'Bronzing', the symptom of iron toxicity in rice, is caused by high ferrous (Fe2+) concentrations in flooded soil in many lowlands and swamps in India, West Africa and other regions. In those areas iron toxicity is a major problem in attempts to stabilize or increase rice production. However, information about genetic loci for the tolerance is still rather limited. The aim of the present work was to establish Fe2+ stress in pot experiments and to identify marker loci for iron tolerance.

Seventy F2 lines were used which had been derived individually from the f1 plants of IR36/Nekken 2 by crossing to Miyukimochi (IR36 is tolerant to iron toxicity). These lines had polymorphisms at 22 out of 25 loci detected with isozymes and other markers (Table l).

Table 1. Maeker genes and their Chromosomes

a: growth rate of plant length, c: chlorophyll content, d: air-dry weight. t: number of tillers.

Research Notes 133

To simulate the stress condition we filled into a 4 liter plastic pot 3kg of soil from weathered granite rock in Hyogo, Japan. Then, 5g glucose was placed deep into the soil in order to accelerate the soil reduction process. Glucose is known to reduce flooded soil and maintain low or no oxygen in pot trials (Yamane et al. 1964). In our test, typical oily film was visible on the soil surface as in actual iron-toxic soils.

Four plants for each F2 line were transplanted in a pot (1 plot) at four week seedling stage. Both the stress and control plots were replicated twice. Data were recorded for plant length (each week for 5 weeks), culm length difference (the distance between the tallest culm and the second one in each plant), number of tillers, chlorophyll content, air-dry weight, and heading date.

In the iron stress condition, almost all plants showed moderately discolored leaves. Growth retardation was confirmed by most measurements but not correlated with heading time. Tolerance indices were calculated as the ratio of the control to stress plot for each measurement for each F2 line and fell into 10 classes. All of the tolerance indices for the F2 lines were pooled into two genotypes of polymorphic markers. The differences of mean tolerance scores were tested between the two genotypes and significant differences were found for some marker genotypes (Table 1). The allelic differences at the Est-I and Est-2 loci were significantly correlated to the differences in the level of tolerance. To examine cumulative effects of the two putative loci, four groups classified by the two-locus genotypes were compared for tolerance. Additive effects of the markers were detected on the growth rate of plant length and air-dry weight (Table 2). For both esterase loci, the lines with IR36-derived alleles were more tolerant than those with Japonica derived alleles. The role of the two putative loci remains to be confirmed.

Table 2. Additive effects of Indica-type markers on tolerance to iron-toxicity

134 Rice Genetics Newsletter Vol. 13

Yamane, I. and K. Sato, 1964. Decomposition of glucose and gas formation in flooded soil. Soil Sci. and