Maurice Ouko, Folkard Asch, Mathias Becker:
Nitrate Reductase Activity in Rice as Related to Weed Competitiveness


University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany

Traditional and improved rice cultivars (Oryza sativa L.) belonging to two groups of Asian rice (japonica-upland and indica-lowland), were studied for weed competitiveness. Rice was grown under controlled conditions in hydroponics using a modified Yoshida nutrient solution. As an indicator for weed competitiveness we used the activity of the enzyme nitrate reductase (NRA) which allows the rice to take up nitrogen under aerobic upland conditions. NRA of the 4 rice genotypes was tested under different nitrogen nutrition regimes representing upland (40 ppm of NO3 ), lowland (40 ppm NH4 ) and hydromorphic (20ppm NH4 and 20ppm NO3 ) rice cropping systems.

Rice varieties appeared to be adapted to the ecological conditions for which they were selected. The upland adapted genotypes showed higher NRA under NO3 nutrition and little NRA with NH4 nutrition whereas the lowland adapted varieties showed relatively low NRA with both NH4 nutrition and NO3 nutrition.

Improved lowland types showed low NRA when grown in either NH4 or NO3 mediums, however very high levels of enzyme activity were observed for improved upland adapted types.

Lowland weed types such as Echinochloa crus-pavonis and Crotolaria spp. show limited NRA under NH4 or NO3 nutrition, with slightly higher activity observed when the medium contained both NO3 and NH4 . Those weeds adapted to upland conditions such as Euphorbia heterrophyla and Zea mays showed relatively high NRA in NO3 medium as compared to NH4 . However, in all cases the NRA of the traditional rice cultivars was higher than that of the weeds. The NH4 NO3 treatment resulted in highest NRA for the broad leafed weeds, while the in the narrow leafed varieties NRA responded to NO3 treatments. All enzyme activity was measured in vivo in leaf tissue.

In all cases, higher RNA resulted in greater accumulation of plant biomass indicating better nutrient acquisition efficiency. This study concludes that it is possible to relate leaf NRA to plant biomass accumulation, giving NRA the potential of a screening tool for weed competitiveness in rice.

Keywords: Enzyme activity, rice cropping systems, screening, selection, upland weeds


Contact Address: Maurice Ouko, University of Bonn, Plant Nutrition in the Tropics and Subtropics, Am Jesuitenhof 1/012, 53117 Bonn, Germany, e-mail: maure
Andreas Deininger, 2003