Adaptation and growth response of <i>Samanea saman</i> (Jacq.) Merr seedlings to iron exposure

One adaptive responses of plants to heavy metal exposure, including iron (Fe), is the exudation of organic acids. This study aims to determine the adaptation and mechanism of Samanea saman seedlings when exposed to Fe. The experiment used a completely randomized one-factor design (Fe concentration) with five levels (0, 0.5, 1, 1.5, and 2 mM Fe), with three replications per treatment. Measured parameters included growth (height, root length, dry weights of roots, shoots, and total seedlings), chlorophyll content, malondialdehyde (MDA) levels, tolerance index, and the secretion and accumulation of organic acids. The findings revealed that Fe exposure significantly influenced the growth of S. saman seedlings. A concentration of 0.5 mM Fe enhanced root length (65 cm), root dry weight (0.28 g) and shoot dry weight (0.78 g). The tolerance index increased at 0.5 mM Fe (121.8%), surpassing the control, but declined with higher Fe concentrations. Chlorophyll and carotenoid contents were unaffected by Fe exposure, while MDA levels varied significantly across treatments and peaked at 2 mM Fe (40129.03 nmol/g FW). Fe exposure significantly impacted the secretion of organic acids, including lactic, acetic, and citric acids. Lactic acid, showing a linear increase with Fe concentration (R² = 0.8907), is suggested as a key Fe chelating agent. Malic acid secretion was observed only at 1.5 (0.246 mg/l) and 2 mM Fe (0.215 mg/l), while oxalic acid was secreted exclusively at 2 mM Fe (0.026 mg/l). The accumulation of acetic, lactic, malic, and oxalic notably higher at 2 mM Fe, with acetic acid showing the highest accumulation among the organic acids. These fundings suggest that the secretion of organic acids serves as an adaptive mechanism enabling S. saman seedlings to cope with Fe exposure.