Abstract

Media summary

Key Words

Fe-tolerance, Fe-excluding power of roots, Fe- toxic soil, rice

Introduction

Iron (Fe) toxicity is one of the most serious problems in irrigated or rainfed soils. Although the International Rice Research Institute (IRRI) has developed many lines and varieties to improve this problem, the physiological mechanisms of Fe toxicity tolerance are not well understood. It has been reported that rice roots have Fe-retaining, Fe-oxidizing and Fe-excluding powers that reduce the amount of Fe in shoot (Tadano 1976). These powers can be explained as follows. Firstly, Fe is retained in the root and not transported to the shoot. Secondly ferrous ion oxidizes to ferric oxide that is not active. And finally, Fe may be excluded by absorbing less Fe than is present in soil solution. The present study was aimed at analysing the Fe-excluding power of rice roots as a physiological mechanism of breeding lines with Fe tolerance.

Methods

In the 2003 wet season, field experiments were conducted at San Dionisio, Iloilo City and Panay Island, Philippines, to examine Fe toxicity. IR64 (check variety) and four lines of rice(Oryiza sativa L.) were used in this experiment. Two lines (FTB7 and FTB11) were near isogenic lines (NILs) of IR64 selected under a solution culture in a green house of IRRI for Fe tolerance. The other two lines (Fe0013 and Fe0014) were selected as Fe-tolerant lines in Iloilo field trials. As a reference under normal conditions, IR64 and the 4 lines were cultivated also at IRRI, Los Baños, Philippines. Plots were laid out in a randomized complete block design with four replications. Pre-germinated seeds were sown in seedling trays to produce uniform seedlings. Fourteen (IRRI) or 21 (Iloilo) day-old seedlings were transplanted on 27 August 2003 in Iloilo and on 25 July 2003 in IRRI at a spacing of 20 × 20 cm. Growth periods after transplanting of IR64, FTB7 and FTB13 in IRRI and those in Iloilo, Fe0013 and FE0014 in IRRI and those in Iloilo were 93, 91, 108 and 106 d, respectively. Basal fertilizer was applied as 30 kg N ha^-1, 20 kg P ha^-1and 20 kg K ha^-1, incorporated at puddling. Nitrogen was topdressed at 20 kg ha^-1 each at midtillering and panicle initiation. During the experiments, plant and soil solutions were sampled periodically. To collect the soil solutions, a porous cup, 10 cm long, was set in the soil vertically around 2 cm below the soil surface. The cup was connected to a flexible plastic tube, and the soil solution obtained was introduced into a 10 mL evacuated test tube (Nozoe et al. 1999). The amount of Fe in soil solution was determined by the colorimetric method using o-phenanthroline (Loeppert and Inskeep 1996).

Results

Table 1 shows the grain yields of IR64 and 4 lines at IRRI and Iloilo. In IR64 and all lines, the yields in Iloilo were lower than those at IRRI. Although FTB7 and FTB11 were selected as Fe tolerance under solution culture, this tolerance was not expressed at the Iloilo field site because the values of B/A of IR64, FTB7 and FTB11 were 51.2, 48.0 and 29.2, respectively. However, the values of B/A of Fe0013 and Fe0014 were 84.2 and 70.3, respectively, indicating that the tolerance of Fe0013 and Fe0014 was expressed in the Iloilo field.

Table 1. Grain yields (t/ha) at IRRI and Iloilo

SD: Standard Deviation

Figure 1. Shoot Dry Matter of IR64 and 4 lines at Iloilo. Values on the bars followed by the same letter are not significantly different at 5% level of significance.)

Figure 1 shows the amount of dry matter of shoots at Iloilo. At 28 days after transplanting (DAT), the shoot dry matter of IR64, FTB7 and FTB11 was smaller than in F0013 and F0014. At 50 DAT, the growth of IR64, FTB7, FTB11 recovered, but at 73 DAT, the growth of IR64, FTB7, FTB11 was inhibited again. These findings suggest that the yield reduction of IR64, FTB7, FTB11 (Table 1) was associated with growth inhibition especially during the late stage of growth.

Figure 2 shows the Fe concentrations in roots at Iloilo. The Fe concentration increased uniformly (Fe0013) or remained constant (Fe0014) after flowering stage, while Fe from the plots of IR64, FTB7 and FTB14 increased rapidly after flowering. As indicated in introduction, the root of rice has the power to exclude Fe so as not to transport Fe to shoot. The mild increase in Fe in the roots of elite lines suggested the operation of this tolerance mechanism in these lines.

Figure 2. Changes in Fe concentration in roots at Iloilo. Vertical bars indicate the standard errors. FL: Flowering stage.

Figure 3. Changes Fe concentration in soil solution at Iloilo. Vertical bars indicate standard errors.

The concentrations of Fe in soil solution ranged from 40 to 140 (mg/L) throughout the growth period (Figure 3). This level has been shown to be enough to inhibit rice growth (Yoshida 1981). Especially in the late stage of growth, the concentration of Fe in soil solution from the plots of elite lines was greater than in plots of IR64 and its NILs. If the rice root excludes the absorption of Fe, the Fe content of soil solution around root can be expected to increase. Therefore, this result supports the presence of iron excluding power in the elite lines.

Figure 4. Rice lines, Fe0013(left) and FTB7(right) before harvesting at Iloilo

One of the symptoms of Fe toxicity in rice is the presence of brown spots on the leaf. As shown in Fig 4, FTB7 showed these symptoms. This finding indicates the difference of susceptibility for Fe toxicity between the lines. However, the colour difference also reflects the difference in time of maturity, Fe0013 being later than FTB7.

Conclusion

The difference in susceptibility to Fe toxicity among IR64 and the lines developed in IRRI was confirmed in the field experiments. The yield reduction by Fe toxicity was associated with the growth inhibition, especially at the later stages of growth. During this period, the Fe content of roots of tolerant lines increased more slowly than those of susceptible lines. Also, the Fe content in soil solution sampled from plots of tolerant lines was higher than in those of susceptible lines. These findings suggest that a Fe exclusion mechanism is operating in the roots of tolerant lines.

References

Dobermann A and Fairhurst T (2000). In 'Rice: Nutrient Disorders & Nutrient Management '. (Ed. A Dobermann and Fairhurst T), pp121-125 (International Rice Research Institute, Manila).

Loeppert RL and Inskeep WP (1996). In 'Methods of Soil Analysis. Part 3- Chemical methods'. (Ed. J. M. Bigham), pp659-661 (Soil Science Society of America, Madison).

Nozoe T, Nishibata Y, Sekiguchi T and Inoue T (1999). Effects of the addition of Fe-containing slag fertilizers on the changes in Eh in paddy soil. Soil Science and Plant Nutrition 45, 729-735.

Tadano T (1976). Studies on the methods to prevent iron toxicity in lowland rice. Memories of Faculty Agriculture Hokkaido University 10, 22-68.

Yoshida S (1981). In 'Fundamentals of Rice Crop Science'. pp156-160 (International Rice Research Institute, Manila).