In regions characterized by high rainfall, iron toxicity presents a significant challenge to rice cultivation, impacting crop health and productivity. This study conducted in College of Agriculture, Central Agricultural University, Imphal, focuses on assessing the iron toxicity tolerance of 17 rice genotypes, aiming to identify cultivars suitable for cultivation under iron toxicity. Five levels of iron (0, 300, 600, 900 and 1200 mg/L Fe2+) were tested through controlled experimentation, simulating conditions typical of iron toxicity, the genotypes were evaluated for their response to varying levels of Fe2+. Results reveal substantial genotype-specific variations in iron toxicity tolerance, with certain cultivars exhibiting resilience to elevated iron levels while others demonstrate increased susceptibility.

Fe2+ toxicity, Rice genotypes, tolerance.

In the eastern Himalayan region, where rice cultivation is integral to agricultural practices and livelihoods, understanding the nuances of rice plant responses to various environmental factors is crucial for ensuring sustainable crop production and food security (Baishya et al., 2015). Among these factors, the influence of iron (Fe2+) levels on rice growth and development has garnered significant attention due to its potential impact on yield and crop health (Butsai et al., 2022). Iron, an essential micronutrient for plants, plays a vital role in various physiological processes, including chlorophyll synthesis, photosynthesis, and enzyme activities (Rout and Sahoo 2015). In the context of Eastern Himalayan region, which is characterized by high rainfall (Deka et al., 2016) the soil is replaced by hydrogen ions (H+). Soil acidity increases with the buildup of H+ and Al3+ or with the leaching out of bases cations such as potassium (K+), calcium (Ca2+) etc., and replaced by H+ (Agegnehu et al., 2021). This excessive levels of iron in the soil can lead to toxicity, adversely affecting plant growth, nutrient uptake, and ultimately, yield (Zahra et al., 2021). Therefore, recognising available genotypes within the region, tolerant to Fe toxicity, pose as one of the cost-effective managements for problem soil.

The study was conducted in Department of Genetics and Plant Breeding, College of Agriculture, Central Agriculture University, Imphal during 2021, to screen the genotype response towards iron toxicity. The experiment was conducted in Randomized Block Design (RBD), with 17 rice genotypes (Table 1) subjected to 5 levels of Fe2+ (0, 300, 600, 900 and 1200 mg/L Fe2+) and replicated thrice. Ferrous Sulphate heptahydrate (FeSO4.7H2O) was used as a source of Fe2+. The experiment was conducted in a hydroponic system and the growth chamber was set at 26°C with16/8 light/dark hour duration. Yoshida solution was prepared using the standard procedure as described by Yoshida et al. (1976) and was supplemented to rice along with different levels of iron on every alternate day. Data on germination percentage was recorded on the 7th Day after sowing (DAS), whereas shoot length, shoot dry weight, root length, root dry weight was recorded after 14 DAS. Standard procedures were followed for recording of data and visual scoring of iron-toxicity symptoms was done in accordance with Standard Evaluation System (SES) of rice (IRRI, 1996).

Table 1: List of rice genotypes used for the experiment.

A. Root count

Mean value of root count of all the genotypes has been presented in Fig. 1. The highest mean value for root count under control was found in Local basmati (15.33) while the least root count was found in Geturaj and Lahi emmo (7.33). Under 300mg/L Fe2+, Local basmati (10.33) was least affected while Geturaj (4.00) was much affected. Under 600 mg/L Fe2+, Lal dhan and Local basmati (7.00) was least affected while much effect of iron stress was seen in Geturaj (3.33). Under 900mg/L Fe2+ stress level of iron, Local basmati (4.66) was least affected while Geturaj (1.33) was much affected. And in the last treatment, 1200mg/L Fe2+, Local basmati and Pikhi (2.33) was least affected while Lailo and Twisa (0.66) was much affected by the stress level for the concerned genotype.

B. Germination percentage

Data on germination percentage is presented in Table 3. Among the genotypes, Pasighat was not affected by the iron stress exhibiting cent percent germination at each level of stress, while Lailo has shown linear reduction along the stress levels, indicating its sensitivity to iron stress. This genotype also registered lowest germination percentage (66.66%) among the genotypes. This was followed by Twisa which also registered lower germination percentage at extreme iron stress while showing linear reduction along the stress. Deku, Gaksum, Kala Joha, Lahi emmo and Pasighat on the other hand have exhibited lower germination at higher stress only. While upto 900mg/L Fe2+ there was no to little reduction in germination.

C. Shoot length

Data on shoot length as given in Table 3 shows that, in control, the highest root length was observed in Local Basmati (22.68 cm), while the lowest shoot length was observed in Lailo (14.35 cm). In stress of 300mg/L Fe2+, least affected genotype was Local Basmati (16.48 cm) while the most affected genotype was Lailo (10.35 cm). In stress of 600mg/L Fe2+, the most affected genotype was Lailo (7.18 cm) while the least affected genotype was Kala Joha (12.14 cm). In stress of 900mg/L Fe2+, most affected genotype was Twisa (5.54 cm) while the least affected genotype was Kala joha (10.61 cm). In stress of 1200mg/L Fe2+, the most affected genotypes were Lailo and Twisa (2.81cm) while the least affected genotype was Gaksum (5.76 cm).

D. Shoot dry weight

Mean value of shoot dry weight is given in Table 3. Highest shoot dry weight under control was observed in Pasighat (0.804g) and the least was observed in Chasa upland (0.39g). In stress condition of 300mg/L Fe2+, the least reduction in shoot weight was found in Damda Aamo (0.69 g) while the most reduced under the same stress level was found in Pikhi (0.26g). The pattern of least reduced was observed in Pasighat under stress level of 600mg/L Fe2+ (0.43g), as well as in 900mg/L Fe2+ (0.33g) and 1200mg/L Fe2+ (0.017g). While for the most reduced shoot fresh weight was observed in Pikhi, and Chasa upland at 600mg/L Fe2+ (0.020g); chasa upland, Lailo and Twisaat  and  900mg/L Fe2+ (0.11g), while in 1200mg/L Fe2+ most reduction in shoot fresh weight was observed in Twisa (0.005g).

E. Root length

Data on root length as presented in Table 4 suggests that the highest root length in control treatment was that of Local basmati (14.71 cm) while the shortest root length was observed in Chasa (upland) (7.43 cm). In stress of 300 mg/L Fe2+, the least affected genotype was Gaksum (11.38 cm) while the most affected genotype was Chasa (upland) (5.28 cm). In stress condition of 600mg/L Fe2+, the least affected genotype was Gaksum (7.48 cm) while the most affected genotype was Lailo (3.63 cm). In stress condition of 900mg/L Fe2+, Gaksum was least affected (5.53 cm) while Lailo was the most affected genotype (2.00 cm). In stress condition of 1200mg/L Fe2+, Gaksum showed more tolerance to stress (3.61 cm) while the most affected genotype was Simoi (0.91 cm).

F. Root dry weight

Mean value of root dry weight is given in Table 4. Under control treatment, the highest dry weight was observed in Damdaaamo (0.070g), while the least was recorded in Chasa upland (0.025g). For 300 mg/L Fe2+ application, the highest (0.050g) was recorded in pasighat and the lowest (0.016g) shared by Chasa upland and Twisa. At application of 600 mg/L Fe2+, Damdaaamo and Pasighat displayed the highest dry weight (0.033g), while Lailo and Twisa shared the lowest recorded root dry weight (0.011g). Under 900 mg/L Fe2+ application, Pasighat displayed the highest value (0.021g) and chasa upland, Lailo and Simoi showed the lowest value (0.006g). At 1200 mg/L Fe2+ application, Chamyak showed the highest root dry weight (8.43g) and Simoi and Twisa displayed the least dry weight (0.430g).

G. Leaf Score Index

From data on leaf score index (Table 4), it is evident that the highest mean value under 300mg/L Fe2+ stress level of iron was found in Chasa lowland, Kala joha and Lailo (1.81) indicating that these genotypes showed more symptoms of iron stress while Lahi emmo (1.09) under the same stress condition showed less symptoms of iron stress. Mean value of leaf score was observed high in Chasa lowland, Damda Aamo and Pikhi (2.34) under the stress condition of 600mg/L Fe2+ indicating that these genotypes were much more affected by stress level of iron while least leaf score was found in Gaksum, Local basmati and Simoi (1.44) indicating that these genotypes were least affected by the iron toxicity. Mean value was found high in Kala joha (2.73) under the iron stress of 900mg/L Fe2+ indicating that genotype is much affected by iron toxicity while Gaksum (2.02) was least affected by iron toxicity. Under iron stress of 1200mg/L Fe2+, high mean value was seen in Chasa upland, Kala Joha and Simoi (2.85) indicating that these genotypes were very much affected by iron toxicity; while Chamyak, Damdaaamo, Gaksum, Geturaj, Lahi emmo, Local basmati, Pikhi (2.6) genotypes showed least symptoms to iron toxicity indicating that iron stress had less impact on these genotypes.  

H. Analysis of Variance:

The pooled ANOVA implied that there were significant differences among the genotypes as shown in Table 2. The sum of squares due to iron levels was found to be significant for all the characters implying that the genotypes were affected by iron levels. Except for leaf score index (LSI), significant differences for the interaction between iron and genotypes were also observed indicating that the genotypes have not responded linearly to the changing iron stress, i.e., differential response of the genotypes to the iron stress was observed. The findings were closely similar to prior works done by Jahan et al. (2016); Dufey et al. (2009).

I. Correlation

All the parameters studied under the experiment, such as shoot length, root length, shoot dry weight and root dry weight has high significant positive correlation with each other; while all the above-mentioned characters are negatively correlated to leaf score index. The results were found to be closely similar with Amaranatha (2016); Joseph (2015).

Fig. 1. Effect of iron toxicity on number of roots.

Table 2: Pooled analysis of variance for seedling character at different levels of iron concentration.

Table 3: Effect of iron concentration on germination percentage, shoot length and shoot dry weight of different genotypes.

Table 4: Effect of iron concentration on root length, root dry weight and leaf score index of different genotypes.

Table 5: Correlation analysis.

1Since there was no variation found in all the traits in iron level of control (0mg/L Fe2+)

2Since there was no variation found for all the traits in iron level of 300mg/L Fe2+)

***Significant at 0.1% level**Significant at 1% level*Significant at 5% level

The future scope of research in iron toxicity tolerance of rice genotypes holds promise for advancing sustainable rice cultivation practices. By integrating multidisciplinary approaches encompassing genetics, physiology, and agronomy, we can accelerate the development and deployment of resilient rice varieties, thereby enhancing food security and livelihoods in iron-toxic regions.