental impacts of agriculture, thereby contributing to the health of land-based ecosystems.

2. What specific targets under those SDGs can be identified based on the article’s content?

   • Target 2.4 (under SDG 2): “By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production…”

     The article’s investigation into the physiological and biochemical responses of citrus seedlings to different nitrogen sources is fundamental to developing more productive agricultural practices. Understanding these mechanisms allows for the optimization of fertilization strategies to maximize plant growth (measured as “plant biomass” and “leaf and root growth characteristics”) and photosynthetic capacity, which are precursors to increased food production.

   • Target 12.2 (under SDG 12): “By 2030, achieve the sustainable management and efficient use of natural resources.”

     The research directly addresses the efficiency of nitrogen use, a key natural resource in food production. The experiment measures “N concentration, accumulation, and distribution” within the plant, as well as the activity of “N metabolism enzymes.” These measurements provide direct insights into how efficiently the plant utilizes the supplied nitrogen, aiming to reduce waste and promote more sustainable resource management in agriculture.

   • Target 15.3 (under SDG 15): “By 2030, combat desertification, restore degraded land and soil…and strive to achieve a land degradation-neutral world.”

     While not a direct study of soil, the research contributes to this target by providing knowledge to mitigate a key driver of soil degradation. Inefficient nitrogen fertilizer use can lead to soil acidification and nutrient imbalances. The article’s focus on optimizing nitrogen uptake (by measuring “NH4+-N and NO3−-N concentrations” in tissues) can inform practices that reduce excess nitrogen in the soil, thus helping to prevent land degradation and maintain soil health.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

   • Indicators for Productivity and Plant Health (Target 2.4):

     The article explicitly details methods for measuring several indicators of plant productivity and resilience.

     • Plant biomass: Measured as the sum of root, stem, and leaf sections, this is a direct indicator of agricultural productivity.
     • Net photosynthetic rate (Pn): Measured using a portable photosynthetic system, this indicates the plant’s capacity for growth.
     • Chlorophyll (Chl) content: Its determination is described as an indicator of the plant’s photosynthetic potential.
     • Root activity: Quantified using tetrazolium chloride staining, this indicates the health and nutrient-absorbing capacity of the plant.
   • Indicators for Resource Use Efficiency (Target 12.2):

     The article outlines several metrics that serve as direct indicators for the efficient use of natural resources like nitrogen and water.

     • Nitrogen (N) concentration, accumulation, and distribution: The methods describe digesting plant tissues to measure total N, which directly quantifies how much of the applied nutrient was taken up by the plant.
     • Water Use Efficiency (WUE): The article specifies that WUE is calculated as the ratio of net photosynthetic rate (Pn) to transpiration rate (Tr), providing a clear indicator of how efficiently the plant uses water.
     • N metabolism enzyme activity: The activity of enzymes like nitrate reductase (NR) and glutamine synthetase (GS) are measured to assess how effectively the plant is processing and assimilating the nitrogen it has absorbed.
   • Indicators for Reduced Environmental Impact (Target 15.3):

     The article implies indicators that can be used to measure the potential for reduced environmental pollution from agriculture.

     • NH4+-N and NO3−-N concentrations in plant tissues: By measuring the amount of nitrogen within the plant, one can infer the efficiency of uptake. Higher efficiency implies less residual nitrogen in the soil, reducing the risk of runoff and leaching that degrades land and water ecosystems.
     • Use of dicyandiamide (DCD): The article mentions incorporating DCD into the nutrition solution “to prevent nitrification.” This is an indicator of a specific agricultural practice aimed at managing nitrogen forms in the soil to improve efficiency and reduce the loss of nitrogen to the environment (e.g., as nitrous oxide, a greenhouse gas).

Summary of SDGs, Targets, and Indicators

Source: bmcplantbiol.biomedcentral.com