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Field experiments involving eight cultivars were conducted in 1998 and 16 cultivars in 1999 to study the ability of rice ( Oryza sativa L.) to suppress Monochoria vaginalis (Burm. f) Kunth through light competition. Dry weights of M. vaginalis shoots in early season culture exceeded those in normal season culture of any rice cultivars. The relative photosynthetic photon flux density (R-PPFD), which was calculated as the ratio of the photosynthetic photon flux density (PPFD) below the rice canopy to that measured above the rice canopy, varied according to rice cultivar. A strong linear correlation was observed between the mean R-PPFD at 29-35 days after transplanting (DAT) ( r 2 = 0.80; p < 0.01 in 1998; r 2 = 0.63, p < 0.001; and r 2 = 0.93, p < 0.001 in 1999), or 36-42 DAT ( r 2 = 0.66, p < 0.05 in 1998; r 2 = 0.72, p < 0.001; and r 2 = 0.97, p < 0.001 in 1999), and the dry weight of M. vaginalis shoots at approximately 60 DAT. Data from the three experiments could be pooled into one regression line because intercepts and regression coefficients were not significantly different. The r 2 values of the combined regression were highest when R-PPFD was expressed as the mean of measurements taken during 14 days (from 29 to 42 DAT; r 2 = 0.81, p < 0.001). The shortest period for measuring mean R-PPFD in order to obtain a meaningful relationship with M. vaginalis shoot dry weight was 7 days (from 29 to 35 DAT; r 2 = 0.78, p < 0.001). For that same period, relationships between M. vaginalis shoot dry weight at 60 DAT and rice tiller number or leaf area index (LAI) at ground level were weak. However, there were negative relationships between M. vaginalis shoot dry weights at 60 DAT and rice LAI measured 20 cm above the ground, plant heights or rice shoot dry weight, but these coefficients of determination were smaller than those calculated by R-PPFD for the same period. Thus, the ability of rice to suppress M. vaginalis can be evaluated more accurately by measuring mean R-PPFD below the rice canopy for 7 days (from 29 to 35 DAT) than by measuring rice LAI, plant height and shoot dry weight.

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Dry matter and seed productivity of Sachiyutaka, an early maturing soybean (Glycine max Merr.) cultivar with a short stem, were investigated under four kinds of interrow spacing (30, 40, 60 and 80 cm) compared with a cultivar, Fukuyutaka under 80 cm of interrow spacing as the conventional culture in southwestern Japan. In Sachiyutaka, the numbers of pod and seed per square meter increased by narrow row spaceing of 30 and 40 cm, and consequently, the seed yield was increased by 30 g m-2 compared with Fukuyutaka under conventional culture. The leaf area index (LAI) at fl owering time and that at the seed-fi lling stage greater under narrow-row-dense-planting culture in Sachiyutaka, were as large as 5. 0 4. 4, which were approximately equivalent to those in Fukuyutaka under conventional culture. In addition, the light extinction coeffi cient of 50-cm upper layer was low at the seed-fi lling stage in narrow-row-dense-planting culture, showing excellent light-intercepting characteristics. As a result, maintaining a suffi cient level of LAI at the seed-fi lling stage under the narrow-row-dense-planting condition in Sachiyutaka was considered as a factor to obtain a high seed yield equal to or superior to that in Fukuyutaka under the conventional culture.