The APTI is built from four biochemical indicators that together describe how well a leaf copes with oxidative stress. Ascorbic acid, the reduced form of vitamin C, acts as a powerful antioxidant that neutralizes the reactive oxygen species generated when pollutants such as sulfur dioxide, nitrogen oxides, and ozone penetrate leaf tissue. Total chlorophyll content reflects the integrity of the photosynthetic apparatus, which is among the first casualties of pollutant exposure. Leaf extract pH indicates the degree of cellular acidification caused by dissolved acidic gases, since a declining pH disrupts enzymatic activity and stomatal function. Finally, relative water content measures how much water the leaf retains, a trait tied to stomatal behavior, membrane stability, and the plant’s overall hydration status under stress. The index combines these four parameters in a weighted formula originally proposed in the early 1990s, and higher values signal greater tolerance.

Between June and August 2023, the team collected leaf samples monthly from eight species growing along the same roadside transect: Prunus cerasifera, Nerium oleander, Nandina domestica, Photinia fraseri, Ligustrum lucidum, Photinia serratifolia, Paeonia suffruticosa, and Eriobotrya japonica. For each species in each month, the researchers prepared three site-level composite samples, allowing statistical comparison among species within every sampling period while describing monthly trends from the observed values. This design ensured that every plant experienced essentially the same ambient pollution load, so any differences in the index could be attributed to intrinsic biochemical differences among species rather than to variation in exposure.

The headline finding is striking: all eight species fell within the sensitive category under the applied APTI thresholds, meaning none of them can be considered truly tolerant of the pollution levels along Huaxia Road. Yet within that sensitive band, the ranking was clear and consistent enough to guide planting decisions. Prunus cerasifera, the purple-leaf plum, topped the list with a three-month mean APTI of 9.842, followed closely by Nerium oleander at 9.570. Nandina domestica, Photinia fraseri, Ligustrum lucidum, and Photinia serratifolia occupied the middle of the ranking with values between 9.49 and 9.20. At the bottom sat Paeonia suffruticosa, the tree peony, at 8.983, and Eriobotrya japonica, the loquat, at 8.414, making the loquat the most pollution-sensitive species in the entire set.

The monthly dynamics added an important layer of nuance. Prunus cerasifera ranked first in both June and July, but its tolerance proved volatile: the species peaked at an APTI of 10.57 in July before dropping sharply to 8.85 in August, the most dramatic fluctuation recorded in the study. In August, Nandina domestica overtook it to claim the highest value of the month. By contrast, Nerium oleander displayed the steadiest profile, maintaining consistently high tolerance levels throughout the summer. Eriobotrya japonica remained the lowest performer in all three months, an unambiguous result that marks it as a poor candidate for exposed roadside positions. These temporal shifts underscore that pollution tolerance is not a fixed property of a species but a dynamic physiological state that responds to seasonal conditions such as temperature, water availability, and the aging of leaves.

Perhaps the most instructive result came from the correlations between individual traits and the overall index. Relative water content showed a significant positive correlation with APTI in every one of the three sampling months, identifying it as the key physiological driver of apparent tolerance. Part of this relationship is structural, since RWC is one of the four components of the index itself, but the consistency of the correlation across months suggests that a plant’s capacity to retain water under roadside stress genuinely dominates its calculated resilience. Species that keep their tissues hydrated maintain stomatal function, sustain photosynthesis, and dilute absorbed pollutants more effectively than those that wilt under the combined pressures of heat, drought, and exhaust-derived gases.

Individual traits, however, did not always track the overall ranking, revealing that the index can mask meaningful biochemical complexity. Paeonia suffruticosa recorded exceptionally high ascorbic acid content, which even declined over the summer months, yet the species still posted one of the lowest mean APTI values, indicating that a strong antioxidant arsenal alone cannot compensate for weaknesses in the other three parameters. Photinia serratifolia showed comparatively high total chlorophyll content while remaining firmly in the sensitive category. Meanwhile, leaf extract pH stayed acidic for all eight species, ranging roughly between 3.0 and 6.5, a signature of the acidifying influence of the roadside atmosphere, and total chlorophyll content peaked in July for most species before declining as the summer wore on.

For urban foresters and landscape designers, the study translates directly into planting strategy. Species such as Prunus cerasifera and Nerium oleander, with the highest relative APTI values, are better suited to the most exposed roadside positions, where they can endure chronic pollution while contributing to particulate capture and microclimate moderation. Conversely, sensitive species such as Paeonia suffruticosa and Eriobotrya japonica may serve a different and equally valuable purpose: as bioindicators. Because their leaf biochemistry responds visibly to pollution stress, they can function as living sensors, alerting city managers to deteriorating air quality before instrumental networks register a problem. This dual framework, using tolerant species for buffering and sensitive species for monitoring, mirrors the approach increasingly adopted in greenbelt development programs across Asia and beyond.

The research also fits into a rapidly expanding global literature on phytomonitoring and greenbelt design, with comparable APTI assessments conducted around brick kilns in Pakistan, gas plants in Nigeria, industrial zones in Iran and India, and traffic corridors in Nepal, Bangladesh, and Saudi Arabia. What distinguishes the Luoyang study is its controlled single-transect design and its monthly resolution, which together expose how unstable tolerance rankings can be when measured only once. A species that appears moderately tolerant in June may slump by August, and a single-season snapshot could easily mislead a planting program. The authors’ work was supported by the Natural Science Foundation of Henan Province, the National Natural Science Foundation of China, and the Henan Provincial Science and Technology Research Project.

As cities worldwide grapple with the health costs of traffic-related air pollution, the humble leaf is emerging as both a shield and a diagnostic tool. The Luoyang findings remind us that green infrastructure is not interchangeable: a hedge of loquat will not do the work of a hedge of oleander. With all eight species classified as sensitive, the study also delivers a sobering note about the intensity of urban roadside pollution in the region, suggesting that even the city’s toughest ornamentals are operating near their physiological limits. Choosing the right plant for the right place, informed by biochemical evidence rather than aesthetics alone, may be one of the simplest and most cost-effective steps a city can take toward cleaner air.

Subject of Research: Air Pollution Tolerance Index of urban roadside plants and their leaf biochemical indicator profiles

Article Title: Interspecific differences in the Air Pollution Tolerance Index (APTI) of plants in an urban roadside environment: relationships with biochemical indicator profiles

Article References: He, C., Liu, J., Zhang, Z., Zhang, W., Wei, C., Zhang, L., Zhang, Q., & Hong, Z. (2026). Interspecific differences in the Air Pollution Tolerance Index (APTI) of plants in an urban roadside environment: relationships with biochemical indicator profiles. Environmental Monitoring and Assessment, 198(11), Article 1148. https://doi.org/10.1007/s10661-026-16014-5

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16014-5

Keywords: Air Pollution Tolerance Index, APTI, urban roadside plants, ascorbic acid, relative water content, chlorophyll, leaf extract pH, greenbelt development, bioindicators, Luoyang, plant physiology, air pollution