Dioxins [polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and coplanar polychlorinated biphenyls (Co-PCBs)] are of public concern because they exhibit marked mammalian toxicity, such as immunotoxicity, carcinogenicity, and adverse effects on reproduction, development, and endocrine function.  Dioxins are released into the environment through incineration of rubbish and waste.  Recently, we conducted an intensive survey of the dioxin levels in Japanese paddy soils, and found that relatively high levels of dioxins have accumulated in these soils.  This is because commercial herbicides such as pentachlorophenol (PCP) and 2,4,6-trichlorophenyl 4-nitrophenyl ether (CNP), which in the past were in common use in paddy fields, contain dioxins as impurities.  Dioxins are chemically and biologically stable in soils and therefore accumulated in our farm soils over a long period of time.  Our aim was to investigate whether rice plants could absorb these dioxins from the contaminated paddy soils via their roots.

Rice plants were cultivated in a greenhouse in 2 soils with different isomer profiles and dioxin concentrations.  The dioxin concentrations were 2100 (soil A) and 36 (soil B) pg-TEQ g^-1.  Hepta and octa CDDs (HpCDD, OCDD), and hexa, hepta, and octa CDFs (HxCDF, HpCDF, OCDF) had dominantly accumulated in soil A, whereas tetra and octa CDDs (TeCDD, OCDD) were the major dioxin isomers in soil B.  Co-PCBs were barely detectable in either soil.

Although we used 2 soils that were different in terms of both isomer profile and dioxin concentration, the concentrations and isomer profiles of dioxins in the husks and leaves of the rice plants grown in both soils were almost the same.  Co-PCBs, which were not contained in either soil but are major isomers of atmospheric dioxins, were dominant in both leaves and husks of plants grown in both soils (Fig. 1).

In another experiment, a rice plant was cultivated on a dioxin-contaminated soil (120 pg-TEQ g^-1), and the distribution of dioxins in its top parts (leaves, stems, husks, and grain) was traced.  The dioxin concentrations decreased in the following order: leaves > husks > stems > grain, with an increasing tendency toward the outer parts (leaves and husks), which may be more exposed to atmospheric gases than the inner parts.  The dioxin concentration in the grain was very low, mainly because the grain was covered by the husk.  No dioxins were detected in the xylem sap (less than 0.0001 pg-TEQ mL^-1) (Fig. 2).

These results indicate that dioxins are not absorbed from contaminated soil by the roots of the rice plant, but that contamination occurs mainly from the atmosphere. (T. Otani, M. Kuwahara, R. Uegaki, and N. Seike)

Soybean is a major staple that is widely consumed in Japan in processed foods and even as a fresh vegetable.  Soybean seeds accumulate relatively high levels of cadmium (Cd) compared to grains of paddy rice and wheat, and it is important that we try to minimize their content of this heavy metal.

We planted more than 30 promising lines or varieties of soybean with different tendencies of seed Cd accumulation in 2 soils contaminated with Cd (soil A: Gray lowland soil, total Cd 0.9 mg kg^-1; soil B: Andosol, total Cd 7.4 mg kg^-1).  A non-Cd-polluted soil was used as a reference (soil C: Andosol, total Cd 0.2 mg kg^-1).  En-b0-1-2, a mutant of the cultivar Enrei, had the lowest seed Cd concentration, followed by 'Enrei' on all these soils.  The cultivars Harosoy and Suzuyutaka had the highest and second-highest seed Cd concentrations.  The cultivar Hatayutaka, which was produced by crossing 'Enrei' with 'Suzuyutaka,' showed a seed Cd concentration in between those of its parent cultivars (Table 1, Fig. 1).  This order of seed Cd concentrations among the 5 cultivars was always the same, regardless of the planting year and the soils.

Four of the cultivars - En-b0-1-2, Enrei, Hatayutaka, and Suzuyutaka - were cultivated hydroponically to maturity, and Cd concentrations were then measured separately in the top parts and roots of the plants.  The Cd concentrations of the top parts (seed, stem, and pod) of En-b0-1-2 were the lowest of the 4, followed by 'Enrei.' However, the Cd concentrations in the roots of En-b0-1-2 and 'Enrei' were substantially higher than those of 'Hatayutaka' and 'Suzuyutaka.' This suggests that low-seed-Cd cultivars such as Enb0-1-2 and 'Enrei' possess an elegant mechanism for retaining Cd in their roots and preventing its translocation to the top parts (Table 2).  To verify this hypothesis, we grafted a scion of 'Enrei' onto rootstocks of 'Suzuyutaka,' 'Hatayutaka,' or En-b0-1-2.  The Cd concentrations in the seeds of 'Enrei' grafted with Suzuyutaka' or 'Hatayutaka' were substantially higher than in those of the ungrafted cultivar (Fig. 2).  These results have encouraged us to screen and breed soybean lines with the aim of strengthening our capacity for cost-effective risk management of food Cd levels.

Reference
Arao T., Ae N., Sugiyama M. and Takahashi M.  (2003) Genetic differences in cadmium uptake and distribution in soybeans.  Plant and Soil 251, 247-253.

The GM corn variety StarLink was bred for use in animal feeds, but in 2000 it was discovered in the United States in foods intended for human consumption.  To minimize the likelihood of such undesirable outcrossings occurring, it is important that we improve our knowledge of the relationship between pollen dispersal distance and outcrossing rate in Zea mays.  This research was begun in 2001 but has been extended to cover the years 2002 to 2005.

We selected 2 commercial sweet corn varieties with different grain colors.  They were 'honey bantam' (pollen donor; yellow grain) and 'silver honey bantam'(pollen recipient; white grains).  'Honey bantam' was planted to windward of 'silver honey bantam.' We selected 10 rows and harvested recipient plants from each row.  We used the xenia phenomenon to determine the percentage of outcrossing, as shown by the presence of yellow grains on the female ears of the recipient (Fig. 1).  The mean outcrossing rate on female ears of recipients bordering the donors was about 23%.  A continuous outcrossing zone was observed to nearly 5 m from the donor.  Far away from the zone, plants of low outcrossing rate were irregularly scattered (Fig. 2).

The outcrossing rate decreased sharply within the zone to 7 m, but no further decrease was observed over a longer distance, and the minimum rate finally converged to 0.28% according to the calculations used (Fig. 3).  In a female recipient ear with 500 grains there were, at most, 1.5 yellow grains at a 0.28% outcrossing rate.

This investigation was conducted on a small scale (in a field of about 14 ares) with a maximum distance of only 50 m between recipient and donor plants.  Further studies have since been initiated in a larger field (4.5 ha) to determine the outcrossing rate over a standard isolation distance of 200 m.  These investigations were initiated in 2002 as a collaborative study between NIAES and the Tsumagoi Station of the National Center for Seeds and Seedlings.  Our 2002 results showed that the outcrossing rate decreased to an average of less than 0.1% over a distance of 100 m from the donor plants.

Because flooded rice cultivation is an important source of atmospheric methane (CH₄), a major greenhouse gas (GHG), there is an urgent need to establish useful strategies for mitigating CH₄ emission from paddy fields.  It is known that intermittent drainage has a strong effect in reducing CH₄ emission during the rice cultivation period.  However, in some cases these drainage practices promote increased emissions of nitrous oxide (N₂O), another major GHG.  We conducted simultaneous measurements of CH₄ and N₂O emissions from paddy fields under conventional Japanese water and fertilizer management.  The experiment was carried out at the GHG controlling facility of NIAES by using automated monitoring systems for CH₄ and N₂O fluxes (Photo 1).

Figure 1 shows the seasonal courses of CH₄ and N₂O fluxes and water depth monitored in the experimental plots.  CH₄ flux gradually increased after the first flood-irrigation of the field and reached about 150 mg CH₄ m^-2 day^-1 at the beginning of July.  After the first summer drainage, however, CH₄ flux dropped rapidly to almost zero within a few days.  CH₄ flux then gradually increased again with intermittent flood-irrigations, but these increases were much smaller than that before the first drainage.  The results indicate that intermittent drainage in summer greatly helps to mitigate CH₄ emissions in the latter half of the rice cultivation period; this agrees with the results of a number of other studies.

A recognizable level of N₂O flux was observed before the first flood-irrigation of the field.  After that, a large peak of N₂O emission was observed, although it lasted for only a few days.  N₂O flux then rapidly decreased and remained close to zero during the subsequent rice cultivation period, with the exception of a small peak just after the top-dressing of additional fertilizer.  The highest peak of N₂O flux was observed after the harvest of rice plants and lasted for about 2 weeks.  The total CH₄ emission rate during the rice cultivation period, from first-flood irrigation to rice harvest, was 3180 ± 1728 mg m^-2, and that of N₂O was 2.6 ± 10.1 mg N m^-2; these values are lower than most previously reported ones.

Our relatively low N₂O emission rate is attributable to the following 2 factors.  First, there was a comparatively low rate of nitrogen fertilizer application.  Large N₂O emissions have been observed in experiments in which nitrogen fertilizer is applied at a rate of 300 kg N ha^-1 or more, whereas we used less than 100 kg N ha^-1.  The other factor is the practice of frequent drainages and irrigations over summer.  Although drainage may stimulate N₂O emission by both resumption of the nitrification process and stimulation of incomplete denitrification, irrigation within a few days likely prevents significant N₂O emission.

In conclusion, our results indicate that strategic water management and fertilizer application can have strong influences on N₂O and CH₄ emissions from flooded paddy fields.  Frequent intermittent mid-season drainage of fields and the use of low application rates of nitrogen fertilizer mitigate both CH₄ and N₂O emissions during the rice cultivation period.