Excessive Iron Effects on the Gray Mangrove, Avicennia marina, Seedlings Under Saline Condition

Excessive iron availability (EIA) in marine ecosystems often occurs as a result of anthropogenic activities and represents a stressful and damaging condition for many marine organisms. The gray mangrove Avicennia marina, the dominant vegetation along the Red Sea shoreline, is frequently exposed to such conditions; however, its physiological responses remain unknown. The current results indicate that, under controlled conditions, EIA had contrasting effects on the growth of A. marina seedlings, depending on the salinity of the medium. Under saline conditions (600 mM NaCl), excessive iron reduced total dry weight (by 35%), photosynthetic assimilation rate (by 20.6%), stomatal conductance (by 40%), transpiration rate (by 46%), iron accumulation in leaves (by 42%), stems (by 18.4%), and roots (by 36.1%), as well as iron absorption efficiency (FeAE) (by 39.4%). In contrast, under nonsaline conditions, EIA enhanced total dry weight (by 39.8%), net photosynthetic assimilation rate (by 19%), and photosynthetic pigment content, while having no significant effect on iron accumulation in different plant organs or on iron use efficiency. Salt excretion occurred only under saline conditions, and EIA had no effect on the rate or composition of the excreted salt crystals. In conclusion, A. marina seedlings appear to cope with excessive iron by sequestering excess Fe in the roots and enhancing root growth and photosynthetic activity under nonsaline conditions. Under saline treatment, however, the plants adopt a restrictive Fe uptake strategy, and despite that, the detrimental effects of EIA were noted only under saline conditions.