Characterisation of lowland rice varieties for tolerance to iron toxicity in Uganda.
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Iron toxicity in flooded soils is attributed to high iron content in the soil, low soil pH, low soil fertility as well as accumulation of harmful organic acids and/or hydrogen sulphide. Effective measures to ameliorate iron toxicity include periodic surface drainage, liming and good fertilizer management. If iron toxicity is not severe, the use of tolerant rice varieties alone may serve as an alternative to these measures. Therefore, development and selection of rice varieties with tolerance to iron toxicity is an important aspect of management of soil degradation due to iron toxicity. In this study, field trials and glass house experiments were conducted at NaCRRI, with the following objectives: (1) to screen local and introduced germplasm for tolerance to Fe toxicity; 2) to determine the effect of iron toxicity on uptake and distribution of nitrogen, phosphorus and potassium in plant tissues; 3) to determine traits associated with tolerance to iron toxicity. In the glass house experiment, rice seedlings were grown in pots filled with fine sand, supplemented with growth nutrients. Four weeks after sowing, seedlings were subjected to 300 (control) and 1000 mg L-1 Fe(II) (applied as FeSO4) as treatments. In the field trial, seedlings were transplanted 15 days after seeding in 5 rows of 20x20 cm spacing. In both cases, data was collected on leaf symptoms score, plant height, dry biomass, tissue Fe, N, P and K content and yield. Differences between genotypes were observed following exposure to Fe(II) 1000 mg L-1 based on visual determination of leaf symptoms, iron uptake and partitioning within plant tissues (chemical analysis), growth parameters and yield measurements. The highest symptom score was recorded in Supa, Kibuyu and IR64 while low bronzing score values were recorded in IR80310-12-B-1-3-B and EG8. The amount of Fe retained in the leaves was highest in Kibuyu (2.48 mg g-1), Supa (1.48mg g-1) and WITA 3 (1.46 mg g-1). Thus, the high bronzing scores of these genotypes was probably due to high iron concentration in the shoot. Tolerant cultivars, K98, PNA and WITA 4 retained more iron in the root tissue suggesting that the retention of Fe in root tissue is rather more efficient than stem retention as an avoidance/exclusion mechanism. Some exceptions, however, were noted in IR 47686-9-2-B that retained more iron in the root tissue yet recorded high toxicity scores. Further researches are required to determine mechanisms that influence such behaviour in some genotypes. In the field trial, leaf bronzing symptoms were neither observed in Fe2+ toxic field nor in control plots in any genotype. Traits other than leaf bronzing symptoms showed significant differences in the field trial. Shoot dry matter yield, tiller number and grain yield were significantly reduced in iron toxic field (p<0.05). Grain yield was reduced by 34.5% under field conditions and 26.8% under glass house conditions. Some genotypes with less bronzing symptoms; IR 80310-12-B-1-3-B and K98, also had significant yield reduction under iron toxicity, suggesting that leaf bronzing alone does not adequately represent the detrimental effect of iron toxicity. The nutrient uptake ability under iron stress and synthesis of other protecting compounds could probably be involved in cultivars -PNA, IR 73678-20-1-B and WITA 4 that showed less variability in all the traits measured. These cultivars could serve as candidates for iron toxicity tolerance genes. Growth and nutrient uptake showed negative correlation with iron content in the leaves suggesting that iron toxicity impacted both rice growth and nutrient uptake. Nutrient concentrations in plant tissues were highly variable and may not adequately explain the difference in sensitivity among the genotypes tested. However, the correlation between P and K (r=0.76), which when combined with existing knowledge of root biochemistry, suggests that P has a significant role in the uptake of K under iron toxic conditions.