Induction and regulation of flowering in rice bean [Vigna umbellata (Thunb.)]

Abstract

The present study entitled ―Induction and regulation of flowering in rice bean [Vigna umbellata (Thunb.)]‖ was carried out in the Department of Genetics and Plant Breeding, College of Agriculture, Vellayani during 2022-24. This study aimed to regulate flowering in rice bean through hormonal application and analyze the upregulation of flowering-related genes. The research programme comprised of two experiments. The first experiment was hormonal application to regulate flowering and second experiment included flowering gene expression analysis. The experiment was laid out in randomized block design with ten treatments and three replications. The plot size of 3m2 was followed in the field experiment. The experimental materials for the present study were fodder rice bean varietes Surabhi from KAU and Bidhan-2 from BCKV, Kalyani. The treatments comprised of control, seed inoculation with Piriformospora indica and different doses of foliar sprays (Salicylic acid at 100 and 150 ppm, Gibberellic acid at 200 and 300 ppm, Paclobutrazol at 10 and 20 ppm and KNO3 at 1 and 1.5%) at 30 days after sowing (DAS). The observations on quantitative characters and qualitative characters showed significant differences among treatments, except for field emergence in variety Surabhi. Early flowering was observed in Surabhi treated with SA at 100 ppm (65.36 DAS), followed by GA₃ at 300 ppm (67.72 DAS), SA at 150 ppm (67.81 DAS), and Paclobutrazol at 10 ppm (69.87 DAS), while control flowered last (80.96 DAS). No flowering was observed in Bidhan-2. Seed inoculation with P.indica improved fodder qualities like LAI, number of branches, plant height, number of pods per plant and green fodder yield per plot in rice bean variety Surabhi. The highest seed yield was also recorded in plants treated with P.indica (81.88 g) and lowest in plants treated with Salicylic acid at 150 ppm (47.88 g). A significant positive correlation was exhibited by days to first flowering with days to 50% flowering (0.992) and days to maturity (0.979), whereas number of branches 95 (-0.537) and number of seeds per plant (-0.41) exhibited significant negative correlation. A significant positive correlation was recorded by green fodder yield per plot with LAI (0.997), dry fodder yield per plot (0.962), plant height (0.949), number of branches (0.746) and field emergence (0.335). Path analysis revealed that days to maturity exhibited highest direct effect on days to first flowering (0.912). Rest of the characters showed a negligible direct effect on days to first flowering. The highest positive indirect effect on days to first flowering was observed by days to maturity through days to 50% flowering (0.905). Principal component analysis showed 85.88% contribution of first 4 PC groups towards the total variance and the squared cosine diagram showed that days to first flowering exhibited maximum value in PC1 which accounted for 34.22% of total variance. The RNA isolated from the flower buds of control and the treatment of which showed earliest flowering (SA at 100 ppm) were converted to complementary DNA (cDNA). The expression levels of genes ELF3 and FT in the selected treatments were studied using the quantitative real time PCR. The relative expression of control and treated plants demonstrated the varying degree of flowering gene expression. The relative expression profile using ELF3 gene showed 1.42 fold increase in treated plants (SA at 100 ppm) as compared to control indicating the upregulation of flowering gene ELF3 in treated plants. In conclusion, foliar application of Salicylic acid at 100 ppm was found to be the best treatment in rice bean variety Surabhi for inducing early flowering. Treated plants flowered 15 days before control. Plants treated with GA3 at 300 ppm, Paclobutrazol at 10 ppm and KNO3 at 1% flowered 13, 11 and 3 days respectively before control. Seed inoculation with P.indica resulted in flowering 7.5 days before control in Surabhi. Seed inoculation with P.indica resulted in improvement of fodder qualities like LAI, number of branches, plant height, number of pods per plant and green fodder yield per plot in both the rice bean varieties (Surabhi and Bidhan-2). Seed yield was also highest (Surabhi) in plants treated with P.indica. No flowering was observed in variety Bidhan-2. The 96 differential expression analysis using RT-qPCR revealed an upregulation of ELF3 genes in treated plants which correlated with an accelerated flowering response. Future studies could explore the different range of hormonal concentrations to determine the optimal dose for inducing early flowering in rice bean, as well as any potential toxic effects at higher doses.