Tolerance Level of Several Rice (Oryza sativa L.) Genotypes to Fe Toxicity Based on Morpho-Physiological Traits

Iron (Fe) toxicity is one of the limiting factor in rice cultivation in the sub-optimal land such as tidal swampy land. The tolerance level to Fe toxicity of plants depend on the genotypes. Plants in the Fe excess condition have inclusion and exclusion strategies to tolerate the toxicity. Evaluation of plant tolerance to Fe toxicity can be done by hydroponic system or planting in the controlled nutrient culture solution. Morpho-physiological traits can also be used to determine the tolerance level of plants to Fe toxicity.

The objectives of this research were to obtain the most effective nutrient culture solution as a screening medium in the Fe toxicity study and to classify the tolerance level of rice under Fe toxicity condition using morpho-physiological traits. The study consisted of three experiments conducted in the greenhouse and laboratory. The first experiment was conducted by comparing the effectiveness of three nutrient culture solutions, namely HSY (Yoshida Half Strength), HSYA (Yoshida Half Strength with 0.2% agar), and FSYA (Yoshida Full Strength solution with 0.2% agar) as an evaluation medium of Fe toxicity on rice. The second experiment was performed by observing the leaf bronzing level, plant growth responses, Fe content in the shoot tissues and in the root plaques, and root histochemical detection of 10 rice genotypes. The traits that observed in the second experiment  were used to classify those genotypes based on their tolerance level to Fe toxicity using Principal Component Analysis (PCA) and cluster analysis. The third experiment was performed by observing several physiological characters (chlorophyll, carotenoids, and malondialdehyde (MDA)) of three selected genotypes that representing each tolerance groups based on the previous PCA and cluster analysis.

The results showed that HSYA's nutrient culture solution was the most effective and efficient compared to the others to evaluate the variation of rice response to Fe toxicity compared to the other nutrient culture solutions. The presence of treatment of Fe 400 ppm for 10 days using hydroponic conditions caused inhibition of rice growth both morphological and physiological traits. Fe toxicity inhibited the root and shoot growth, leading to Fe plaque formation at the root surface and causing bronzing on the leaves.

The cluster analysis divided ten rice genotypes into three groups based on their tolerance levels to Fe toxicity. The sensitive group consisted of IR64 and Inpara 5. The moderate group consisted of Mahsuri, IRH108, Danau Gaung, Indragiri, Inpara 2, and Inpara 6. The tolerant group consisted of Hawara Bunar and Pokkali. In addition, Fe toxicity decreased chlorophyll and carotenoid concentration and increased the lipid peroxidation level in both the root and shoot. Based on bronzing character in leaves, Fe concentration in shoot tissues, Fe concentration in root plaques, there were three tolerance type to Fe toxicity, namely includer, excluder, and includer-excluder. The includer type consist of Mahsuri, Indragiri, Inpara 6, and Inpara 2. The includer-excluder type consist of Hawara Bunar, Pokkali, Danau Gaung, dan IRH108. Among the tolerance and moderate group that investigated in this research, there were no genotypes with only excluder type to tolerate the Fe toxicity. Furthermore, this study can be seen that morpho-physiological traits could be used to distinguish the tolerance level and tolerance type of rice under Fe excess condition.

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