Allelopathic effects of leachates from two alien mangrove species, Sonneratia apetala and Laguncularia racemosa on seed germination, seedling growth and antioxidative activity of native mangrove species Sonneratia caseolaris

To evaluate the impacts of two alien mangrove species Laguncularia racemosa and Sonneratia apetala on the native mangrove plants on Hainan Island, we chose a native mangrove species, Sonneratia caseolaris, because it has similar ecological requirements to the two alien species and they are usually planted close to it. The effects of aqueous leachates from the two alien species on S. caseolaris were analysed by measuring the germination rate of seeds and antioxidative activity of seedlings. The activities of SOD, POD and APX in the seedlings of S. caseolaris were inhibited by 0.2 and 0.4 g/mL concentration of leachates from the two exotic mangrove species, but the CAT activity increased, when the concentration of leachates increased. The leachates from S. apetala, increased the activities of SOD, POD, APX and CAT. However, the activities of SOD and CAT increased, while POD and APX were inhibited < 0.1g/mL concentration of the leachates from L. racemosa. The changes in the antioxidative activity may be an adaptive regulatory strategy in S. caseolaris seedlings in response to the allelochemicals of L. racemosa and S. apetala. Furthermore, the chemical components in aqueous leachates from mixed fresh branches, leaves and fruits of L. racemosa and S. apetala were characterized by GC-MS. Octadecanoic acid, 3-[(1-oxododecyl)oxyl-1,2-propanediyl ester was found at relatively high contents in every leachate of both of the exotic species, which may play an important allelopathic role in the leachates.