Zinc Oxide Nanoparticles Shield Pepper and Watermelon From Heat Stress

As heat waves intensify across the world’s vegetable-growing regions, scientists are racing to find practical ways to keep crops alive when temperatures climb beyond what plants can tolerate. A new study from Hatay Mustafa Kemal University in Türkiye offers a striking glimpse of what nanotechnology might do for two of the most heat-sensitive warm-climate crops: pepper and watermelon. Researchers Seher Toprak and Ömer Faruk Coşkun report that a simple foliar spray of zinc oxide nanoparticles, applied at a concentration of just 100 milligrams per liter, partially rescued seedlings exposed to a sustained 35 degrees Celsius heat regime, restoring chlorophyll levels and biomass in some genotypes to near control values. The work, published in BMC Agriculture, is notable not only for its positive results but also for its honesty about where the nanoparticles failed.

The experimental design is what sets this study apart from much of the earlier literature on nanomaterials in agriculture. Rather than testing a single variety of a single species, the team grew three pepper genotypes and three watermelon genotypes side by side under identical conditions, subjecting them to three treatments: a 25 degree control, a 35 degree heat stress, and heat stress combined with the nanoparticle spray. Pepper belongs to the nightshade family while watermelon belongs to the cucurbits, two lineages with very different canopy architectures and water-use strategies, yet the two crops are frequently rotated or relay-cropped in the same fields. By running the comparison under matched regimes, the researchers could disentangle effects that are species-specific from those that depend on the individual genotype, a distinction that most prior studies could not make.

The heat treatment itself was calibrated to mimic an acute field episode without killing the plants. Stress began when pepper seedlings had six to seven fully expanded leaves and watermelon seedlings had five to six, and it lasted fifteen consecutive days. The zinc oxide nanoparticles used were commercial products with a primary particle size below 50 nanometers and a purity of at least 99 percent, delivered in a working solution containing a small amount of the surfactant Tween-20 to help the spray adhere to and penetrate leaf surfaces. Fifteen days after the treatments began, the team measured a battery of traits: the SPAD chlorophyll index, leaf number, stem diameter, shoot and root lengths, leaf dimensions, and both fresh and dry weights.

The damage inflicted by heat was consistent and statistically significant across both species. Chlorophyll readings fell, shoots shortened, and biomass declined in every genotype, confirming that 35 degrees is genuinely stressful for these crops. In pepper, the most dramatic recovery occurred in genotype P2, a heat-stressed line whose SPAD value collapsed to 56.50 under heat but rebounded to 76.07 when the nanoparticle spray was added, a recovery of roughly 35 percent that brought the pigment status back almost to control levels. Dry weight in the same genotype rose by about a third compared with heat-stressed plants that received no spray. In P1, fresh weight under heat plus nanoparticles actually exceeded the unstressed control, hinting at a growth-promoting effect that goes beyond simple stress relief.

Watermelon produced perhaps the most eye-catching numbers of the entire study. In genotype W1, shoot length under heat climbed from 21.4 centimeters to 30.0 centimeters with the nanoparticle treatment, and fresh weight more than tripled relative to the heat-stressed control, an increase of over 200 percent. Genotype W3 held its dry weight close to control levels when sprayed under heat and maintained or slightly improved its leaf width. But the third genotype, W2, told a very different story. It was the most heat-sensitive line in the experiment, showing the lowest SPAD, shoot length, and stem diameter under stress, and the nanoparticles did nothing to help. Worse, root length in W2 actually declined further when the spray was applied, echoing earlier reports in mungbean and tomato where excess zinc impaired root growth. The authors describe W2 as a non-responder or negative responder, likely a zinc-sensitive genotype that would require a lower dose.

Why would spraying zinc onto leaves help a plant survive heat? The mechanism, as the authors explain, rests on zinc’s well-established biochemical roles. Zinc is an essential cofactor for antioxidant enzymes such as superoxide dismutase, catalase, and peroxidase, the enzymatic machinery that detoxifies the reactive oxygen species that flood plant cells when membranes destabilize and proteins begin to denature under heat. Zinc also sits at the heart of chlorophyll biosynthesis pathways, which helps explain why the most consistent benefit of the spray was the stabilization of pigment levels. There is evidence too that zinc availability supports auxin biosynthesis through tryptophan-dependent pathways, which could account for the recovery of shoot elongation observed in several genotypes, since cell expansion and turgor maintenance depend on both hormone signaling and water status.

The root responses complicate this tidy picture and reveal a bell-shaped dose relationship that the authors treat seriously. In pepper genotype P1, the nanoparticle spray reduced root length compared with heat stress alone, a pattern the researchers attribute to two non-exclusive causes: a shoot-biased partitioning of carbon once leaf-level stress is relieved, or localized zinc sensitivity at the root meristem, where excess zinc can inhibit cell division and elongation at the tested dose. Similar genotype- and dose-dependent phytotoxic thresholds have been documented in the wider literature, particularly for roots, which are the first tissue to encounter accumulated nanoparticles. This means the same spray that rescues one genotype’s canopy can quietly poison another’s root system, a caution that any grower considering nanofertilizers should take to heart.

Beyond the univariate statistics, the team deployed multivariate tools to make sense of the correlated trait data. Principal component analysis explained 73.1 percent of the variance in the first two components, with the first axis loaded primarily on growth and biomass traits and the second on photosynthetic and morphological measures such as SPAD and leaf number. On the scatter plot, control plants clustered in one region and heat-stressed plants in another, and crucially, the nanoparticle-treated plants shifted visibly toward the controls, with the effect strongest for pepper P2 and watermelon W1. Hierarchical clustering using Ward’s method reinforced this picture, grouping several heat-plus-nanoparticle treatments closer to unstressed controls than to their unsprayed heat-stressed counterparts, suggesting that the nanoparticles partially restore the normal phenotypic profile rather than inducing an entirely new one.

A supervised partial least squares analysis ranked the traits by their importance for predicting nanoparticle responsiveness, and the results were clear: fresh weight, SPAD, and dry weight topped the list with variable-importance scores above one, while root length and leaf number contributed little. In plain terms, zinc oxide nanoparticles protect these crops mainly by preserving chlorophyll and sustaining biomass accumulation, not by reshaping plant architecture. The authors are candid that the dataset is small and that leave-one-out cross-validation of the model yielded a negative R-squared, so they present these multivariate findings as exploratory rankings rather than robust predictive tools. That transparency is refreshing in a field where machine-learning claims often outrun the data behind them.

The broader implications stretch from the greenhouse to the climate debate. Field-scale evidence from other laboratories has shown that foliar zinc oxide nanoparticles can sustain grain yield in rice under actual heat waves, possibly through complementary interactions with the phyllosphere microbial community, which lends agronomic weight to the controlled-environment trends seen here. But the Turkish team is careful about translation: they call for wider genotype coverage, formal dose-response curves across phenological stages, field trials with fruit yield endpoints, independent particle characterization, and quantification of ecotoxicological risks, including soil persistence and zinc accumulation in edible tissues, before any broad deployment. Their conclusion is measured but hopeful. Zinc oxide nanoparticles are not a universal shield against heat, and in the wrong genotype at the wrong dose they can do harm. Yet as a genotype-informed supplement to breeding and agronomy, they may offer a fast, cheap, and surprisingly effective way to buy vulnerable crops time in a warming world.

Subject of Research: Effects of foliar zinc oxide nanoparticles on heat stress tolerance in pepper and watermelon genotypes

Article Title: Heat stress mitigation by zinc oxide nanoparticles in pepper and watermelon

Article References: Toprak, S., & Coşkun, Ö. F. (2026). Heat stress mitigation by zinc oxide nanoparticles in pepper and watermelon. BMC Agriculture, 2(1), Article 2. https://doi.org/10.1186/s44399-025-00024-8

Image Credits: AI Generated

DOI: 10.1186/s44399-025-00024-8

Keywords: zinc oxide nanoparticles, heat stress, pepper, watermelon, Capsicum annuum, Citrullus lanatus, chlorophyll, biomass, genotype-specific response, abiotic stress, nanotechnology, plant physiology