Evaluation of the suppression ability of rice (Oryza sativa) on Monochoria vaginalis by measuring photosynthetic photon flux density below rice canopy

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 inter-cepts 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.