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Development of interspecific hybrids between urdbean & mungbean

Interspecific hybridization was performed between five genotypes of urdbean & three genotypes of mungbean to check the crossability relationship. Thirteen cross combinations of urdbean x mungbean were successfully developed. Interspecific seeds showed no germination under soil conditions so various growth media were used for germination. Interspecific seeds germinated only on the salt solution. F 1 seeds of cross UG-218 x Suketi exhibited highest crossability and showed maximum response on salt solution. The study put emphasis on the different kinds of fertilization barriers. In future genetic improvement studies can be carried out with the genotypes showing substantially high percent of crossability.


Introduction
Protein is an important source of individual diet which is mostly taken form milk, eggs and pulses. The fourth most important food legume in India is Urdbean [Vigna mungo (L.) Hepper] also known as blackgram, black lentil, mash, mungo bean belongs to family Leguminoseae and subfamily Papilionaceae, domesticated from V. mungo var. silverstris (Bhareti et al., 2011). Urdbean is short duration, self-pollinated crop and found in most of parts of India. India is center of genetic diversity for urdbean with its secondary center of origin in Central Asia (Singh et al., 2016). Urdbean is valuable source of protein, minerals and amino acids like lysine and methionine, as well as vitamins like niacin, riboflavin and thiamine, as well as phosphorus and iron (Gill et al., 2017). Plant parts are used as fodder for animals and green manuring. India is the world's greatest producer and consumer of urdbean, producing 2.93 million tonnes per year from 4.49 million hectares of land, with an average productivity of 500 kg per hectare (Anonymous, 2019). In Himachal Pradesh, it is mainly cultivated on low and mid hills & grown as intercrop with maize as well as a monocrop. However, yield potential of urdbean is low as compared to other grain legumes. Narrow genetic base of urdbean often results in low yield and productivity due to poor plant type, cultivation in marginal and harsh environment, common ancestry of various superior genotypes and its vulnerability to abiotic and biotic stresses viz., Cercospora leaf spots (Cercospora canescens, C. cruenta), anthracnose (Colletotrichum truncatum), powdery mildew (Erysiphe polygoni), and Mung Bean Mosaic Viruses (Ali et al., 2006). The related species V. radiata (mungbean) has been found to be nutritive, easily digestible & early maturity as compared to urdbean. As urdbean is self-pollinated crop to get its better understanding of crossability relationship among the species is helpful in chosing methods for making successful crosses. Interspecific seeds developed shows no germination under normal soil condition due to which different media composition were attempted in the present investigation to check the efficacy of growth media's on germination of interspecific seeds and for studying the crossability of V. mungo with V. radiata.

Material and Methods
For the study, a total eight different genotypes i.e. five of blackgram (HPBU-111, Him Mash-1, Palampur-93, UG-218 & PDU-1) taken as female and three of mungbean (Suketi, SML-668 & ML-818) taken as male were used to study the crossability relationship & germinability of their hybrids (Table 1). media. Petri plates with sterile F 1 seeds were placed in incubator at 25±1ºC for four to five days and salt solution was changed every day under sterile conditions. On second transfer on fresh salt solution seed coat of imbibed F 1 seeds were removed and allowed to develop on salt solution for one or two days. Four to five days old seeds showing radicle formation/seedling were transferred to paper cups having mixture of sand + cocopeat + vermicompost which were then transferred to field and glasshouse after the development of cotyledonary leaves ( Figure 1). Data were recorded with respect to:

Simple t-test
Simple t-test was performed to test the mean difference of radicle and hybrid plant production in the study.

Results and Discussion
Interspecific hybridization is a promising tool to transfer the desirable traits and to widen the gene pool of any crop. However, wide crosses are not always successful because of the existence of pre and post fertilization barriers that are operative at various stages of development and also various incompatibility barriers limit the potential for recombining the important characters for improving production and adaptation. The present study was taken with the objective to study the crossability relationship between urdbean & mungbean and to study the efficacy of different growth media on germination of interspecific seeds. There is high incidence of abscission of crossed flowers within 72 hours from pollination & young fruits dropping between 3 to 30 days after pollination implying the presence of fertilization barriers. Some of pods harvested had no seed or had very minute seeds.  Lekhi et al., (2017) in V. mungo x V. radiata with 5.50 to 24.10 %. The percent crossability among different sets of crosses varies from species to species due to substantial heterogeneity in the genetic architecture of species involved in interspecific hybridization, resulting in differences in cross compatibility. Some of the pods which were formed were without seed or had shriveled seeds with ruptured seed coat. In the crosses which had HPBU-111 as one of the parent had large number of empty pods. F 1 seeds developed were of two types, viz. highly shriveled, minute, brown coloured and the second was bold and comparatively brown coloured but very weak as compared to self-ones. The number of seeds per pod in the interspecific hybrids varied from 1-4. The results are in agreement with the earlier studies of Sehrawat et al. (2016a) for number of F 1 seeds per pod in interspecific crosses of urdbean and ricebean. The F 1 seeds obtained from all cross combinations were small, wrinkled and shrunken ( Figure 2). The F 1 seeds were small in size and shriveled because of the poor development of the endosperm and embryo which is due to incompatibility between the two parental genomes or due to the failure of embryo to reach maturity (Rashid et al., 1987). Even though crossability barriers were present, few interspecific hybrids were produced. The seeds obtained from the interspecific crosses under study shows no germination under normal soil conditions so attempts were made to grow them on various media. To study the response of different growth media on germination of interspecific seeds, 30 seeds comprising of all the cross combinations of urdbean & mungbean were put on different growth media i.e. Salt solution, Gamborg B5 media and    Table 4). The results are in agreement with the findings of Bindra et al., (2020), they reported germination percentage upto 59.34 % in V. mungo x V. umbellata hybridization whereas, Basavaraja et al., (2018) found germination percentage of 36.84 % in interspecific crosses between V. radiata & V. umbellata, Lekhi et al., (2017) noted germination percentage upto 30.56 % in interspecific crosses of urdbean and mungbean. Some of the F 1 seeds did not imbibe, some showed distorted cotyledons, poor root development whereas in some cases roots developed but died before shoot formation so success rate in germination was low. Mittal et al., (2005) reported similar results in interspecific crosses of urdbean & ricebean. The parents involved in interspecific hybridization showed differential genotypic response which indicates the use of more number of genotypes and large number of crosses should be attempted to get more F 1 plants. Differential genotypic response of parents involved in interspecific hybridization also reported by Mittal et al. (2005).

Conclusion
The study put emphasis on the different kinds of fertilization barriers that are responsible for complete sterility to low fertility. Even though the fertilization barriers were predominant, some interspecific hybrids were produced. Salt solution was most efficient growth media for germinability. The parents involved in interspecific hybridization showed differential genotypic response which indicates the use of more number of genotypes and large number of crosses should be attempted to get more F 1 plants. In future genetic improvement studies can be carried out with the genotypes showing substantially high percent of crossability.