Abstract: The potted grape vines of‘Manicure Finger’ (Vitis vinifera L. ) were used for studying the photosynthesis under overcast and rainy days and the physiological characteristics of isolated intact chloroplasts under hypotonic swelling . The results of research showed that the net photosynthetic rate decreased under overcast and rainy days and thatwould increase when it was cleared up. Both water potential (Ψw ) and stom atal resistance (Rs) and glycolate oxidase activity negatively correlated with the net photosynthetic rate ( Pn) ( r = - 0.96109, r = - 0.95609, r = - 0.94613 ). When the osmotic pressure of the medium was decreased by lowering sorbitol concentration from 0.33 mol·L ^{- 1} to 0.18 mol·L ^{- 1} , the percentage of intact chloroplasts decreased slightly but the volume of chloroplasts swelled obviously and the activity of phosphatase was strongly decreased. The swelling of chloroplast and the decreasing of the activity of phosphatase were significantly inhibited the photosynthesis of grapes in the overcast and rainy days. The volume of chlorop lasts would restored to normalwhen the osmotic pressure of the medium was becoming to normal. Itwas the one of the reasons for the photosynthesis increasing after the rain.

Key words: Grape, Photosynthetic characteristics, Chloroplast