Abstract: The effect of magnesium deficiency on photosynthesis of longan (Dimocarpus longana Lour.) seedlings was studied using hydroponic experiment. Magnesium def iciency led to the decline of the contents of chlorophyll and carotenoid, the capability of light energy absorption of chloroplast as well as maximum photosynthesis at both light and CO₂ saturation conditions. On the other hand, the apparent quantum yield, carboxylation efficiency and both light and CO₂ compensat ion points turned out to become higher as magnesium become deficient . The results of chlorophyll a fluorescence induction measurement showed a decrease of Fv/ Fo, Fv/Fm and Fd/ Fs in magnesium deficient longan leaves, indicating that primary light energy conversion ability and potential photosynthet ic activities of PSI were inhibited. The decrease in F₆₈₄/ F₇₃₆ under magnesium deficiency condit ion suggested that the regulat ion of excitation energy distribution between PS 􀀂 and PSII was reduced. The discussion was made on the responsive mechanism of longan to magnesium deficiency.

Key words: Longan, Magnesium, Photosynthesis