INTRODUCTION

Jasmine (Jasminum spp.) is one of the remuneratively prized and significant traditional flower crops of India. It belongs to the family Oleaceae and is one of the aromatic flowers cultivated since times immemorial and is considered as the most revered flower in our country for its attractive and fragrant flowers. Jasmine flowers are popularly used in preparation of garlands, hair adornments for women, used in religious and ceremonial occasion and also for extracting perfumery oil (Sanchita et al., 2018) which is used in the cosmetic and perfumery industries. India is the largest exporter of jasmine oil in the world accounting for over 40 per cent of total world export. It has extensive application in aromatherapy as jasmine fragrance is effective in treating depression, nervous exhaustion and stress. It is also widely used in the medicinal and pharmaceutical industries (Green and Miller, 2009). Exceptional increase in the consumption of jasmine flowers by the Indian population settled in Middle East countries and the United States of America has led to the augmentative export demand for flower strings of J. sambac (Jawaharlal et al., 2012). The genus Jasminum is reported to comprise of around 200 species (Bailey 1958). The commercially cultivated jasmine species in Tamil Nadu, Karnataka, Andhra Pradesh, Uttar Pradesh and some parts of Bihar and West Bengal are J. sambac, J. grandiflorum, J. auriculatum and J. multiflorum. Exclusive of these commercially important species, lesser-known species namely, J. nitidum, J. calophyllumand J. flexile also acquire economic importance as they produce flowers which are suitable for use as loose flower, besides being ideal garden plants (Raman, 1969; Ganga et al., 2015).

Hybridization leads to the development of new adaptive traits allowing the expansion of new habitats (Johnston et al., 2004), fitness enhancement (Burke et al., 1998), or the origin of new hybrid lineages (Grant, 1981; Arnold, 1997). Hybridization can also reinforce reproductive barriers through natural selection for conspecific gene flow (Arnold, 1992), the creation of stable hybrid zones (Barton and Hewitt, 1985), or the formation of introgressive races. Hybridization is not the general outcome whenever congeneric species come into contact because there often are pre- and post-mating barriers that prevent hybridization.

Pollination is the result of pollen being transferred from the anther to the stigma of another flower. Landing of pollen on stigma is no guarantee for seed- set. Failure of fertilization after self-pollination in self- sterile or self-incompatible plants may also be due to the inability of the pollen to germinate on its own stigma. Pollination in many crops has manifested a major influence on the number of fruit set, fruit length, fruit girth and fruit shape (Nirmalaruban et al., 2020). Jasmine varieties released till date are only clonal selections and mutants. Hence there is a dire necessity to evolve hybrids in jasmine using commercial and under-utilized species. Prior attempts at hybridization have failed because of the compatibility and fertilization barriers present among the species. Considering the above, the present study focuses on the method of pollination and its potency on fruit set in jasmine.

MATERIALS AND METHODS

The study was carried out at the Department of Floriculture and Landscape Architecture, Horticultural College and Research Institute, Tamil Nadu Agricultural University, Coimbatore during 2019- 2021. Ten-year-old plants of J. auriculatum cultivars CO.1 Mullai, CO.2 Mullai, Parimullai and J. grandiflorumcultivars CO.1 Pitchi and CO.2 Pitchi were selected as female parent and Jasminum genotypes like J. auriculatum cv CO.1 Mullai, J. grandiflorum cv CO.1 Pitchi, J. sambac, J. multiflorum, J. nitidum, J. calophyllum and J. flexile were used as the pollen source. The experimental layout was a complete randomized design with three replications of each crossing combination. The pollen from the previously bagged flowers was collected from the male parents during 6:00 to 8:00 am in the morning of the succeeding day. Similarly in the female parent, fully opened flowers and mature buds at pre anthesis stage were emasculated between 7.00 to 10.00 am. The pollen collected from the pollen source was dusted on the stigmatic surface of the respective emasculated female parent and the flowers were bagged with butter paper cover and then labelled. For the self-pollination treatment hundred flowering shoots were randomly selected on the plants and one third of the flowering bunches were bagged without any emasculation. The remaining flowering shoots (five per plant) were tagged and the flower bunches on each shoot were thinned to five buds (approx. 24 h before anthesis) for open pollination the flowers were left untreated without any bagging. The observations on days to fruit initiation, fruit set percentage, duration of fruit retention, shape of the fruit, colour of the fruit, fruit intensity and season of fruit set were recorded. Colour was assessed for each fruit using RHS colour chart. For the analysis of embryo viability, the longitudinally dissected fruits were treated with 2, 3, 5 triphenyl tetrazolium chloride and after 3 hours of incubation the stained embryos were examined for viability. Fruit set and fruit quality characters were evaluated by variance analysis using SPSS 28.0 software.

RESULTS AND DISCUSSION

Evaluation of the best possible cross combination with the varied method of pollination is the base factor that decides the success of a hybridization programme. The method of pollination plays a significant role in the fruit set of the plant and is influenced by various factors such as morphology of the flower, pollen-pistil interaction, nutrients and environmental parameters. As observed from the data (Table 1), among the possible cross combinations of pollen parents with completely opened flowers of J. auriculatum cultivars, the cultivar CO.1 Mullai as female parent recorded good response with male parent J. Flexile by recording the earliest fruit set (38 DAP), highest number of fruits set at 60 DAP (52), fruit set at maturity (31), highest fruit set (38.75%) and maximum fruit retention in the plant (30 days). Similarly, CO.2 Mullai produced best results as female parent when crossed with J. flexile with maximum fruit set (46.25%) but recorded delayed fruit set (47 days). The cultivar Parimullai as female parent also responded with J. flexile as pollen parent showing maximum fruit set of 48.75% and the duration of fruit retention clocked up to 33 days.

The results furnished (Table 1) signify the best cross combination for the bud pollination of J. auriculatum cultivars with various pollen parents. CO.1 and CO.2 Mullai as female parents exhibited best results with the combination of J. flexile as pollen parent while Parimullai responded well with the pollen parent J. grandiflorum CO.1 Pitchi. The results emphasize the fact that the pollen source and quantity influence the fruit set. Aggregated results from the Table 1 indicate that there is significant amount of fruit set failure from the fruit set at 60 DAP till maturity. The failure in fruit development or the malformation of fruits accounts to the low fertilization rate (Koubouris et al., 2010) or loss of pollen viability (Deng et al., 2017) or inadequate nutrient availability (Nyomora et al., 1999). Competition between fruits for assimilates and growth regulators are the factors that are responsible for different fruiting behaviour of the assessed cultivars.

In crosses involving J. grandiflorum cultivars as female parents, CO.1 Pitchi and CO.2 Pitchi evinced best results with J. flexile as the pollen donor. Maximum fruit set (80.10 and 83.75% respectively in CO.1 Pitchi and CO.2 Pitchi) with the earliest fruit set initiation of 22 days were recorded for the crosses effected with hand pollination of open flowers. For the bud pollination, CO.1 Pitchi had best compatibility with J. flexile while CO.2 Pitchi proved the best results with the combination that entailed J. multiflorum as the pollen parent (Table 2). The major drawback in the crosses involving J. grandiflorum as seed setting parent is the abnormal fruit set. The initiation of the fruit set is expressed by the bulging of the ovary proving the development of the fruit but as time progresses the ovary fails to develop completely causing misshapen fruits further arresting the growth of the embryo resulting in the loss of fruit set. Existence of pre-fertilization barriers like low pollen viability, early senescence of pistil cells and low pistil receptivity are the possible barriers in hybrid set (Deng et al., 2016). Early and rapid senescence of pistils is harmful for pollen adhesion and germination resulting in the arrest of pollen tube growth after it enters the stigma. Hybrid sterility can also be accounted due to the structural changes in the chromosomes (Sharma and Sharma, 1958).

None of the crosses involving J. sambac as male parent resulted in fruit set both in hand pollination and bud pollination implying that prevalence of pre- fertilization barriers hinders the fruit set. Low pollen fertility, pistil receptivity and pollen-stigma compatibility, ovule sterility (Deng et al., 2010; Sua´rez et al., 2012) have been enumerated as major reasons responsible for the hampered hybrid set. With respect to open pollination J. auriculatum cv Parimullai recorded maximum fruit set of 76.43% with the earliest fruit initiation of 32 days and retained the fruits up to 28 days (Table 3.) while J. grandiflorum cv CO.2 Pitchi proved best with the highest fruit set (83.40%), earliest initiation of fruit set (38 days) and longest duration of fruit retention(55 days) although malformation of the fruits occurred during their growth stage. The favourable fruit set in J. auriculatum may be attributed with as the absence of embryo antagonism (Veluswamy et al., 1981) and better source-sink relationship supporting the nutrient availability (Keshavarz et al., 2011). Failure in the fruit development and maturity can also be caused due to the abnormalities in the endosperm. Irregularities in the endosperm result in embryo starvation leading to distorted embryo sac (Veluswamy et al., 1981). Along with pre-fertilization barriers, obstructions post fertilization also poses a threat in hybridization.

Data in Table 3 are pertinent to self-pollination in J. auriculatum and J. grandiflorum. The cultivars CO.2 Mullai, CO.1 Mullai and Parimullai of J. auriculatum recorded fruit set rates of 20.86 %, 8.16 % and 2.14% respectively. Thus, the results revealed that J. grandiflorum exhibited better self-pollination efficiency in comparison with J. auriculatum but the fruit malformation in J. grandiflorum stands as a stumbling block the hybridization attempts involving this species.

Data furnished in Table 4 demonstrated that fruits evolved from crosses involving J. multiflorum and J. nitidum exhibited oblate shape while those from the crosses involving J. grandiflorum cv CO.1 Pitchi, J. calophyllum and J. flexile expressed spherical shape. Fruit intensity was profuse for most of the cross combinations in bud pollination when compared to hand pollination of the open flowers. Peak season of fruit set concurred with June to November under both the pollination methods. J. flexile and J. multiflorum as pollen parents responded well with Parimullai as female parent in terms of fruit growth (Table 5). Colour of the fruit varied from light green to yellow green and medium green and turns black on maturity. Fruits of J. auriculatum yielded from open pollination performed better in terms of growth as well as the intensity of the fruit set while self-pollinated fruitsproved better in terms of fruit growth with bolder fruits though the fruit set was poor. The efficiency of the fruit set depends upon the flowers that have pollen- laden anthers that appear to set fruit far better when cross- pollinated than when fertilized with their own pollen (Ortega et al., 2006). Despite the lack of complete fruit development in J. grandiflorum, peak season of the fruit set was observed during February to April. The fruits were conical in shape and yellow- green in colour (Table 6). In terms of method of pollination, open pollination contributed the most for the successful fruit set followed by bud pollination (Fig 1.). Results pertaining to fruit growth and quality

parameters (Fig 2.) revealed that bud pollination followed by hand pollination of open flowers ensured significantly superior fruit set.

Among all the possible cross combinations J. flexile corresponded well with all the cultivars of J. auriculatum and can be considered as the best pollen donor parent for the successful hybridization of the crop. J. auriculatum cv. Parimullai provided best results among all the cultivars in terms of fruit set and intensity , thus proving to be an elite female parent amongst the cross combinations. This study indicates the failure of fruit set and fertilization barrier prevailing in jasmine upon hybridization. Understanding the type of the barriers prevailing in jasmine facilitates the integration of conventional approaches with biotechnological tools to overcome the complications and obtain interspecific hybrids with desirable traits.

Acknowledgments

The financial support extended by DUS testing scheme on jasmine funded by PPV&FRA, Govt. of India, New Delhi to carry out the research is obliged and also the author expresses gratitude to the staff of the Department of Floriculture and Landscape Architecture and Tamil Nadu Agricultural University for their immense support to implement this research work.

REFERENCES

Arnold, M. L. 1992. Natural hybridization as an evolutionary process. Annual Review of Ecology and Systematics, 24: 521-524.

Arnold, M. L. 1997. Natural hybridization and evolution. Oxford University Press, New York, New York, USA.

Bailey, L.H. 1958. Manual of cultivated plants. Pub:Macmillan and Co., New York.

Bartolini, S., Viti, R. and Guerriero, R. 2000, September. Observations on the fertilization process in self-pollinated flowers of cultivar “Leccino”. In IV International Symposium on Olive Growing. 586: 521-524.

Barton, N. H., and G. M. Hewitt. 1985. Analysis of hybrid zones. Annual Review of Ecology and Systematics, 16: 113-148.

Burke, J. M., S. E. Carney, and M. L. Arnold. 1998. Hybrid fitness in the Louisiana irises: analysis of parental and Fi performance. Evolution. 52: 37-43.

Deng, Y. M., Teng, N. J., Chen, S. M., Chen, F. D., Guan, Z. Y., Song, A. P. 2010. Reproductive barriers in the intergeneric hybridization between Chrysanthemum grandiflorum (Ramat.) Kitam and Ajania przewalskii Poljak. (Asteraceae). Euphytica.174: 41–50.

Deng, Y., Jia, X., Liang, L., Gu, C. and Sun, X., 2016. Morphological anatomy, sporogenesis and gametogenesis in flowering process of jasmine (Jasminum sambac Aiton). Scientia Horticulturae, 198: 257-266.Usha et al

Deng, Y., Sun, X., Gu, C., Jia, X., Liang, L. and Su,J. 2017. Identification of pre-fertilization reproductive barriers and the underlying cytological mechanism in crosses among three petal-types of Jasminum sambac and their relevance to phylogenetic relationships. PLoS One, 12(4), p.e0176026.

Ganga, M., Madhumalar, P., Jawaharlal, M. and Ranchana, P. 2015. Assessment of the horticultural significance of lesser-known jasmine (Jasminum spp.) wealth. Journal of Ornamental Horticulture, 18 (1 and 2): 17-24.

Grant, V. 1981.Plant speciation. Columbia University Press, New York, New York, USA

Green, P. and D. Miller. 2009. The genus Jasminum in cultivation. Kew Publishing, Royal Botanic Gardens, Kew.

Ghosh, S. 2018. Assessment of mutagenic sensitivity in jasmine (Jasminum spp.) to chemical mutagen. Electronic Journal of Plant Breeding, 9(3):1002-1011.

Jawaharlal, M., Thamaraiselvi, S.P. and Ganga, M. 2012.Packaging technology for export of jasmine (Jasminum sambac Ait.) flowers. Journal of Horticultural Sciences, 7(2): pp.180-189.

Johnston, J.A., Donovan, L.A. and M.L Arnold. 2004. Novel phenotypes among early generation hybrids of two Louisiana iris species: flooding experiments. Journal of Ecology92: 967-97

Keshavarz, K., Vahdati Samar, M., Azadegan, B. and Brown, P.H. 2011. Foliar application of zinc and boron improves walnut vegetative and reproductive growth. Hort Technology, 21(2):181-186.

Koubouris, G. C., I. T. Metzidakis, and M. D. Vasilakakis. 2010. Influence of cross - pollination on the development of parthenocarpic olive (Olea europaea) fruits (shotberries). Experimental Agriculture, 46(1): 67–76,

Nirmalaruban, R., Ganesan, N.M. and Anantharaju, P., 2020.Breeding system study in sunnhemp (Crotalaria juncea L.)A stepping stone in crop improvement. Electronic Journal of Plant Breeding, 11(1):204-209.

Nyomora, A.M., Brown, P.H. and Krueger, B. 1999. Rate and time of boron application increase almond productivity and tissue boron concentration. Hort. Science, 34(2):242-245.

Ortega, E., P. J. Martinez-Garcia and F. Dicenta. 2006. Pollination control in olive orchards. Olivea. Hort. Abst. 72:2231

Raman, K. R., A. Shanmugam and H. Ahmad Shah. 1969. Studies on the flowering habits and flower yields of some Jasminum species. South Indian Hort, 17:18-27

Ruiz, D., Campoy, J.A. and Caballero, J.E. 2010.Ovule development at anthesis in Japanese plum (Prunus salicina Lindl.) cultivars. Spanish journal of agricultural research, (1):151-158.

Sharma, A. K. and A. Sharma. 1958. Analysis of chromosome morphology and possible means of speciation in Jasminum. Cytologia, 23:172-185

Sua´rez C, Castro A. J, Rapoport H. F, Rodrý´guez– Garcý´a M. I. 2012. Morphological, histological and ultrastructural changes in the olive pistil during flowering. Sex Plant Reprod 25: 133–146.