Taming the world's worst weed: population dynamics, flooding adaptation mechanisms and management of purple nutsedge (Cyperus rotundus L.) in rice-vegetable cropping systems

Purple nutsedge (Cyperus rotundus L.), a major weed infesting dryland crops, is increasing in populations and now threatening to be a major weed problem in flooded rice (Oryza sativa L.) grown in rotation with dryland crops. Apparently, the continuous wet-dry rotation pattern over the years has selected for an ecotype of purple nutsedge that can grow in flooded soil. The wetland purple nutsedge plants are about 50 percent taller and bigger than the dryland plants and produce bigger tubers. DNA studies using RAPDs technique showed genetic variations between wetland and dryland ecotypes. Seven-day old germinating purple nutsedge seedlings subjected to hypoxia showed down regulation of alcohol dehydrogenase and pyrubate decarbosylase, enzyme of ethanol fermentation, the pathway commonly used in oxygen-deficient plants. Down-regulation of ADH and PDC during anaerobic respiration, coupled with high amylase activity and high soluble sugar content in tubers of wetland plants prior to germination appear to be the main adaptation mechanisms of the wetland ecotype to flooding. Results of on-farm studies for 8 years showed that indirect control strategies (cultivation, tillage) aimed at reducing tuber survival are more cost-effective than direct control strategies aimed at reducing the production rate of new tubers (herbicides, handweeding). The former requires an understanding of the weeds biology and population dynamics of its shoots and tubers. Stale seedbed technique, a cultural control strategy which makes use of the principles of tuber dormancy and sprouting, reduced tuber and short populations faster and used less inputs than current farmers' practices using herbicides and handweeding. This results in higher net profits for farmers in small, highly diversified tropical farms like those in the Philippines, where 40 percent of its rice-growing areas are rainfed rice-based multi-crop rotation systems. Results also support the new paradigm shift from single season to multi-season approaches aimed at reducing input costs in long-range management of weed populations.