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Yukio Kujira
Faculty of Education, Kanazawa University, Kakuma-machi, Kanazawa 920-11, Japan
Introduction
Worldwide, irrigated rice cultivation is considered to be among the largest sources of atmospheric methane. Methane is a radiatively active gas with ~25 times more infrared absorbing capacity per molecule than carbon dioxiside as a source of potential grobal warming (Rodhe, 1990). Wetland rice cultivation is the major anthropogenic CH4 source and emissions into the atmosphere are estimated between 60 and 170 Tg year-1 which represents about 25% of worldwide CH4 release (USEPA, 1991). On the other hand, farmers apply fertilizer to crops to produce maximum economic returns. To attain this, it is necessary to make clear that genetic and environmental factors affecting on root morphology of rice plant. Because, it is important that fertilizer requirements be accurately estimated, and the prevention of excessive use of fertilizer on economic grounds has also the beneficial consequence of maximizing environmental pollution of groundwaters through the leaching of unused fertilizer.
Genetic factors affecting on root system
Effect of semi-dwarf gene on the spreading angle of root system
The inheritance of root system characteristics in rice was reported by Chang et al. (1982) and Armenta-Sota et al. (1983). The expression of long roots and high root number were controlled by dominant alleles in one parent. Different genotypic cultivars of rice, Fujiminori presenting a tall type, Reimei, which is a mutant of Fujiminori representing a semi-dwarf type and Dee-geo-woo-gen, another semi-dwarf type, were used to analyse the effect of semi-dwarf gene on the spreading angle of root system. Root boxes and seed pack growth pouches were used to determine the lateral spreading angle. Thus, a semi-dwarf gene that affects the plant type, did not influence the spreading angle of root system.
Inheritance on the spreading anlge of root systems
The morphological characteristics of root systems in rice cultivars were examined employing different nutrition levels. Norin 3 had a small spreading angle, on the other hand, Rikuu 132 showed a large spread. The dry weight of the top including tiller dry weight, leaf blade and leaf sheath dry weight, did not significantly differ between these cultivars (Kujira, 1987). F1 plants from the cross between Norin 3 and Rikuu 132 had an intermediate spreading angle of root system between that of each parent. In the rhizotron experiments we have observed that F2 segregating rice populations the rooting patterns are controlled by a single dominant gene. We are continuously selecting germplasm that has deeper rooting habit. Deeper rooting helps the plant withstand lodging and leads to higher yields.
Environmental factors affecting on root morphology
Many Japanese cultivars and Indica-Japonica Hybrid rice were used to assess the morphological root system using seed pack growth pouches and hand-maded root boxes. Moreover, Monolith method was used to investigate the root morphology under the field condition. In this experiment, significant differences on the root system were recognized. Root thickness and spreading angle were controlled by genetic factors, and amount of root dry weight would be changed by different method of fertilizer applications or different method of agronomical technique (Kujira,1989, 1990, 1992).
Effect of calcium sulfate addition on root system and methane evolution
Nitrogen was applied to the rice as a basal dressing at a rate of 40
kg-N ha-1 of LPSS 100 and 20 kg-N ha-1 of LP70 which
meant using polyolefin-coated (urea:40%) slow release fertilizer. No-tillage
and traditional transplanting treatment were used with Koshihikari and
Bluebonnet. Each experimental plot was divided into two plots and calcium
sulfate (mined gypsum)was applied at 0 (control) and 400 kg ha-1.
The application of calcium sulfate to no-tillage plot, compost plus LP
fertilizer plot and LP fertilizer plot of Koshihikari reduced methane evolution
by 53%, 35% and 78%, respectively, compared to the control on Aug. 4(Kujira
et al, 1996). Also, the addition of gypsum reduced methane evolution by
76% in the plot of Bluebonnet compared to control. Methane reduction due
to calcium sulfate appeared to be greatest during the mid-growing season
in rice (Lindau, 1994). The same results was shown in this experiment.
The ratio of deeper root (below a 10 cm depth) dry weight per total root dry weight was increased with gypsum application (Kujira et al. 1996).
To get the stable yield of rice under the different cultivation technique, it is necessary to promote the root growth from surface to deeper layer in the soil. There is a significant relationship between root dry weight within a surface an yield under the limited yield of 6t ha-1 (Kawada et al., 1978). We suggest that an integrated germplasm enhancement and agronomic management of paddy production could lead to sustained increased yields and environmental protection.
References