In this study, the effect of flooding on plant growth and photosynthetic activity of grafted watermelon were investigated. The watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] cv. 'Crimson Tide' was grafted onto Lagenaria siceraria SKP (Landrace). Grafted and ungrafted watermelon plants were flooded at the soil surface for 20 days. For every 5 days, three plants were sampled to determine plant fresh and dry weight, leaf number and main stem. length. Leaf colour, single leaf CO(2) exchange rate (CER), stomatal conductance (SC) and transpiration rate (Ts) were determined at T days interval. Flooding caused chlorosis on both grafted and ungrafted plants but such effect was more pronounced on ungrafted watermelon plants. CER, SC and Ts began to decrease from the 4th day of the flooding in both grafted and ungrafted plants as compared with non-flooded controls. However, grafted plants showed higher tolerance to flooding and had two-folds more CER, SC and Ts. Plant growth rate was also significantly lower in flooded plants than when compared to unflooded controls. Ungrafted plants had lower dry weight than grafted plants under flooding conditions. At the end of the experiment, decrease in fresh weight of plants was about 180% in ungrafted and 50% in grafted watermelons. Dry weight also decreased about 230% in ungrafted and 80% in grafted watermelons. Similar results were found in leaf number and main stem length. Adventitious roots and aerenchyma formation were observed in grafted watermelon but not in ungrafted watermelon under flooding. Adventitious root formation began from 3rd or 4th day of flooding and adventitious roots grew towards the soil surface. Flooding tolerance of watermelon could be improved by grafting onto L. siceraria. (c) 2005 Elsevier B.V. All rights reserved.