SELECTION OF HEAT TOLERANT PARENTS AND THE DEVELOPMENT OF SUPERIOR HYBRIDS IN BOTTLE GOURD [Lagenaria siceraria (Mol.) Standl.]

Abstract

Rising summer temperatures in Bangladesh severely constrain bottle gourd productivity, necessitating the identification of heat-tolerant parents and the development of superior hybrids. To address this challenge, a comprehensive research program was conducted from 2022 to 2025 at the BRAC Vegetable Research Center, Dinajpur, involving fifty-eight (58) genetically diverse bottle gourd genotypes. The study aimed to assess genetic variability and screen heat-tolerant genotypes, identify superior parents through combining ability, and develop heterotic hybrids under heat stress. Initially, the 58 genotypes were evaluated under both optimum (Rabi) and heat-stress (Kharif) environments for morphological, yield contributing, and quality traits. Wide and significant genetic variability was observed for days to first female flower appearance, node number at first female flower appearance, days to first edible fruit harvest, fruit length, fruit diameter, number of fruits per plant, fruit weight, fruit yield per plant, total soluble solids, dry matter content, and ascorbic acid content. High heritability coupled with high genetic advance for most traits indicated the predominance of additive gene action and substantial scope for genetic improvement through selection. Correlation and path analyses identified number of fruits per plant, fruit weight, and fruit diameter as primary yield determinants, with earliness contributing indirectly, establishing these traits as key selection criteria under heat stress. Heat tolerance was assessed using combined stress tolerance indices of major yield-contributing traits. Based on indices values, ranking, and cluster analysis, genotypes were classified into 21 heat-tolerant, 18 mid-tolerant, and 19 susceptible groups. From these, 20 tolerant and two mid-tolerant genotypes were selected for further evaluation through line × tester analysis under heat stress conditions. Combining ability analysis identified several parents with desirable general combining ability (GCA) effects for earliness, fruit size, yield components, and yield per plant. Based on per se performance and GCA ranking, six genotypes with BG-01, BG-16, and BG-21 as strong general combiners and BG-42, BG-55, and BG-53 as moderate but trait-specific good combiners were selected for a 6 × 6 full diallel mating design. Diallel analysis conducted during Kharif 2025 under heat stress showed highly significant GCA, SCA, and reciprocal effects for all studied traits, confirming the involvement of additive, dominance, and maternal genetic components, as well as the importance of cross direction in hybrid performance. Among the parents, BG-53 and BG-21 were identified as superior general combiners for fruit yield and major yield determinants. Heterosis analysis revealed significant positive heterosis over mid-parent, better-parent, and standard checks for yield and yield-contributing traits under heat stress. The hybrid BG-53 × BG-16 exhibited maximum heterosis for number of fruits per plant and fruit yield per plant. BG-53 × BG-01 recorded the highest heterosis for average fruit weight along with substantial yield advantage. BG-55 × BG-42 showed significant heterosis for fruit diameter along with favorable yield and quality traits. Crosses BG-01 × BG-21 and BG-21 × BG-53 combined desirable earliness with high yield heterosis, indicating strong dominance and over-dominance effects under heat-stress conditions. Genotypes BG-53 (mid-tolerant, early, and prolific) and BG-21 (tolerant, with higher fruit weight and yield) were identified as key parental lines for heat-stress breeding programs. The hybrids BG-53 × BG-16, BG-53 × BG-01, BG-01 × BG-21, BG-21 × BG-53, and BG-55 × BG-42 were selected as promising for potential commercial hybrid development under heat stress in Bangladesh.