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Genetic Variations of Root Development Traits Under Different Concentrations and Forms of Nitrogen in Bangladeshi Rice (oryza Sativa L.) Accessions

M. M. Emam Ahmed
Biological Resources and Post-harvest Division, Japan International Research Center for Agricultural Sciences, Tsukuba, Japan & Plant Breeding Division, Bangladesh Rice Research Institute, Gazipur, Bangladesh

M. Ashik Iqbal Khan
Plant Pathology Division, Bangladesh Rice Research Institute, Gazipur, Bangladesh

Md. Shahjahan Kabir
Director General, Bangladesh Rice Research Institute, Gazipur, Bangladesh;

Yoshimichi Fukuta
Tropical Agriculture Research Front, Japan International Research Center for Agricultural Sciences, Ishigaki, Japan

Mitsuhiro Obara
Biological Resources and Post-harvest Division, Japan International Research Center for Agricultural Sciences, Tsukuba, Japan

Abstract/ Executive Summary:

Genetic variations in the root traits of rice remain unexplored in relation to varietal groups and diverse ecosystems. Therefore, we evaluated 257 Bangladeshi accessions, which included landraces and improved varieties belonging to four ecotypes (Aus, Aman, Boro and Jhum), to understand genetic variations in the total root length (TRL), maximum root length and root number under different concentrations of ammonium and nitrate nitrogen in hydroponic culture. There were large variations in all traits across all nitrogen levels. Active elongation of individual roots of the landrace type across all nitrogen conditions suggested a higher potential for the uptake of nutrients by extending roots distribution in any nitrogen environment, while the active development of primary root primordia in the improved type in the presence of nitrate suggested a higher potential for the uptake of nitrate from the soil surface by extending roots distribution around soil surface. Cluster analysis classified the examined accessions into two groups according to their TRL: cluster I with a longer TRL and cluster II with a shorter TRL across all nitrogen conditions. Each of these groups was further divided into two sub-clusters: sub-cluster Ia, in which the longer TRL resulted from active development of primary root primordia, sub-cluster Ib, in which the longer TRL resulted from active elongation of individual roots, sub-cluster IIa, in which the shorter TRL resulted from severe reduction of the elongation of individual roots, and sub-cluster IIb, in which the shorter TRL resulted from a moderate reduction in the elongation of individual roots. Further investigation revealed a difference in the composition of these four sub-clusters among the four ecotypes, indicating that Aman was the most diverse ecotype, followed by Aus. The root traits of these promising accessions could be exploited in the future to understand genetic and physiological mechanism(s) of adaptation to diverse ecosystems.

Keywords: Accession, Diversity Ecotype, Development, Varietal group

Location: Rice Gene Bank of the Bangladesh Rice Research Institute (BRRI), Gazipur, Bangladesh

Start Date:

End Date:

Program Area: Variety and Species

Commodity: Rice

Objectives:

To discuss possible relationships how these genetic variations are related to the selection environments and existing cultivation zones in Bangladesh.

Methodology:

A total of 257 Bangladeshi rice accessions and two controls (IR 64 and Kasalath) were used in this study to investigate the levels of genetic variation in root development traits under hydroponic culture. The rice accessions were provided by the Rice Gene Bank of the Bangladesh Rice Research Institute (BRRI), Gazipur, Bangladesh (Khan et al. 2017), and were categorized based on available information about the varietal groups (landrace and improved types) and ecotypes (Aus, Aman, Boro and Jhum) in Bangladesh. The varieties IR 64 and Kasalath were used as controls for hydroponic culture In this study, 8-day-old rice seedlings were used to measure three root development traits with three spatial replications: total root length (TRL), maximum root length (MRL) and root number (RN). Well-filled seeds were germinated at 30? for 2 days following two-step seed stabilization, which involved soaking the seeds for 10 minutes in water at 60? and then for 20 minutes in a 2% (v/v) sodium hydrochloride solution, as described previously (Obara et al. 2014). The germinated seeds were then sown on a nylon net (160 × 220 mm) (three seeds per accession; eight nets for each nitrogen application). After sowing, a set with the four nylon nets with germinated seeds were kept afloat in a plastic container (50 × 37 × 27 cm) containing 40 L of distilled water mixed with basal nutrient solution. The same basal nutrient solution was used as described previously (Obara et al. 2014), and 5 mM 2-(N-morpholino) ethanesulphonic acid was added as a buffer to maintain the pH of the nutrient solution at approximately 5.5 to 5.8 during the experimental period. The seedlings were raised under natural light conditions in a greenhouse located at Tsukuba, Ibaraki, Japan (36°05′N, 140°09′E, 25 m above sea level), that was maintained at approximately 28?. To characterize the genetic variations in root development traits under different form and concentration of nitrogen that was available (ammonium or nitrate), plants were grown under four different conditions: in the presence of NH4Cl or KNO3 at concentrations of 5 μM (nitrogen deficient) or 500 μM (nitrogen sufficient, representing the soil nitrogen concentration in paddy fields). The hydroponic medium was renewed at 4 and 6 days after seed sowing. Evaluation of the root development traits-The root development traits TRL, MRL, and RN were evaluated to investigate genetic variation in the 257 Bangladeshi rice accessions. First, the longest root length of each 8-day-old seedling was measured using a ruler and defined as MRL. The seedlings were then placed in a plastic bag and stored at −30? until measurement of the individual root length and number of roots. Following storage, the seedlings were placed outside for 30 min to bring them up to air temperature, following which the roots were separated from the seedlings by cutting the seedlings at the base. The primary roots length of each seedling was then measured to calculate TRL and the number of primary roots was counted to obtain RN. To estimate contribution of dissected traits MRL and RN in TRL, the measured value for MRL was used as an indicator for root elongation, while the value for RN was used as an indicator for development of primary root primordia in the whole manuscript. Broad-sense heritability (hb2) was estimated from one-way analysis of variance (ANOVA) with the following formula: hb2 = σ2g/?(σ2e/r)+ σ2g?, where σ2g is the genetic variance, σ2e is the environmental variance, and r is the number of data employed (Kobayashi and Koyama 2002). Cluster analysis- To classify the 257 rice accessions and two control varieties based on their root development traits under different form and concentration of nitrogen that was available, cluster analysis was performed using Ward’s hierarchical clustering method (Ward and Joe 1963) with the program JMP 11.2.0 for Windows (SAS Institute, Inc., Cary, NC, USA). Differences in the mean value between the two groups were statistically analyzed using the unpaired t-test (two-tailed), and differences among multiple groups were compared using the Tukey–Kramer test (probability<0.01).

Source of Information: SOIL SCIENCE AND PLANT NUTRITION, 2021, VOL. 67, NO. 1, 31–40

Article Link (Online Journal): https://doi.org/10.1080/00380768.2020.1843370

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The aim of this study was to reveal the levels of genetic variation in hydroponically grown root development traits under different nitrogen conditions in the landrace and improved type rice varieties of Bangladesh, as well as in the four rice ecotypes Aus, Aman, Boro, and Jhum. We observed large variations in all nitrogen conditions, allowing them to be classified into four sub-cluster groups (Ia, Ib, IIa, and IIb) according to TRL, MRL, and RN. The compositions of these four sub-clusters differed among the four ecotypes, indicating a possible relationship with cultivation zone. Aus was mainly grouped into sub-clusters Ib and IIb, Boro and Jhum were mainly placed in sub-cluster IIb, and Aman was distributed across sub-clusters Ia, Ib, and IIb in almost equal proportions, indicating that Aman accessions have conserved the most variation, followed by Aus. Further research using specific rice accessions from the speculated cultivation zones would confirm the nature of the root traits for each ecotype. In addition, a large number of landraces from these ecotypes were characterized by the presence of longer TRLs resulting from active development of primary root primordia and active elongation of the individual roots, allowing nomination of landraces as donor parent which were adapted to native conditions in breeding program of Bangladesh. Therefore, further physiological and genetic studies on relationship between rice ecotypes and cultivation zones could improve our understanding of genetic and physiological mechanism(s) of adaptation to diverse ecosystems.

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