Abstract: By the methods of sequential extraction and chlorophyll fluorescence, a laboratory experiment with no sediments was conducted to study the cadmium (Cd) accumulation in different organs of tropical seagrass Thalassia hemprichii and the effects of Cd on the photosynthesis of the seagrass. The Cd accumulation in T. hemprichii organs was in the order of leaf > root > rhizome. In all treatments, the Cd in the T. hemprichii organs was dominated by the 1 mol·L^-1 NaCl-, 2% HAc-, and 0.6 mol·L^-1 HClextractable Cd, occupying 80%-99% of the total Cd, indicating that the less active and weakly migrated forms were the main forms of Cd in T. hemprichii. Under the exposure of increasing concentration Cd, the proportion of 0.6 mol·L^-1 HClextractable Cd in different organs of T. hemprichii decreased by 7%-24%, while the proportion of 1 mol·L^-1 NaCl and 2% HAcextractable Cd increased by 2%-24%. On the other hand, the 80% ethanol and dH₂Oextractable Cd, the high active Cd forms, decreased in T. hemprichii leaf but increased in root and rhizome. These results suggested that T. hemprichii could weaken the toxicity of Cd by changing the proportion of different Cd forms in cells and/or organs. Under the exposure of high concentration Cd, the effective quantum yield (ΔF/F_m′) and the chlorophyll a, chlorophyll b and total chlorophyll contents declined significantly, suggesting that the photosynthesis of T. hemprichii was seriously impaired. It seemed that the protective mechanisms evolved by T. hemprichii could play an important role in Cd detoxification when exposed to relatively low concentration Cd, but became restricted under higher Cd stress.

Key words: nitrapyrin-N fertilizer, summer maize, N₂O emission, yield, N utilization.