A. D. Munshi

Effect of salt stress on cucumber: Na+–K+ ratio, osmolyte concentration, phenols and chlorophyll content

A pot experiment with 17 diverse genotypes of cucumber with four levels of salt stress viz., 0, 2, 4 and 6xa0dSxa0m−1 was carried out during 2006. ANOVA revealed significant differences amongst genotypes and genotypexa0×xa0salt stress interaction indicating the genetic variability and differential response of the genotypes to different salt stress levels. The salt stress adversely affected the biochemical parameters; effects were severe under 4xa0dSxa0m−1. No genotype could survive at 6xa0dSxa0m−1. Sodium content, Na+–K+ ratio, proline, reducing sugars, phenol and yield reduction (%) increased significantly as the salt stress increased. Potassium, chlorophyll, membrane stability index and fruit yield decreased significantly under salt stress in all genotypes. However, the genotypes CRC-8, CHC-2 and G-338 showed lower accumulation of sodium, lesser depletion of potassium, lower Na+–K+ ratio and higher accumulation of proline, reducing sugars, phenols, better membrane stability and lower yield reduction (%) under salt stress, while CH-20 and DC-1 were sensitive to salt stress. Thus, a combination of traits such as higher membrane stability, lower Na+–K+ ratio, higher osmotic concentration and selective uptake of useful ions and prevention of over accumulation of toxic ions contribute to salt stress tolerance in cucumber. These traits would be useful selection criteria during salt stress breeding in cucumber.

Cytotaxonomical analysis of Momordica L. (Cucurbitaceae) species of Indian occurrence

Somatic chromosome number and detailed karyotype analysis were carried out in six Indian Momordica species viz. M. balsamina, M. charantia, M. cochinchinensis, M. dioica, M. sahyadrica and M. cymbalaria (syn. Luffa cymbalaria; a taxon of controversial taxonomic identity). The somatic chromosome number 2n = 22 was reconfirmed in monoecious species (M. balsamina and M. charantia). Out of four dioecious species, the chromosome number was reconfirmed in M. cochinchinensis (2n = 28), M. dioica (2n = 28) and M. subangulata subsp. renigera (2n = 56), while in M. sahyadrica (2n = 28) somatic chromosome number was reported for the first time. A new chromosome number of 2n = 18 was reported in M. cymbalaria against its previous reports of 2n = 16, 22. The karyotype analysis of all the species revealed significant numerical and structural variations of chromosomes. It was possible to distinguish chromosomes of M. cymbalaria from other Momordica species and also between monoecious and dioecious taxa of the genus. Morphology and crossability among the dioecious species was also studied. Evidence from morphology, crossability, pollen viability and chromosome synapsis suggests a segmental allopolyploid origin for M. subangulata subsp. renigera. The taxonomic status of the controversial taxon M. cymbalaria was also discussed using morphological, karyological and crossability data.

Genetics of resistance in Luffa cylindrica Roem. against Tomato leaf curl New Delhi virus

Sponge gourd is a popular vegetable grown throughout India. Tomato leaf curl New Delhi virus, the causal virus of tomato leaf curl disease, has recently been reported to be associated with sponge gourd, causing up to 100% crop loss under epidemic conditions. We have collected 30 genotypically diverse genotypes of sponge gourd from different parts of India, screened these for resistance under natural epiphytotic conditions, and then confirmed the results through challenge inoculation with a purified strain of the virus under insect-proof greenhouse conditions. The minimum vulnerability index was recorded in genotype DSG-6 (3.33), followed by DSG 7 (6.0) under the challenge (whitefly-populated) inoculation conditions. Two susceptible genotypes (‘Pusa Sneha’ and NSG-1-11), both possessing desirable fruit characters, were crossed with the two most promising resistant lines (DSG-6 and DSG-7) and the disease reaction of segregating and backcross generations studied through challenge inoculation with a purified strain of virus under insect-proof greenhouse conditions. A chi-square (χ2) test of frequency distribution based on the vulnerability index of the F2 progenies of the two resistantxa0×xa0susceptible crosses revealed monogenic dominant Mendelian ratio 3(R):1(S) to be the best fit in all crosses. This monogenic dominant model was further confirmed by the 1(R):1(S) ratio found to be best fit for the test cross with the susceptible parent. These results reveal that resistance to Tomato leaf curl New Delhi virus associated with yellow mosaic disease of sponge gourd is controlled by a single dominant gene in the genetic background of the resistant parents (DSG-6 and DSG-7) and that these two lines can be effectively utilized for the development of high-yielding and yellow mosaic disease-resistant varieties/hybrids of sponge gourd. This is the first conclusive identification of a resistant source and the inheritance of resistance against Tomato leaf curl New Delhi virus in sponge gourd.

Molecular diversity and phenetic relationship of Momordica spp. of Indian occurrence

Twenty-one RAPD and twelve ISSR primers were used for assessment of genetic diversity and establishing phenetic relationships among 35 genotypes of six currently cultivated Indian Momordica species and five genotypes of two Luffa species. A total of 436 RAPD and 230 ISSR scorable fragments were produced of which 99.8% fragments showed polymorphism among the species and varieties of Momordica and Luffa. The level of polymorphism detected by the 33 random primers was higher among the species (99.8%) of Momordica than that estimated among the varieties (61.3%). The varieties belonging to dioecious Momordica species (75.6%) showed a higher level of polymorphism as compared to monoecious species (50.3%). A significant level (68.6%) of polymorphism however was detected by the two marker types among the Indian varieties of monoecious M. charantia species. A wider range of molecular diversity (16–95%) detected by both RAPD and ISSR markers reflected presence of high level of genetic variation among the species and Indian varieties of Momordica and Luffa. The level of inter-specific diversity was maximum (90%) between annual monoecious M. charantia and perennial dioecious M. cochinchinensis whereas the extent of intra-specific diversity was highest particularly in dioecious species (51%) as compared to monoecious species like M. charantia (38%). Wider divergence of the taxon of controversial identity, M. cymbalaria from the other Indian cultivated Momordica species and their evolutionary closeness with Luffa species was evident. The clustering pattern obtained among the 40 genotypes belonging to different Momordica and Luffa species corresponded well with their morphological, cytological and taxonomic classification, which was further supported by high boot-strap values and PCA analysis. Species and genotype-specific fragments detected by the random markers would be useful in introgression breeding for genetic improvement of Momordica cultivated in India. A smaller set of 28 informative random markers screened in this study could precisely differentiate the Momordica genotypes from each other and thus would be of use in many marker-based genotyping applications in Momordica.

Gynoecious inbred with better combining ability improves yield and earliness in bitter gourd (Momordica charantia L.)

This study was the only report for exploiting gynoecy for studying combining ability and hybrid development in bitter gourd. Thirty-six F1 hybrids and nine inbred lines including one gynoecious line were used to assess combining ability for earliness and yield in bitter gourd Momordica charantia L.). Among parents, gynoecious parent; DBGy-201 showed maximum GCA effect in desirable direction for node to first female flower, days to first picking, numbers of fruits and yield per plant. The parent, P6 (Priya) exhibited highest GCA effect for fruit length, weight and diameter. These parents were good general combiners for yield and earliness and could be used to improve these traits in bitter gourd breeding programmes for the accumulation of favourable genes. The parent P1 (DBGy-201) also exhibited earliest in per se performance with first female flower at ~7th node and the gynoecious hybrids DBGy-201xa0×xa0Pusa Vishesh and DBGy-201xa0×xa0Pusa Do Mausami had female flower at lower node (~8th node). The best gynoecious hybrids were DBGy-201xa0×xa0Priya for numbers of fruits per plant and DBGy-201xa0×xa0NDBT-12 for fruit yield per plant. The findings of second experiment (i.e. Linexa0×xa0Tester mating design) confirmed the potentiality of gynoeciuosxa0×xa0monoeciuos hybrids for high yield and earliness. The gynoecious inbred, DBGy-201 showed maximum GCA effect in desirable direction for node to first female flower, days to first picking, numbers of fruits and yield per plant and additive gene action is more important for breeding by selection.

Cyto-morphological evidence for segmental allopolyploid origin of Teasle gourd (Momordica subangulata subsp. renigera)

Teasle gourd [Momordica subangulata Blume subsp. renigera (G. Don) de Wilde, 2nxa0=xa056] exhibits morphological characters found in both M. dioica (2nxa0=xa028) and M. cochinchinensis (2nxa0=xa028). Morphological analysis of M. subangulata subsp. renigera suggests an allopolyploid origin. We present evidence elucidating the genomic relationships between M. dioica, M. cochinchinensis and M. subangulata subsp. renigera. A triploid M. dioicaxa0×xa0M. subangulata subsp. renigera hybrid had an average of 12.76 bivalents, 13.84 univalents and 0.88 trivalents at metaphase I, while the M. cochinchinensisxa0×xa0M. subangulata subsp. renigera hybrid had an average of 13.08 bivalents, 12.96 univalents and 0.96 trivalents. F1 hybrids of the two diploid species (M. dioicaxa0×xa0M. cochinchinensis) showed an average of 9.12 bivalents and 9.76 univalents, suggesting that the genomes of these species are only partially homologous. A higher number of bivalents in the triploid hybrids suggests that M. subangulata subsp. renigera is a segmental allopolyploid of M. dioica and M. cochinchinensis and that its genomes have diverged from the parental genomes.

Genetics of resistance to Cucumber mosaic virus in Cucumis sativus var. hardwickii R. Alef

The genetics of resistance to Cucumber mosaic virus (CMV) in Cucumis sativus var. hardwickii R. Alef, the wild progenitor of cultivated cucumber was assessed by challenge inoculation and by natural infection of CMV. Among the 31 genotypes of C. sativus var. hardwickii collected from 21 locations in India the lowest mean percent disease intensity (PDI) was recorded in IC-277048 (6.33%) while the highest PDI was observed in IC-331631 (75.33%). All the four cultivated varieties (DC-1, DC-2, CHC-1 and CHC-2) showed very high PDI and susceptible disease reaction. Based on mean PDI, 8 genotypes were categorized as resistant, 13 as moderately resistant, 9 as moderately susceptible and one as susceptible. A chi-square test of frequency distribution based on mean PDI in F2 progenies of six resistantxa0×xa0susceptible crosses revealed monogenic recessive Mendelian ratio 1(R):3(S) to be the best fit. This monogenic recessive model was further confirmed by 1(R):1(S) ratio as the best fit for back cross with resistant parent and no fit for either 3:1 or 1:1 in the back cross with the susceptible parent. The results revealed that CMV resistance in C. sativus var. hardwickii was controlled by a single recessive gene. Considering the cross compatibility between C. sativus var. hardwickii and cultivated cucumber, the resistance trait can be easily transferred to cultivated species through simple backcross breeding.

Hermaphrodite inbreds with better combining ability improve antioxidant properties in ridge gourd [Luffa acutangula (Roxb.) L.]

An experiment was conducted with the objectives to evaluate the breeding potential of hermaphrodite lines and to suggest suitable breeding approaches following half-diallel mating design for genetic improvement of antioxidant content and activity in ridge gourd [Luffaacutangula (Roxb.) L.] fruits. Twenty-eight genotypes, including seven parental lines and 21 crosses, were grown in randomized block design with three replications for fruit sample collection. The highly significant mean squares due to parents, hybrids and parents versus hybrids; and GCA and SCA for yield and antioxidants (ascorbic acid, total carotenoids, total phenolics, DPPH-RSA, ABTS-RSA and CUPRAC assay) indicate the existence of abundant genetic variation. The per se performance and combining ability of hermaphrodite parents (Satputia Long and Satputia Small) and hybrids of “monoeciousxa0×xa0hermaphrodite” cross were found to be superior for antioxidants along with yield potential. The cross combinations with superior per se performance coupled with high SCA estimates and having at least one hermaphrodite parent would be useful for concentrating desirable alleles to improve the antioxidants and yield simultaneously. Thus, hermaphrodite lines in combination with monoecious counterpart have enormous potential to breed “genotypes for higher antioxidants” without compromising yield in ridge gourd.

Induction and Morphological Characterization of Hermaphrodite Flowers in a Gynoecious Line of Bitter Gourd by Silver Nitrate, Gibberellic Acid, and Silver Thiosulfate

Gynoecious sex form in bitter gourd (Momordica charantia L.) has importance in hybrid breeding, but lack of easy and economic techniques to maintain gynoecious lines limits its use. Sex modification with chemicals can be an option in this regard. Bitter gourd is a typical monoecious (both male and female flower in same plant at separate nodes) plant. The gynoecious line was isolated from its related wild form M. charantia var. muricata L. Effects of concentrations of silver nitrate (SN), gibberellic acid (GA3), and silver thiosulfate (STS) for induction of male flowers in the gynoecious variety DBGy-201 were determined. The STS at 3 and 6 mm induced staminal tissue in female flowers of gynoecious bitter gourd and GA3 at 2.9 and 4.3 mm induced increased vegetative growth and rudimentary stamen production without viable pollen. The SN at 1.2 and 1.5 mm had no effect. The highest percentage (57.63%) of altered sex form was in plants treated with STS at 6 mm. Stamens were observed in female flowers that matured between 7 and 15 days after treatment. Induced hermaphrodite flowers had ovaries, stigma, styles, stamens, anthers, calyx, corolla, and petioles that were double the size of normal female flowers. Silver nitrate applied at 6 mm was best for inducing hermaphrodite sex form in the gynoecious line of bitter gourd.

Genetics and molecular mapping of gynoecious (F) locus in cucumber (Cucumis sativus L.)

Gynoecious is an important economic trait of cucumber for determinant of earliness and yield, yet genetic mechanism is not well understood for this trait. The experiment was conducted using F₂ mapping population by crossing of PPC-2, a gynoecious and parthenocarpic line with Pusa Uday (monoecious and non-parthenocarpic cultivar). Out of 179 SSR markers screened, 39 markers differentiated the gynoecious and monoecious parents. However, only 17 markers were segregating with F₂ mapping population, those were used for genotyping and linkage map analysis and these markers were placed along with F locus on chromosome 6 covering a total distance of 100.4cM. The SSR markers, SSR13251 and UW020605 were found to be closely linked to gynoecious (F) locus at 1.0 and 4.5 cM, respectively. The segregation of F₂ population of PPC-2 × Pusa Uday and GPC-1 × Punjab Naveen and test crosses for sex type herein suggested that single dominant gene controlled the gynoecious sex expression in cucumber particularly in gynoecious genotypes PPC-2 and GPC-1. Therefore, the monogenic dominant nature of gynoecious sex identified in the present experiment and SSR markers closely linked to the F locus will be useful in marker-assisted backcross breeding for transfering gynoecious trait into horticulturally desirable varieties.