CHLOROPHYLL DEGRADATION UNDER SMOG EXPOSURE: UNVEILING THE MOLECULAR AND ECOLOGICAL CONSEQUENCES

Abstract

The current study revealed that exposure to smog posed a major source of environmental stress and reduced photosynthesis and vigor in plants. Chlorophyll breakdown is a major aspect of smog sensitivity triggered by altered oxidative stress, an integral aspect of photosynthesis. Therefore, the general objective of this study is to evaluate the molecular and ecological impacts of chlorophyll degradation in Arabidopsis thaliana plants under smog conditions as evidenced by oxidative stress markers, hormonal regulation, and chlorophyllase activity. The Arabidopsis thaliana plants were cultivated in a growth chamber under optimal conditions of temperature, and light/dark regime, and after acclimatization the plants were exposed to artificial smog for a total of 48 hours NO₂ 50 ppm, O₃ 120 ppm, and PM10. Untreated plants or control plants were arranged in similar photoperiod regimes and with no exposure to smog. Chlorophyll a and b, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), SOD, abscisic acid (ABA), cytokinins, gibberellins, auxins, potassium (K⁺), chloride (Cl⁻), calcium (Ca²⁺), hydrogen (H⁺), chlorophyllase, and ROS were assessed. The treatment outcomes were compared with appropriate control using p-values to determine the significance level of change between control and smog-exposed plants. Smog led to decreases in chlorophyll content (from 1.20 ± 0.05 to 0.75 ± 0.03 mg/g) and chlorophyll b content (from 0.50 ± 0.02 to 0.30 ± 0.02 mg/g; p-values = 0.015 and 0.019, respectively). There were also increased levels of oxidative stress MDA (1.45 ± 0.10 nmol/g), H₂O₂ (4.50 ± 0.12 µmol/g), and SOD (21.6 ± 1.53 U/mg protein) compared to the control plants (15.2 ± 1.37 U/mg protein, p-values = 0.023, 0.022, and Phytohormones responded to the smog treatments by increasing the ABA contents (85.79 ng/g FW) while decreasing the cytokinin content to 40.63 ng/g FW. Concentrations of potassium, chloride, calcium, and hydrogen ions were changed in guard cells, where the difference was significant between control and smog-exposed plants, potassium ions (85.32 mM), chloride ions (70.27 mM), calcium ions (45.17 mM) and hydrogen ions (0.0156 mM). Chlorophyllase activity in smog-exposed plants was higher than that of controls: 0.30 ± 0.02 µmolg-1h-1 compared to 0.15 ± 0.01 µmolg-1h-1; p < 0.004. ROS levels were higher and the fluorescence intensity (280.22 ± 18.33 AU) associated with smog-exposed plants was statistically significant (p = 0.003). This work proves that smog impacts negatively on the chlorophyll content of Arabidopsis thaliana by increasing oxidative stress levels, changing hormonal regulation, and upregulating chlorophyllase activity. The findings reveal molecular processes involved in plant stress responses and show that smog-induced chlorophyll loss impacts plant health and ecosystem services.