This study was conducted to screen hydroponically grown various gourd genotypes to obtain tolerant rootstocks for sustainable watermelon production under contrasting pH levels in relation to physiological and root morphological characters under greenhouse conditions. Two commercial watermelon cultivars (Crimson Tide and Crisby) and 20 different gourd genotypes includes Lagenaria siceraria and Cucurbita maxima and C. moschata interspecific hybrids were screened in nutrient solution under three different pH (High: 8.5, Neutral (control): 6.5 and Low: 4.5) levels. Shoot and root dry matter, leaf area, total root length and leaf mineral element compositions were significantly (P<0.001) affected by different levels of pH, genotype and pH x genotype interaction. Plants usually showed significantly lowest performances in shoot and root growth under both low and high pH conditions. However, some of the interspecific hybrids C. maxima x C. moschata (Nun9075, Ferro, Ercole and Jumbo) responded significantly positive by enhancing the shoot and root growth under both low and high pH conditions. Therefore, these genotypes are characterized by 'acid-tolerant' and 'alkaline-tolerant' which are highly appropriate to be used as rootstocks for the watermelon under both contrasting pH conditions. Because, two watermelon cultivars (Crisby and Crimson Tide) exhibited significantly lowest performance in all measured parameters under three pH levels as compared to twenty gourd genotypes. The tolerance mechanism of the determined gourd genotypes was closely associated with vigorous root system and large leaf area formation. These traits could be useful characters for the selection and/or breeding of 'pH-tolerant' gourd rootstocks for sustainable watermelon production in the future. However, this should be examined by grafting watermelon cultivars onto the 'acid-tolerant' and `alkaline-tolerant' characterized gourd genotypes in further study.