Nitrogen and Biostimulant Timing as a Physiological Hypothesis for Internal Heart Greening in Globe Artichoke (Cynara scolymus L.)

1. Introduction

Greening of the artichoke heart (the floral receptacle) is a physiological disorder that affects marketable quality, particularly in export markets that require a uniform, light color in the flower heads. The disorder is linked to several nutritional and environmental factors that influence the plant’s physiological balance during head formation.

Research in plant physiology and crop nutrition indicates that nitrogen management and the timing of biostimulant application are two key factors that influence the balance between vegetative and reproductive (flowering) growth. This paper synthesizes that literature into an explicit, falsifiable hypothesis for the artichoke heart-greening phenomenon, and proposes the field trial design needed to test it directly.

The motivation for this hypothesis arose from direct field observations by the author of internal heart greening in commercially processed artichoke heads, which prompted a review of the underlying physiological literature presented here.

The author is professionally employed in the artichoke export industry, which provided the field context for the observations motivating this study. The author declares no financial, commercial, or advisory relationship with any manufacturer of nitrogen fertilizers or biostimulants discussed in this work.

2. Physiological Basis

2.1. Role of Nitrogen in Chlorophyll Formation and Vegetative Growth

Nitrogen is an essential element in chlorophyll formation: it is incorporated into the porphyrin ring at the core of the chlorophyll molecule, and it is also required for the synthesis of amino acids, proteins and nucleic acids. Increased nitrogen supply to the plant is therefore associated with:

• Increased vegetative growth
• Higher chlorophyll content in plant tissues
• Increased enzymatic activity
• Delayed maturity in some crops

Marschner (2012) and Taiz, Zeiger, Møller & Murphy (2015) note that abundant nitrogen supply is associated with increased vegetative biomass and chlorophyll content in plant tissues.

2.2. Effect of the Carbon-to-Nitrogen (C/N) Ratio

The internal carbon-to-nitrogen ratio influences the direction of plant growth. A higher nitrogen status lowers the C/N ratio, which favours vegetative growth and can delay floral differentiation. Mengel & Kirkby (2001) note that nutritional balance is a decisive factor in the plant’s transition from the vegetative to the reproductive phase.

2.3. Effect of Biostimulants (Amino Acids and Seaweed Extracts)

Seaweed extracts contain compounds with hormone-like activity, such as cytokinin- and auxin-like substances, which may enhance cell division and vegetative growth. Free amino acids may also improve nutrient uptake efficiency and physiological activity, particularly under stress conditions. Craigie (2011) reports that seaweed extracts can stimulate vegetative growth in a range of crops, with the effect depending on growth stage and dose applied.

3. Crop-Specific Evidence and the Proposed Mechanism

Based on the physiological principles above, the use of nitrogen sources or biostimulants during head formation may plausibly contribute to continued vegetative activity within the floral tissue, increased chlorophyll content in the cells of the floral disc, delayed completion of floral differentiation, and visible internal green color that affects marketable quality.

This reasoning is not based solely on general plant-physiology principles: crop-specific field evidence in globe artichoke itself supports the underlying mechanism. In a controlled two-cultivar field trial (Violet de Provence and Tema 2000) with three nitrogen rates (0, 200 and 400 kg N ha⁻¹), Lombardo et al. (2020) measured chlorophyll content (SPAD units) directly in artichoke plant tissue and found that N200 sustained the highest, optimal chlorophyll reading regardless of measurement time — a direct, quantitative link between nitrogen supply and chlorophyll accumulation in the artichoke plant itself, though this trial measured leaf chlorophyll rather than chlorophyll within the receptacle (heart) tissue specifically. Complementing this, Lombardo et al. (2017) linked nitrogen fertilization rates (0, 200 and 400 kg N ha⁻¹) to overall head quality in minimally processed artichoke, and Montesano et al. (2022) demonstrated, in a two-season field trial on cvs. Opal and Madrigal, that both nitrogen rate and foliar seaweed-based biostimulant application altered the polyphenolic profile of the receptacle tissue itself — the same tissue in which greening is observed, providing more direct (though still indirect, since polyphenols are not chlorophyll) support for a receptacle-level physiological response to these inputs.

Statement of the hypothesis and its current evidentiary status. Even with this crop-specific support for the underlying mechanism, and with Ierna et al. (2012) noting, in a separate field trial on globe artichoke, that nitrogen fertilization affected the green color degree of the heads (a general observation, not a study of the specific internal-greening defect discussed here, and noted in passing within the broader pre- and postharvest review by El-Mogy et al., 2024), no published study to date directly documents nitrogen or biostimulant use as the proven cause of internal heart greening in artichoke as a named, visually assessed disorder; a systematic search did not return a study describing this specific defect by name in the artichoke literature. It therefore remains most accurate to describe internal greening as a plausible, mechanistically well-supported outcome of nitrogen/biostimulant timing — within a broader system that also includes elevated temperature, excessive irrigation and poor fertilization timing — rather than a conclusively proven causal chain. A dedicated artichoke trial that scores visible internal greening directly (not just SPAD chlorophyll or polyphenol content) is needed to close this gap; see Section 6.

4. Why the Green Color Becomes Visible After Boiling

Chemical studies of chlorophyll suggest that boiling does not create new color; rather, it reveals chlorophyll that was already present within the tissue. During boiling, cell walls break down, chloroplasts are released from disrupted cells, and the internal chemical environment of the tissue changes. This results in a green color that becomes clearly visible, having been previously masked.

Two factors help explain why the color is not obvious before cooking: the chlorophyll is contained within intact chloroplasts inside undamaged cells with intact cell walls, and the presence of natural acids in the raw tissue mutes the color, so the heart appears creamy or white. Boiling breaks the cell walls, releases the chlorophyll, and shifts the internal chemical environment, so the previously masked green becomes visible — a pattern also seen in green beans, whose color intensifies after boiling. Schwartz and von Elbe (1983) studied the heat-driven conversion of chlorophyll to pheophytin/pyropheophytin in vegetables, a related but distinct thermal transformation (an olive-brown color shift from chlorophyll breakdown, rather than the release of already-present, intact chlorophyll proposed here); the cell-wall-breakdown and acid-masking mechanism described above is the authors’ own physiological reasoning and has not been confirmed for artichoke heart tissue specifically.

A simple field observation supports the physiological explanation over a purely cooking-related one: if boiling itself caused the greening, all boiled artichoke hearts would turn green. In practice, the discoloration is observed mainly in heads that received late nitrogen fertilization or biostimulants during head formation — consistent with a pre-harvest physiological origin rather than an artefact of the cooking process itself.

5. Discussion

The synthesis above supports internal heart greening as a physiological, pre-harvest disorder rather than a cooking artefact, driven by a combination of late nitrogen supply, biostimulant timing, and compounding environmental stressors (heat, over-irrigation). This interpretation is consistent with, though not yet directly confirmed by, artichoke-specific field data on chlorophyll and receptacle biochemistry. The principal limitation is the absence of a published trial that scores the named, visually assessed greening defect itself as the outcome variable, rather than a proxy measurement (SPAD chlorophyll, polyphenol profile).

6. Proposed Trial to Test the Hypothesis

To move from hypothesis to confirmed mechanism, we propose a controlled field trial with the following minimum features:

• Factorial design crossing nitrogen rate/timing (including a late-application treatment during head formation) with biostimulant application (present/absent, timed at head formation)
• At least one commercial artichoke cultivar relevant to export production, with adequate replication (≥ 4 blocks) for statistical power
• Direct, blinded visual scoring of internal receptacle greening on a standardized scale, in addition to SPAD chlorophyll readings, as the primary outcome variable
• Recording of temperature and irrigation regime as covariates, given their proposed role as compounding stressors
• Statistical analysis (e.g., ANOVA or mixed-effects models) relating treatment, covariates, and greening score

7. Technical Recommendations (Pending Confirmatory Trial)

• Adjust the fertilization program according to the plant’s physiological stage
• Reduce nitrogen supply at the onset of flower-head formation
• Avoid biostimulant application during floral differentiation
• Base nutrient decisions on soil and plant tissue analysis
• Manage irrigation to avoid excessive vegetative growth

8. Conclusion

Artichoke heart greening appears, on current physiological and crop-specific evidence, to be primarily a pre-harvest physiological disorder linked to poorly timed nitrogen and biostimulant application, rather than an artefact of cooking. This should be treated as a well-supported hypothesis — not a definitively proven mechanism — pending a dedicated field trial that scores the named disorder directly.