Unlocking the black box of plant biostimulants is the intriguing title of an editorial in the academic journal Scientia Horticulturae. The paper argues for a shift in how plant biostimulants are characterised and evaluated, and for that shift to better reflect their role as functional tools in broader fertilisation strategies. Yield cannot be the only indicator of efficacy is the repeated message.

In part this revision has arrived at the right time. Biostimulants are no longer the new kid on the block. “No longer viewed as experimental, they are now seen as essential components of crop management,” the paper says.

But to be generally accepted as a functional tool the mode of action needs to be fully explained, argue the authors. The paper notes that this is not a requirement of some of the current regulatory schemes. The authors view this as a weakness that does not help to build credibility within a marketplace that can be sceptical of biostimulant products.

This knowledge gap is the reason behind the “black box” metaphor in the title of the paper – a black box being a system or device whose inputs and outputs are observable but whose internal workings are unknown. The authors are clear: “…credibility will not come from bold promises, but from transparent, evidence-backed claims that reflect actual agronomic results.”

Begins with Context
Context is everything, as the saying goes. And context is how biostimulants should be framed, and ultimately evaluated argues a new editorial paper in Scientia Horticulturae.

The paper, which has current and previous members of the Biostimulants World Congress Scientific Committee among its list of authors, begins by setting out the foundational concept in how biostimulants are defined: “biostimulants are not defined by what they contain, but by what they do.”

This distinguishes them from fertilizers or pesticides. And introduces an idea of ‘claim-based’ marketing rather than a focus on composition, or the formulation. It also pushes any form of regulation of these products towards a focus on the effects, and the validation of those effects.

What was the motivation for the article?
Corresponding author Petronia Carillo, Full Professor of Agronomy at the University of Campania “Luigi Vanvitelli” Caserta Italy told New AG International:
“The article was directly inspired by the 2025 ISHS HortForum, where different perspectives on biostimulants were openly discussed. Despite the rapid growth of this sector, there is still confusion about definitions, evaluation methods, and how to build credibility. We felt it was important to bring these points together and emphasise a shift from product identity to function, and from yield alone to broader measures of resilience and plant health. Our aim was to move the conversation from marketing narratives to a more evidence-based and functional framework.”
The point about definitions is pertinent when it comes to regulating biostimulants in the marketplace.

Regulation – wording matters
Defining biostimulants by what they do has also passed into regulatory texts.
The internationally adopted ISO/TC 134 definition echoes this concept, describing biostimulants as “substances, microorganisms, or mixtures thereof that support a plant’s natural nutrition processes.”

The paper notes that there are some subtle but potentially significant differences around the wording of definitions. In the United States, a preference has been observed for the definition “support natural nutrition processes,” unlike the EU’s “stimulate plant nutrition.”

The authors feel that this subtle difference in the American definition could lead to the ability to broaden the functional targets that biostimulants can act upon and to strengthen the incentive to determine the functional mechanisms involved. In this way, regulation could better enable the search for mechanistic causes and biostimulant functions.

Yield – not the full story
When considering how biostimulants are evaluated, the paper stresses that yield alone does not tell the full story. “Yield remains the standard for evaluating agricultural inputs, but it often misrepresents the value of biostimulants.”

For the authors, it is evident that plant biostimulants need broader evaluation methods since yield efficiency alone is not adequate as a single indicator.

The paper elaborates that plant biostimulant products do not directly stimulate growth like fertilizers or pesticides. “They support internal functions such as root development, antioxidant defence, and nutrient redistribution. Their effects are subtle, shaped by environmental conditions, and may not lead to immediate biomass increases.”

Some biostimulants act by ‘inducing’ or ‘priming’ stress responses and this plays a part in the yield story too. Induction refers to protective genes being up regulated before stress occurs. Priming means that stress response genes are not activated until the stress occurs, but the rapidity or amplitude of the stress response will be enhanced by the priming stimulus.

In some cases, the enhanced stress tolerance may require some energy that will reduce growth. “The trade-off means that greater resilience does not always translate into higher yield.”

Spot the difference
Any discussion on biostimulants needs to keep in mind the effect they have on different crops and crop types.

The importance of timing and stress exposure is particularly relevant in perennial crops notes the paper. “When applied before events like heatwaves, biostimulants can be partially effective. But in the absence of stress, their impact may be minimal. In long-cycle crops, visible results may take months or even years, while both regulations and markets often demand quick, measurable returns,” the paper explains.

In horticulture, microbial biostimulants can be effective because farmers can adjust their timing and application based on the crop needs. “This flexibility makes it easier to optimize responses and reduce reliance on chemical inputs. In contrast, large-scale systems like maize or soybean offer fewer adjustment opportunities, making it harder to detect subtle physiological benefits.”

As the authors conclude, this reinforces the idea that yield alone is not sufficient to evaluate biostimulants. Functional traits should also become as important – root development, nutrient utilization, production of anti-stress compounds. These are important for long-term performance, rather than a short-term fix. The authors say that physiological traits and metabolic profiles must be considered as complementary indicators. However, evaluating for the long and the short-term – what challenges will that bring?

Evaluation – a two-step process
If biostimulants are doing more than boosting yield, how can this best be captured in the evaluation process over the short- and long-term?

The first stage in any evaluation process is to verify the effects of a biostimulant. This is the stage when ‘claims’ are verified.

The second step in the process is ‘the how’ – in other words, what is the mechanism by which the effect is achieved? This is where real-world conditions come in and what the paper describes as variability.

“However, many products still make bold claims based on weak or non-replicable data, leading to inconsistencies that undermine growers’ confidence. The issue is not necessarily product ineffectiveness, but rather the difficulty of verifying results under real-world conditions,” says the paper.

If a benefit is observed, the next question needs to be how was it achieved. “In most cases, approval is based solely on the demonstration of a measurable agronomic benefit, without the need to clarify the biological mechanisms involved.”

The net result says the paper is that products can enter the market based on observed outcomes, even when the underlying processes remain unclear.

“While such an approach encourages innovation and broadens market access, it may reduce scientific transparency and make it more difficult for growers to distinguish between empirically effective products and those supported by a clear mode of action.”

Credibility Gap
This notion of gaining market access without an explained mechanism leads to another consideration. The problem is referred to as he ‘regulatory gap’ in the paper, or as the paper pertinently expresses: “a blind spot in validation.”

This blind spot also means that the market cannot easily allow for further technical change: “Advanced formulations such as smart polymers and nanoparticle carriers present new challenges. These innovations enhance delivery efficiency but may fall outside the scope of current regulatory definitions.” If observed effects are the only requirement, then new products can gain market access, without little scrutiny of their modes of actions.

This leads to a risk in credibility more than anything, say the authors. They argue that validation should reflect on-farm realities that ensure both credibility and practical adoption. “A clear gap persists between scientific rigor and how products are marketed. When practical results do not align with simplified claims, confidence in individual products declines and trust in the entire category is undermined.”

The authors are clear: “Validation should include both efficacy data and an explanation of how products achieve agronomic benefits.”

This comes back to context. The authors say that the goal is not to prove universal outcomes but to show where and how biostimulants work. “Collecting context-specific evidence is crucial to understanding their function and building credibility in the sector.”

Efficacy given context
One of the omnipresent difficulties in discussing how a biostimulant works is that it can involve a cocktail of components.

“Modern biostimulants act through synergistic mechanisms involving microbes, molecules, and carriers. Their efficacy depends on context, and traditional testing frameworks may be too limited to capture their full effects.”

One area where their full effects are being more greatly appreciated is through widening scope of plant health. “The concept of plant health is evolving beyond the mere absence of disease or nutrient deficiency,” says the paper. This reflects a broader shift in agronomy, the paper notes, from resource supplementation to the management of physiological stability under variable conditions.
The authors see shifting to a functional view of plant health changes how biostimulants should be evaluated and applied. “Yield alone cannot capture their complexity,” the paper says succinctly.

When it comes to ‘synergistic mechanisms’, is it fair to say the science is still catching up?
More from Carillo: “Biostimulants often act through multiple mechanisms at the same time (e.g., microbial interactions, bioactive molecules, nutrient dynamics, stress priming), which means their effects are often not immediately visible and can vary greatly depending on crop type, growth stage, and environmental conditions. Science is progressing quickly, but evaluation methods have not yet fully caught up with this complexity. This is why we believe validation should always connect field results with a sound biological explanation, even if the exact molecular details are not yet completely known.”

Credibility cohesion
The paper concludes by returning to the central definition at the beginning of the paper – that a biostimulant is defined by what it does rather than its composition – and the implication this has when placing products on the market.

“The credibility of biostimulants relies on coherence between claims, biological mechanisms, and supporting evidence. The main risk lies not in regulation, but in the loss of trust among farmers and researchers, a loss that is difficult and slow to recover. This is especially critical for products defined by function rather than input content.”

Scientific coherence does not require exhaustive molecular detail or long-term trials for every product, say the authors. “What matters is that biological plausibility is part of the narrative from the start. In a context defined by environmental variability and complex interactions, both the potential and the limits of a product must be communicated clearly. Credibility will not come from bold promises, but from transparent, evidence-backed claims that reflect actual agronomic results.”

The paper covers a lot of ground – what is the most important idea that readers should take away?
Final word from Carillo: “The key message is that yield alone is not enough to evaluate biostimulants. Their true contribution lies in improving resilience, stability, and quality under variable conditions. They are functional tools that work best when integrated into broader crop management strategies. Credibility depends on coherence: clear claims, mechanistic plausibility, and evidence that reflects real-world conditions.”

Citation
Editorial: Unlocking the black box of plant biostimulants
Scientia Horticulturae 350 (2025) 114281
Patrick du Jardin, Patrick H. Brown, Theodore M. DeJong, Fabricio Cassan, Antonio Ferrante, Vasileios Fotopoulos, George A. Manganaris, Petronia Carillo (1)
(1) Corresponding author