Utilisation of male sterility and polyploidy for genetic improvement in targets spp.

Abstract

Marigold (Tagets spp.) is a multifunctional annual crop which can be grown in wide range of climatic conditions. The area of this crop is being expanded with the wide use of marigold flowers in cosmetic, food coloring, poultry and pharmaceutical industries due to its high carotenoid content. Among 33 Tagetes species, African marigold (Tagetes erecta) and French marigold (Tagetes patula) are highly preferred for the commercial cultivation. Even though these two species together hold a large extent of area under cultivation in the country, prominent steps taken for the development of superior hybrids (inter varietal as well as inter-specific) from public sector is still narrower. One of the major reasons for this might be the complex flower structure of marigold as in any other Asteraceae crop. The way out from this problem is the utilisation of genetic male sterility that exists in Tagetes erecta. In this context, the study entitled “Utilization of male sterility and polyploidy for genetic improvement in Tagetes spp.” was undertaken in the department of Floriculture and Landscaping during 2019-2022. As a preliminary parental evaluation, 20 marigold genotypes with 12 testers (10 African marigold and 2 French marigold genotypes) and 8 male sterile lines were evaluated during 2019-20. The analysis on plant characters showed the superiority of the line KAU - Ms6 in terms of plant height (47.27 cm), plant spread (31.39 cm) and stem girth (2.98 cm) while Ms18 exhibited with the early flowering habit in terms of bud initiation (17.07 days) and 50 per cent flowering (24.33 days). Superiority with respect to floral characters viz., flower size and flower weight was observed in KAU - Ms42 (3.70 cm and 4.30 g respectively). The analysis on yield traits exhibited the superiority of the line KAU - Ms6 in terms of number of flowers (56.77) and yield per plant (119.91 g) while KAU – Ms42 with highest number of viable seeds per plant (60.17). The estimation of total carotenoids revealed that the line KAU – Ms18 produced maximum pigment (2.01 mg/g) while the lutein content estimated was highest in KAU – Ms5 (6.23 ppm). Among testers, KAU - M1 was marked with superiority in terms of plant spread (44.00 cm), stem girth (4.97 cm) and leaf area (47.14 cm2) while French marigold genotypes (KAU - M47 and KAU - M48) were recorded lowest plant height, spread, stem girth and leaf area. However, early flowering was noticed in the French marigold genotypes, KAU – M47 and KAU – M48, in terms of days to bud initiation (19.20 and 17.20 days respectively) and 50% flowering (22.67 and 32.00 days respectively), produced small sized flowers of 3.20 cm and 2.70 cm flower diameter and 1.27 g and 1.04 g flower weight respectively. Among the African marigold testers, KAU - M1 showed delayed flowering, large sized flowers (6.80 cm flower diameter) with highest flower weight (11.87 g). With regard to number of flowers and yield per plant, the genotypes KAU - M46 and KAU - M2 showed better performance (126.16 and 253.96 g/plant; 111.34 and 266.32 g/plant respectively). The estimated total carotenoids was maximum in KAU – M47 (4.86 mg/g) and lutein in KAU – M1 (10.23 ppm). The observations on the stem color and flower color revealed the difference among the genotypes. The flower color was almost similar for all the 8 genotypes (Orange N25 C and Orange N25 B) in male sterile lines, while in the case of testers, among African marigold types, almost all the genotypes were orange colored (Orange – N25 group), except KAU – M1 (Yellow 9 A) and KAU – M46 (White NN155 C). Based on the evaluation of parental genotypes, four lines (KAU - Ms6, KAU - Ms18, KAU - Ms24 and KAU - Ms42) and six testers (KAU - M1, KAU - M2, KAU - M4, KAU - M8, KAU - M40 and KAU - M47) were selected and a crossing block was established during 2020-21 in a line x tester model. The mean performance of twenty four hybrids highlighted the inter-specific hybrid ‘KAU - Ms6 x KAU - M47’ which showed very good hybrid vigour. The combining ability analysis revealed the best general combiners KAU - M47 for number of flowers and yield per plant (108.96 and 128.93 respectively) and KAU - M1 for yield per plant (42.07). Genotypes KAU - Ms18, KAU - Ms24 and KAU - M8 were estimated with highest GCA for total carotenoids (0.09, 0.17 and 0.09 respectively). According to RHS color chart, except in the case of interspecific hybrids, all the other crosses were found to be showing the flower color of ‘Orange – N25’ group with shade of A, B and C. The interspecific hybrids KAU – Ms6 x KAU – M47, KAU – Ms6 x KAU – M47, KAU – Ms24 x KAU – M47 and KAU – Ms42 x KAU – M47 showed double colored flower heads with shades of ‘Orange N25 B and Red 46 A’. The good specific combiners identified for number of flowers and yield per plant were KAU - Ms6 x KAU - M47 (86.17 and 207.65 respectively), KAU - Ms42 x KAU - M2 (41.26 and 125.14 respectively) and KAU - Ms18 x KAU - M40 (19.95 and 93.17 respectively). In terms of total carotenoids, KAU - Ms6 x KAU - M2, KAU - Ms18 x KAU - M4, KAU - Ms24 x KAU - M1, KAU - Ms24 x KAU - M40, KAU - Ms24 x KAU - M47, KAU - Ms42 x KAU - M2, KAU - Ms42 x KAU - M8 and KAU - Ms42 x KAU - M47 were estimated with best SCA. On the estimation of heterosis for number of flowers and yield per plant, the inter-specific cross KAU - Ms6 x KAU - M47 recorded superiority in terms of relative heterosis (RH – 254.87%), heterobeltiosis (HB – 159.48%) and standard heterosis (SH – 380.63%). Other crosses which exhibited significant SH for yield per plant were KAU - Ms18 x KAU - M1 (61.22%), KAU - Ms18 x KAU - M47 (60.74%), KAU - Ms24 x KAU - M1 (47.22%), KAU - Ms24 x KAU - M47 (36.47%), KAU - Ms42 x KAU - M1 (43.73%), KAU - Ms42 x KAU - M2 (70.93%) and KAU - Ms42 x KAU - M47 (38.88%). With respect to total carotenoid, the hybrid KAU - Ms18 x KAU - M4 exhibited highest significant heterosis (RH – 104.29%, HB – 11.89% and SH – 32.0%). Experiment on polyploidisation of male sterile lines (KAU - Ms6) using two different concentration of colchicine (0.05% and 0.1%) for 1, 3 and 5 hours was conducted. Among the treatments, the lower colchicine concentration for short duration (0.05% for 1 hour) resulted in highest survival of seedlings (37.00%) and potted plants (27.02%). The treatment with 0.05% colchicine for 1 hour exhibited the most dwarf, compact and sturdy plants compared to other treatments. The analysis of stomatal characters also revealed larger stomata and low stomatal density with higher concentration and duration of colchicine, with highest stomatal size of 35.97 μm length and 11.49 μm width. The lowest stomatal density was observed for the treatment 0.1% colchicine for 3 hours (107.39). The ploidy levels of the variants through flow cytometry detected tetraploids. From the overall performance the seed treatment with 0.05% for 1 hour could be observed as the best for tetraploid indction which recorded maximum survival of plants, compact sturdy growth and larger sized stomata. Vegetative propagation of male sterile lines through cuttings was experimented as an alternative to the sib-mating method of maintaining the male sterile lines. Herbaceous cuttings collected from KAU - Ms42 were treated with growth regulators NAA and IBA at 100, 200, 300 and 400 ppm concentrations. Among the treatments, of the highest per cent of rooting with good survival of plants in pots was observed with the NAA 300 ppm (50.83% and 74.12% respectively). Considering per cent rooting of cuttings and survival of plants after transplanting to pots, NAA 300 ppm could be recommended as the best treatment. The present investigation on “Utilisation of male sterility and polyploidy for genetic improvement in Tagetes spp.” resulted in two heterotic hybrids viz. KAU - Ms6 x KAU - M47 and KAU - Ms42 x KAU - M2, for loose flower as well as landscaping purpose while three hybrids for total carotenoid content (KAU - Ms24 x KAU - M40, KAU - Ms42 x KAU - M2 and KAU - Ms42 x KAU - M47).