Abstract:In order to understand the response of photosynthesis and ultrastructure of mesophyll cells to drought stress, three cultivars of Jaminum sambac, including ‘Unifoliatum', ‘Bifoliatum' and ‘Trifoliatum', were cultured in pots experiment in greenhouse under different drought treatments (mild drought, moderate drought, severe drought), The effects of drought stress on photosynthetic characteristics and ultrastructure of mesophyll cells in J. sambac were studied. The results showed that the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) in J. sambac ‘Unifoliatum' and ‘Trifoliatum' decreased under mild drought stress, and the ultrastructure of mesophyll cells was only slightly affected. Under moderate drought stress, Pn, Tr, Gs and Ci of ‘Unifoliatum' and ‘Trifoliatum' decreased continuously except Ci of ‘Trifoliatum' increased, while chloroplast structure changed obviously and occurred damage. Under severe drought stress, Pn, Tr and Gs of ‘Unifoliatum' and ‘Trifoliatum' had singnificant decrease compared to those of control (P<0.01), and Ci increased, while chloroplast structure changed tremendously and occurred serious damage. The Pn, Tr, Gs and Ci of ‘Bifoliatum' decreased slightly with increment of drought stress, and ultrastructure of mesophyll cells of ‘Bifoliatum' had not obvious changes. Therefore, the tolerance ability to drought of J. sambac ‘Bifoliatum' is stronger than that of ‘Unifoliatum' and ‘Trifoliatum'.