Breeding for high parthenocarpic expression and improved fruit quality in cucumber (cucumis sativus L.)

Abstract

Cucumber (Cucumis sativus L.) is a widely cultivated cucurbitaceous vegetable and traits like parthenocarpy and gynoecy boost productivity under protected conditions where pollinators are forbidden. Although parthenocarpic cucumber was introduced early in India, public-sector hybrid development remains limited. Kerala Agricultural University launched a breeding program in 2011, leading to the release of KPCH-1, a high-yielding, early-maturing, parthenocarpic hybrid with moderate disease resistance and a cluster-bearing habit, mostly confined to upper nodes. However, poor cluster formation at lower nodes and late-stage fruit deformities limit its potential vis-a-vis popular private sector hybrids. With only few public-sector options available, farmers are forced to rely on costly private hybrids. This highlights the need for improved public sector parthenocarpic hybrids for protected cultivation, prompting the present study on “Breeding for high parthenocarpic expression and improved fruit quality in cucumber (Cucumis sativus L.)”. The study comprised of four experiments aimed at evaluating and identifying cucumber lines with high parthenocarpic expression and yield, developing F₁ hybrids through diallel crossing, assessing their mean performance, combining ability and heterosis and validating the reported SSR markers linked to parthenocarpy. In the first experiment, twelve gynoecious and parthenocarpic cucumber lines were evaluated, revealing significant variation for all traits except ovary length and crop duration. Among the genotypes evaluated, several stood out for specific agronomic and market-preferred traits. CS 134 was notable for its high fruit weight and lack of deformities, while CS 135 excelled in fruit number per plant, indicating strong yield potential. CS 136 demonstrated superiority across multiple parameters, including compact growth, high parthenocarpic fruit set, extended harvesting duration and top yield performance, making it a strong candidate for commercial cultivation and breeding. CS 138 showed early and compact fruiting traits. CS 140 was distinguished by its longer fruits, appealing for market preferences. CS 141 displayed excellence in fruit quality traits such as girth, flesh thickness, and TSS, along with ideal cluster fruiting and absence of deformities. CS 142 and CS 143 were among the earliest to reach harvest and showed favorable flesh quality and shelf-life traits. CS 144 was superior in early flowering and parthenocarpic expression. CS 145 also showed high sweetness levels, aligning with consumer preferences. Overall, CS 136 emerged as the most promising genotype for yield and adaptability under protected conditions, while others offered valuable traits for targeted improvement in breeding programs. High phenotypic coefficient of variation (PCV) and genotypic coefficient of variation (GCV) were observed for traits such as node at which the first female flower emerged, node at which parthenocarpic expression occurred, parthenocarpic fruit set (%), parthenocarpic fruit expansion (%), number of fruits per plant, percentage of deformed fruits and physiological loss in weight, indicating a strong genetic influence and suggesting good potential for selection. High heritability coupled with high genetic advance as percent of mean was recorded for several key traits including node at which the first female flower emerged, node of parthenocarpic expression, parthenocarpic fruit set and expansion percentages, node at which clustered fruit appeared, number of fruits per cluster at middle nodes, fruit length, number of fruits per plant, percentage of deformed fruits, yield per plant, total soluble solids (TSS) and physiological loss in weight. These findings suggest that these traits are primarily governed by additive gene action and, therefore, selection for their improvement would be highly effective. In the correlation study, yield per plant was found significantly and positively correlated with parthenocarpic fruit set percentage, parthenocarpic fruit expansion percentage, node of clustered fruit appearance, number of fruits per plant and number of harvests. Path coefficient analysis revealed that several traits have a positive direct influence on fruit yield per plant. Key contributors included parthenocarpic fruit set, early flowering, fruit number, and fruit size characteristics. Traits that showed both strong positive correlations and direct effects on yield, such as parthenocarpic fruit set, fruit expansion, number of fruits, and frequency of harvest, are particularly promising for selection in cucumber breeding programs targeting improved productivity under protected cultivation. From the twelve gynoecious parthenocarpic lines evaluated, six best performing lines (CS 135, CS 136, CS 137, CS 141, CS 142 and CS 143) were selected as parents for diallel crossing programme based on selection index and the thirty F1 hybrids so developed were evaluated under naturally ventilated polyhouse conditions for various traits along with the parents and two commercial checks – KPCH -1 and Multistar. During the crossing programme, the application of silver thiosulphate (STS) successfully induced male flowers across all parental lines without causing significant variation in the timing of flower emergence or pollen viability. While these traits remained consistent, crossing efficiency and seed set varied notably among genotypes. Based on the general combining ability (GCA) effects for various traits, the parent CS 142 emerged as the best general combiner, exhibiting superior GCA for the maximum number of traits, i.e., eight traits. These include early flowering (days to first female flower anthesis and node at which first female flower emerged), reproductive efficiency (node at which parthenocarpic expression occurred and length of ovary), and fruit quality traits (fruit length and TSS), along with favorable performance in days to first harvest and number of fruits per cluster at upper nodes. This makes CS 142 a valuable parent in cucumber breeding for improving earliness, parthenocarpy and fruit quality. Based on the specific combining ability (SCA) effects observed across traits, the top five hybrids identified were CS 143 × CS 137, CS 135 × CS 141, CS 143 × CS 136, CS 142 × CS 136, and CS 135 × CS 142. The hybrid CS 143 × CS 137 exhibited superior performance for average fruit weight, fruit length, fruit girth, flesh thickness, number of fruits per plant, and total soluble solids, indicating excellent potential for fruit quality and productivity. CS 135 × CS 141 excelled in parthenocarpic fruit set and expansion, fruit length and girth, number of fruits per plant, yield per plant and TSS, making it a strong candidate for yield enhancement through parthenocarpy. CS 143 × CS 136 demonstrated high SCA for parthenocarpic traits, number of harvests, yield per plant, and TSS. CS 142 × CS 136 was notable for earliness, favorable average fruit weight and girth, higher TSS, and lower physiological loss in weight while CS 135 × CS 142 was superior in internodal length, ovary length, fruit girth, TSS and physiological loss in weight. Based on the analysis of mean performance across key traits, the hybrid CS 141 × CS 142 emerged as the most outstanding performer, ranking first with superiority in six traits, including days to first female flower anthesis, node at which the first female flower emerged, node of parthenocarpic expression, flesh thickness, days to first harvest and lowest percentage of deformed fruits. Following this, CS 143 × CS 136 ranked second, excelling in four important traits including parthenocarpic fruit set percentage, fruit expansion percentage, number of fruits per plant and yield per plant, highlighting its strong potential for enhancing both parthenocarpic fruit setting efficiency and yield. Several hybrids exhibited high levels of heterosis for yield and associated traits, highlighting their potential for commercial cultivation under protected environments. Five SSR markers previously linked to parthenocarpy in cucumber (SSR01148, SSR01012, SSR10476, SSR19174, and SSR16226) were tested for validation, but four showed no amplification while SSR10476 lacked polymorphism between parthenocarpic and non-parthenocarpic lines, indicating their ineffectiveness in the current genetic background. The top four hybrids, based on their performance for various traits, high GCA estimates of their parents, high SCA estimates and pioneer heterotic performance were CS 143 × CS 136, CS 141 × CS 142, CS 141 × CS 135 and CS 135 × CS 141. This study also aimed to address key limitations of the commercial hybrid KPCH-1, including limited cluster-bearing at lower nodes and fruit deformities. Among the 30 hybrids developed, CS 137 × CS 141 and CS 142 × CS 141 showed earlier expression of clustered fruits compared to KPCH-1 and hybrids such as CS 137 × CS 136, CS 141 × CS 142, and CS 141 × CS 143 exhibited complete absence of fruit deformities, while several others showed marked reduction. Parthenocarpic fruit set and fruit expansion percentages, showing strong positive correlations, high heritability and direct effects on yield, emerged as key selection criteria in developing high-yielding parthenocarpic cucumber hybrids for protected cultivation.