Iron toxicity in rice : factors affecting the sensitivity and its alleviation | |
| Author | Ahmed, Sheikh Faruk |
| Call Number | AIT Diss no.AS-23-02 |
| Subject(s) | Iron--Toxicology Rice Rainfed lowland rice Silicon |
| Note | A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Agricultural Systems and Engineering |
| Publisher | Asian Institute of Technology |
| Abstract | Rice (Oryza sativa L.) is the staple food for more than 50% of the world’s population, satisfying above 20% of the global calorie requirement. Due to a changing climate, various abiotic stresses, including drought, salinity, flood, heavy metal toxicity, and deficiency or excess of micronutrients, are threatening rice production nowadays. Micronutrient, such as iron (Fe), plays a crucial role in different plant metabolic activities, including photosynthesis, chlorophyll biosynthesis, mitochondrial respiration, nucleic acids synthesis/repair, and maintenance of chloroplast ultrastructure/function. Iron is also an integral component of essential proteins like heme and acts as a co-factor of several enzymes functioning as a proton accepter/donor. Both excess and deficiency of Fe are one of the most widely prevalent micronutrient disorders for lowland rice production. Phytotoxicity by excess ferrous Fe (Fe2+) availability in the soil solution is associated with substantial yield losses, posing a serious threat to global food security. Moreover, the widely practiced rice cultivation system under standing water notably features ideal conditions for Fe2+ toxicity, which demands rapid action toward adjustments of management strategies. Progress in better suited rice cultivar development with improved tolerance and deployment of suitable management strategies for addressing Fe2+ toxicity has been very challenging not only because of its complex nature but also due to lack of adequate research advancements in this regard. Therefore, to address the issue of Fe2+ toxicity for lowland rice production, an approach exploiting the possibilities of natural Fe2+ toxicity tolerance of rice coupled with innovative nutrient management practices were evaluated through sequential polyhouse experiments.In the first polyhouse experiment, the Fe2+ tolerance potential of nine diverse Thai rice genotypes was evaluated against five levels of Fe2+ (FeSO4.7H2O) (0 [control], 150, 300, 600, and 900 mg L−1 ). There were a recognized tolerant (Azucena) genotype and a susceptible (IR64) genotype for comparison. The evaluation was conducted initially by a germination trial in Petri dishes, followed by a polyhouse study on growth, yield, and physiochemical performance. Results showed significant variations in terms of Fe2+ -tolerance across genotypes. Increasing Fe2+ level beyond 300 mg L−1 was equally detrimental for germination and growth of all tested genotypes, although germination responses were negatively affected at 300 mg L−1 . Physiochemical responses in the form of leaf greenness (SPAD value), net photosynthetic rate, membrane stability index, and Fe contents in leaf and root tissues were the most representative of excess Fe2+ -toxicity-mediated impairments on overall growth and yield. Difference in physiochemical responses was effectively correlated with the contrasting ability of the genotypes on lowering excess Fe2+ in tissues. Analysis of average tolerance and stress tolerance index unveiled that the genotypes RD85 and RD31 were the closest to the tolerant-check Azucena and the sensitive-check IR64, respectively. The unweighted pair group method with arithmetic means clustering revealed three major clusters, with cluster II (four genotypes) being Fe2+ tolerant and cluster I (four genotypes) being Fe2+ sensitive. Principal component (PC) analysis and genotype by trait-biplot analysis showed that the first two components explained 90.5% of the total variation, with PC1 accounting for 56.6% and PC2 for 33.9% of the total variation. The identified tolerant rice genotypes show potential for cultivation in Fe2+ toxic lowlands for increased productivity. The findings contribute to the present understanding on Fe2+ -toxicity response and provide a basis for future genotype selection or rice crop improvement programs against Fe2+ -toxicity.The second polyhouse experiment was conducted to evaluate whether exogenous application of silicon (Si) or potassium (K) could alleviate the impairments of Fe2+ toxicity on rice roots. Two independent pot experiments were simultaneously conducted using two rice genotypes (RD85 [Fe-tolerant] and RD31 [Fe-susceptible]) and three Fe2+ (FeSO4.7H2O) levels (0 [control], 600, and 900 mg L−1 ). In Experiment A, soluble Si was applied in four doses (0 [control], 30, 60, and 90 kg ha−1 ) in the form of monosilicic acid. In Experiment B, K was applied in four doses (0 [control], 60, 120, and 180 kg ha−1 ) in the form of potassium chloride. Rice genotypes and Fe2+ levels remained the same in the two experiments. In both experiments, root impairments increased with increasing Fe2+ levels and the application of either Si or K effectively reduced the impairments across all measured parameters regardless of genotypes. Although the extent of mitigation of root growth damage was quite similar for plants grown with either Si or K, the underlying mechanisms related to these responses were distinct between them. The application of Si alleviated at best 26%, 36%, 16%, 28%, 44%, 33%, 34%, and 35% adverse impacts on root number per plant, root volume, root dry matter, membrane stability index, relative cell death, lipid peroxidation rate, free proline content, and total antioxidant activity, respectively, along with 59% reduction in Fe content and 37% increase in Si content in roots. Similarly, K application alleviated at best 22%, 44%, 33%, 15%, 41%, 30%, 30%, and 86% adverse impacts on the same respective parameters along with 39% reduction in Fe content and 50% increase in K content in roots. The major difference in the Fe2+ toxicity alleviation activity between Si and K remained within the amount of Fe uptake for the former and within the total antioxidant activity for the latter. Silicon promoted exodermal Casparian band formation in the root exodermis for effective exclusion of excess Fe2+, whereas K extensively boosted free radical scavenging capacity to attain similar mitigation responses on the other tested parameters. The reported findings of this study would contribute to enhancing present understanding on Fe2+ toxicity alleviation effect of both Si and K as well as help formulate management strategies for successful rice cultivation under Fe2+ -toxic lowland environments.The third polyhouse experiment was conducted to evaluate whether exogenous application of Si or K could alleviate the impairments of Fe2+ toxicity on rice growth and productivity via physio-biochemical adjustments. Two independent pot experiments were simultaneously conducted using two rice genotypes (RD85 [Fetolerant] and RD31 [Fe-susceptible]) and three Fe2+ (FeSO4.7H2O) levels (0 [control], 600, and 900 mg L−1 ). In Experiment A, soluble Si was applied in four doses (0 [control], 30, 60, and 90 kg ha−1 ) in the form of monosilicic acid. In Experiment B, K was applied in four doses (0 [control], 60, 120, and 180 kg ha−1 ) in the form of potassium chloride. Rice genotypes and Fe2+ levels remained the same in the two experiments. In both experiments, growth and yield impairments increased with increasing Fe2+ levels and the application of either Si or K effectively reduced the impairments across all measured parameters regardless of genotypes. Silicon application alleviated at best 8%, 30%, 6%, 64%, 50%, 8%, 17%, 50%, 23%, and 27% adverse impacts on plant height, root length, plant biomass, grain yield, total chlorophyll content, membrane stability index, lipid peroxidation rate, free proline content, total antioxidant activity, and net photosynthetic rate respectively, along with 40% reduction in leaf-bronzing score, 31% reduction in leaf Fe content, and 5% increase in leaf Si content. Similarly, K application alleviated at best 6%, 35%, 19%, 69%, 55%, 11%, 29%, 84%, 19%, and 31% adverse impacts on the same respective parameters along with 30% reduction in leaf-bronzing score, 18% reduction in leaf Fe content, and 25% increase in leaf K content. Although the extent of mitigation responses was similar, the magnitude of these responses varied depending on Si or K treatment. However, noticeable difference between Si- and K-mediated Fe2+ toxicity mitigation was evident by significant reduction of leaf Fe content for Si and boosting of free proline content for K. The reported findings would contribute to enhancing present understanding on Fe2+ toxicity alleviation effect of both Si and K as well as help formulating management strategies for successful rice cultivation under Fe2+ -toxic environments.The fourth polyhouse experiment was conducted to evaluate the potential of combined application of Si and K in mitigating Fe2+ toxicity impairments to rice leaf mesophyll tissue. The experiment was laid out in a factorial combination of two Fe2+ levels (0 and 300 mg L−1 ), two Si levels (0 and 56 mg L−1 ), and two K levels (0 and 200 mg L−1 ) following a completely randomized design. Excess Fe2+ impaired leaf mesophyll performance by negatively affecting all tested parameters; however, Si and K significantly reduced those impairments. The integrated application of Si and K resulted in the maximum mitigation effects as high as 23%, 27%, 14%, 40%, 25%, 37%, 24%, 48%, 34%, 28%, 41%, and 15% for total chlorophyll content, maximum quantum yield of photosystem II, membrane stability index, relative cell death, lipid peroxidation, total protein content, free proline content, total polyphenol content, hydrogen peroxide content, total antioxidant activity, net photosynthetic rate, and plant biomass, respectively, compared with the control, with 29% decrease in Fe content and 4.2-fold and 1.7-fold respective increase in Si and K content of rice leaf mesophyll. Silicon or K enhanced photosynthetic performance by boosting antioxidant activity, proline content, and polyphenol content, thereby reducing oxidative damages from Fe2+ - generated free radicals like hydrogen peroxide; however, K was comparatively more effective than Si. The combined application of Si and K resulted in significantly better Fe2+ toxicity alleviation response over their sole application for the maximum studied traits. The findings would contribute to the present understanding of Fe2+ toxicity alleviation and help in strategizing crop management practices for sustainable rice production in Fe2+ -toxic lowlands. |
| Year | 2023 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Agricultural Systems and Engineering (ASE) |
| Chairperson(s) | Datta, Avishek; |
| Examination Committee(s) | Loc, Thai Nguyen;Zulfiqar, Farhad; |
| Scholarship Donor(s) | His Majesty the King’s Scholarship Thailand; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2023 |