Introduction
Rice is mostly cultivated annually from seed, but this is not always the ideal option for all agricultural circumstances. Jhum, or shifting agriculture, is a method where farmers remove mountainous forested areas by burning the plants, then plant rice or other crops. This practice has a significant negative impact on soil erosion and the quick loss of soil fertility. Production of perennial rice crops could offer a practical solution to addressing these issues. Several wild species of rice, including O. longistaminata, O. rhizomatis and O. rufipogon, exhibit perennial features including rhizomes and stolons which have the ability to regenerate. In rice, perennation frequently takes the form of the plant's ability to ratoon (regrow). Ratoon rice, according to Chen et al. (Reference Chen, He, Wang, Peng, Huang, Cui and Nie2018), is produced by regrowing rice plants from the main crop plants' stubble after the main crop has been harvested. According to He et al. (Reference He, Jiang, Nie, Man and Peng2023), this regrowth may come from the tiller buds at the base of the stubble or from branching buds in the higher nodes of the stubble. Ratoon rice yields about 40–50% of the yield of the main (original) crop while requiring between 50 and 70% less labour and water (Chauhan et al., Reference Chauhan, Lopez and Vergara1989; Munda et al., Reference Munda, Das and Patel2009). According to Wang et al. (Reference Wang, He, Jiang, Sun, Jiang, Man and Nie2020), rice ratooning is a resource- and environmentally friendly method of producing rice. By eliminating the actions of tilling, sowing and transplanting in the second crop season, ratooning ability in rice produces increased rice production with extremely few agricultural inputs (Shen et al., Reference Shen, Zhang and Zhang2021). Rice ratooning for large-scale commercial farming has not been accepted in Asia, perhaps due to a lack of cultivars with strong ratooning potential and management practices. This study is thus aimed to evaluate diversity and screen some Assam rice genotypes with respect to superior ratooning ability.
Experimental
The field research was done at ICR farm of AAU, Jorhat (Assam) from February 2020 to February 2022 which included both main (original) and ratoon crop. Fifty commonly grown Assam rice genotypes were collected from RARS Diphu, RARS Lakhimpur, RARS Karimganj and IARI. These genotypes belonged to distinct classes based on growing season (Ahu, Sali, Boro) and photoperiod sensitivity (sensitive and insensitive) to achieve maximum diversity. Genotypes were planted in a randomized complete block design with three replications following standard practices. Row to row and plant to plant spacing were maintained as 20 × 20 and 10 × 10 cm, respectively, with plot size of 2.4 m × 0.2 m. Harvesting was done when the crop achieved 95% maturity. The plants were cut 15 cm above the ground in accordance with procedures of Santos et al. (Reference Santos, Fageria and Prabhu2003). One day after harvesting, recommended additional amounts of fertilizers (Petroudi et al., Reference Petroudi, Noormohammadi, Mirhadi, Madani and Mobasser2011) were applied to enhance the ratoon growth. In the ratoon crop, five plants from each treatment were randomly chosen for evaluation of 11 morphological traits (Chakrawarti et al., Reference Chakrawarti, Borgohain and Verma2022). To find out diversity among evaluated genotypes, D 2 analysis was done in R studio version 2.0. The genotypes were grouped into cluster according to Tocher's method (Rao, Reference Rao1952). The square root of average D 2 value was estimated to calculate the average intra and inter-cluster D values. The cluster mean for a character was calculated as the sum of mean values of genotypes included in a cluster divided by number of genotypes in that cluster.
Out of all 50 genotypes, 30 were able to produce ratoon yield. These 30 genotypes were assessed for nature and magnitude of genetic divergence based on 11 traits following Mahanolobis D 2 statistics. From the Wilks test table, the varieties under D 2 matrix were found to be highly significant. Cluster analysis based on 11 traits grouped the 30 genotypes (those which recorded ratoon yield) into 10 clusters. Cluster I was the largest cluster with 15 genotypes while clusters VII, VIII, IX and X were monogenotype clusters (online Supplementary Table S1). Cluster VI showed maximum intra-cluster distance (Table 1). The intercluster D 2 value was found maximum between clusters III and X followed by clusters III and IV (Table 1). Mean performance of clusters for the traits is shown in Table 2. Genotypes under cluster III were superior for many ratoon yield attributing traits. Thus, cluster III (Binadhan-11 and Sayjihari) was the best performing cluster for all ratoon yield contributing traits. This cluster was also different from the other clusters in terms of number of days required for ratoon crop maturity which was highest for this cluster while genotypes of cluster VII (Lachit) were earliest to mature. Contribution of various ratoon crop traits towards total divergence revealed that maximum contribution was made by number of basal tillers followed by ratoon plant height and days to ratoon emergence. Iso (Reference Iso1954) proposed that tillers originated from upper nodes have high C:N ratio which reacted like old seedlings whereas tillers originated from the base have low C:N ratio which have the characteristics of young seedlings due to which their productive performance was better. Therefore, development of greater number of lower tiller should produce more yield in ratoon crop.
Table 1. Intra(bold) and intercluster distances
Table 2. Mean performance of clusters
NRT, number of ratoon tillers; NPRT, number of productive ratoon tillers; NLT, number of lodging tillers; NDT, number of dwarf tillers; RPH, ratoon plant height (cm); DRM, days to ratoon maturity; RSF, ratoon spikelet fertility (%); RYPT, ratoon yield per tiller (g); RYPP, ratoon yield per plant (g); DRE, days to ratoon emergence; NNT, number of nodal tillers; NBT, number of basal tillers.
Discussion
The investigation has revealed the presence of diversity in Assam rice genotypes for ratooning ability. From the cluster analysis, it was clear that clusters II and X had maximum intercluster distance. The greater distance between the two clusters indicated wider genetic diversity between genotypes. Thus, the cross-combination between the varieties falling under these clusters may be used in future breeding programmes for utilizing hybrid vigour. The highest intracluster distance in cluster VI indicated that a good cross-combination may be obtained by hybridization between genotypes within the same cluster. Genotypes under clusters III and VI had the most desirable traits for ratoon cropping like number of ratoon tillers and productive ratoon tillers, number of basal ratoon tillers and ratoon grain yield. Traits like the number of basal ratoon tillers with maximum contribution may be prioritized in a hybridization programme to develop genotypes with high ratooning ability.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S1479262124000121
Acknowledgement
The authors are thankful to Assam agricultural university for providing funds and area to conduct the research.
