Plant Biomarkers, Air Pollution and One Health Resilience: Urban Evidence from Psidium guajava in Douala, Cameroon

Abstract

Accelerating urbanization across Africa is intensifying particulate air pollution, threatening ecosystem integrity, urban liveability, and public health within the One Health framework. Urban vegetation provides critical natural mitigation infrastructure by intercepting airborne particulates and improving environmental quality; however, resilience thresholds of tropical species remain insufficiently understood. Generating such evidence is essential for designing climate-resilient African cities and cost-effective nature-based solutions. This study investigated physiological responses of Psidium guajava L. to dust pollution across urban (PK14), residential (Entrée Kotto), and suburban (Lendi) zones of Douala, Cameroon over a 12-month period. Leaf dust load and key stress biomarkers; total chlorophyll, ascorbic acid, relative water content (RWC), leaf extract pH, and Air Pollution Tolerance Index (APTI) were assessed as indicators of plant health and environmental stress. Results revealed a threshold-dependent response. Moderate dust loads (≤2.4%) significantly enhanced chlorophyll accumulation (β = 1.607, p = 0.017) and optimized leaf pH, indicating adaptive compensation under low-to-moderate stress. Beyond
this threshold, elevated dust exposure caused marked physiological decline, with peak dust loads reaching 3.9% in the urban site and significant reductions in water status (RWC; r = −0.74, p = 0.01). APTI values ranged from 6.0–9.7, identifying P. guajava as a sensitive bioindicator suitable for urban air-quality surveillance. Seasonal rainfall moderated dust deposition, whereas highly urbanized locations experienced greater stress intensity.

These findings demonstrate that plant biomarkers can provide low-cost early-warning tools for pollution monitoring in data-scarce cities. They further show that strategic urban greening and species selection can strengthen pollution mitigation, ecosystem quality, and human wellbeing. Integrating vegetation-based interventions into city planning offers a practical pathway for advancing climate adaptation and One Health outcomes across rapidly growing African urban centres.