Ecophysiological responses of the halophyte Lumnitzera racemosa Willd. to heavy metal stress in an ultramafic mining environment

Abstract

Plants are essential components of life, providing food, habitat, and ecological balance. Their leaves harness solar energy, making them critical indicators of environmental health. Leaf size indices provide valuable insights into plant health, transpiration, growth, temperature regulation, and light capture. This study investigated the effects of six heavy metals—Molybdenum (Mo), Manganese (Mn), Lead (Pb), Chromium (Cr), Nickel (Ni), and Cadmium (Cd)—on the morphometric leaf traits of the halophyte Lumnitzera racemosa Willd. in an ultramafic mining site in Claver, Surigao del Norte, with a non-mining control site in Nasipit, Agusan del Norte. Sediment analyses revealed that Cr (4,434 ppm) and Ni (4,234 ppm) in the mining site exceeded WHO and FAO permissible limits (100 and 50 ppm, respectively), while Ni (552 ppm) and Cr (334 ppm) in the control site also surpassed thresholds, indicating diffuse contamination. Morphometric analysis showed significant differences (p < 0.05) in all measured leaf traits between sites. L. racemosa in Nasipit exhibited longer leaves (mean = 5.17 cm) and greater leaf area (mean = 7.53 cm²) compared with Claver (4.17 cm and 6.83 cm², respectively), representing increases of 22% in length and 9% in area. In contrast, leaves from Claver had broader widths (by 12%) and longer petioles (by 15%), possibly reflecting compensatory morphological plasticity under multi-metal stress. The widespread contamination of all six heavy metals likely induced synergistic or antagonistic toxic effects, altering cell division and photosynthetic activity. Leaf macronutrient analysis revealed nitrogen deficiency in both populations, optimum to high phosphorus, and excessive potassium levels, suggesting that leaf size variations were not driven by nutrient imbalance but by metal toxicity and environmental stress. The study concludes that L. racemosa exhibits adaptive morphometric responses to heavy metal exposure, indicating its potential as a bioindicator species for monitoring contamination in ultramafic and mining-affected mangrove ecosystems.