Abstract: The effects ofmagnesium deficiency on photosynthesis characteristic of flowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee) were studied. Net photosynthesis rate (Pn) , stomatal conductance and transpiration rate changed in one-peak curve in daytime, and those decreased with the decrease of magnesium supply. The Pn peak reached earlier in Mg-deficiency leaves. Light compensation point, light saturation point and Pn maximum at saturated light decreased in Mg-deficiency leaves. CO₂ compensation point increased, CO₂ saturation point decreased (Mg 1 mmol·L ^{- 1} ) or increased (Mg 0) , Pn maximum at saturated CO₂ concentration decreased inMg-deficiency leaves. There were little effects in the changes of intracellular CO₂ concentration (Ci) in daytime, the response of Ci to light and CO₂ concentration byMg-deficiency, those indicated that photosynthesis inhibition in flowering Chinese cabbage by Mg-deficiency was mainly due to non-stomatal factors.

Key words: Flowering Chinese cabbage, Mg-deficiency, Photosynthesis