788. Elucidating the Mechanisms Exhibited by Tolerant Rice Genotypes Under Iron-toxic Field Conditions in Madagascar

Iron is one of the essential elements required for plant growth, but its excessive supply can adversely affect plant growth. Iron toxicity is a physiological disorder specific to rice grown under waterlogged conditions and occurs in Southeast Asia and in many regions of Africa, including Madagascar and West Africa. Iron toxicity causes leaf bronzing and growth retardation. In many rice fields affected by iron toxicity, rice yields are known to be reduced by 15% or more, posing food security concerns. Although the problem of iron toxicity has been recognized for more than half a century and research has been conducted to improve crops, many of the genes that help improve tolerance and the physiological mechanisms that confer tolerance have not been identified. It is known that there are large genotypic differences in iron toxicity tolerance, and research has been directed toward the search for factors important for tolerance using a wide range of genetic resources. However, research to date has mainly focused on growing young seedlings in hydroponic solution or in pots under artificial conditions, and on evaluating tolerance using visible symptoms such as leaf bronzing as indicator, but not on physiological factors important for tolerance in the actual field throughout the growing season.
 
JIRCAS, in collaboration with the University of Antananarivo, Madagascar, and Cranfield University, UK, has conducted a study in which iron toxicity tolerant genotypes selected from a wide range of genetic resources and susceptible lines, including IR64, a major rice cultivar in the world, were grown in iron-toxic fields in Madagascar and physiological analyses were carried out at each growth stage to elucidate the mechanisms exhibited by iron toxicity tolerant genotypes. As a result, we found the following:

• As previously known, during the vegetative growth phase, tolerant genotypes were classified into iron ‘excluders’, which suppress excess iron uptake (e.g., Bahia, Nerica L-43, Tsipala), and iron ‘includers’, which retain the uptake of iron in the plant in a less harmful form (e.g., KA-28, X265).
• The iron ‘includer’ tolerant genotype KA-28 had a high total plant iron content, but the proportion of iron distributed to new leaves was low, indicating that the mechanism of minimizing the effect on new leaves by hoarding iron in old leaves was functioning.
•  During the reproductive growth phase, iron uptake of ‘excluders’ increased and the iron concentration was similar to that of ‘includers,’ indicating that the iron exclusion mechanism of these genotypes only functions during the vegetative growth phase.
• No correlation was found between the degree of visible symptoms and yield, or between growth during the vegetative growth phase and yield, suggesting that selection of tolerant genotypes focusing only on visible symptoms or growth during the vegetative phase may not necessarily be effective in selecting genotypes with improved yield in the field.

These results indicate that the multiple physiological mechanisms that confer iron toxicity tolerance function in a growth stage-dependent manner, and that evaluation at the young seedling stage and using visual symptom as indicator are insufficient for selecting genotypes with improved yield. Furthermore, physiological factors such as soil-plant interactions and plant iron partitioning are important for iron toxicity tolerance and these factors are growth stage dependent, indicating that iron toxicity tolerance in the field is controlled by a complex interplay of factors. This study suggests that evaluation of tolerance throughout the growing season is necessary for future crop improvement, and that the multiple tolerant genotypes identified in this study can be used to improve specific physiological functions in the field.

Photo caption

Left: A field affected by iron toxicity in the central highlands of Madagascar (Behenjy). The number of tillers is limited and the tips of the leaves are yellowish brown.

Right: Another field severely affected by iron toxicity, also in the central highlands of Madagascar (Antsirabe). The plants appear to be dying due to the intense stress.

Reference
Rajonandraina, T., Rakotoson, T., Wissuwa, M., Ueda, Y., Razafimbelo, T., Andriamananjara, A., Kirk, J.D.K. (2023) Mechanisms of genotypic differences in tolerance of iron toxicity in field-grown rice. Field Crops Research 298, 108953. https://doi.org/10.1016/j.fcr.2023.108953

Contributors: UEDA Yoshiaki (Crop, Livestock and Environment Division), NAKASHIMA Kazuo (Food Program)