Physiological mechanisms of Carya illinoensis tolerance to manganese stress

Manganese (Mn) is an essential element for plant growth but can be toxic at high levels. Pecan (Carya illinoensis), an important nut-producing species, has been observed to exhibit tolerance to high Mn levels. In this study, pecan seedlings were exposed to a nutrient solution containing either 2 μM (control) or 1000 μM (excess) MnSO₄ to investigate the physiological mechanisms. Despite substantial increases in Mn concentration in all pecan tissues, the presence of excess Mn did not induce visible symptoms of Mn toxicity on pecan leaves, nor did it result in any changes in malondialdehyde (MDA) levels. Photosynthetic rate and chlorophyll fluorescence parameters also remained unchanged. Subsequent examination revealed more cell layers and greater cell numbers in leaf palisade mesophyll tissue of Mn-treated plants compared with the control group. Cell length, and cell area decreased significantly in response to excess Mn, but total chloroplast area was unchanged and chloroplast structure remained intact. Subcellular fractionation analysis demonstrated that the cell walls, and to a lesser extent the soluble fraction, contained the majority of the Mn in leaves. The presence of excess Mn caused a marked increase in leaf concentrations of malic acid and citric acid, potential chelators of Mn. Our results suggest that the majority of Mn was sequestered in the leaf cell walls and may have been present as less-toxic chelated organic acids, thereby safeguarding the primary Mn target, the chloroplast, and ultimately conferring robust Mn tolerance in pecan.