Objectives In this paper we investigated the effects of different magnesium nutrition levels on the chloroplast ultrastructure and photosynthetic characteristics of areca palm seedling leaves, which provided a theoretical basis for the balanced fertilization treatment and high yield and high quality cultivation of areca palm.

Methods The three-leaf age of ‘Reyan No.1’ areca palm seedlings were used in a sand culture experiment. The 1/2 MS nutrient solution was used as control, in which the Mg concentration was 0.75 mmol/L(CK); and the Mg deficient and sufficient treatments were prepared by adding Mg 0 mmol/L (–Mg) and Mg 2.25 mmol/L(＋Mg). After the seedlings grew for 50 days in the treated solution, plant samples were taken. The non-structural carbohydrate contents, the activitiesof sucrose synthase enzymes (SS) and sucrose phosphate synthaseenzymes (SPS), chlorophyll fluorescence kinetic parameters and the chloroplast ultrastructure in the leaves of areca palm seedlings were observed.

Results 1) –Mg treatment resulted in a significant decrease in the relative chlorophyll content (SPAD) of betel nut, the maximum photochemical efficiency F_v/F_m of photosystem Ⅱ (PSⅡ), the actual photochemical efficiency Y(Ⅱ) and the photochemical quenching coefficient qP; but all the above parameters were not significantly different between +Mg treatment and CK. 2) The contents of soluble sugar and sucrose in the leaves under –Mg treatment were significantly higher than those in CK, while the starch content was significantly lower. The starch content in +Mg treatment was significantly higher than in CK, but the soluble sugar and sucrose contents were not significant. 3) Under –Mg treatment, the chloroplast membrane became disintegrated, most of the grana lamellae disappeared, the thylakoid structure was broken, and the number of osmiophilic granule increased. Under +Mg treatment, the chloroplast shape was deformed, the chloroplast membrane was blurred, and the grana lamellae partially disappeared. The osmiophilic granules and starch granules increased.

Conclusions Magnesium deficiency could damage the chloroplast ultrastructure of areca palm seedlings, inhibit the metabolism of chlorophyll and carbon and in turn reduce the photosynthetic efficiency. High level of magnesium supplying is less possible to cause unfavorable effect on areca palm leaves.