Influence of foliar absorption, herbicide metabolism, light intensity, irrigation, and porphyrin biosynthesis on carfentrazone-ethyl selectivity

Abstract

Carfentrazone-ethyl is a new post emergence herbicide for broadleaf control in com and wheat. Low use rates (0.009 kg ha-1) of carfentrazone inhibit protoporphyrinogen oxidase in the chlorophyll biosynthesis pathway, which results in increased levels of protoporphyrin IX and production of singlet oxygen species in the presence of light. Plant death occurs rapidly due to membrane disruption and desiccation. Research was conducted to determine the mechanism of selectivity of carfentrazone and factors that influence crop safety. Carfentrazone absorption, translocation and metabolism were determined using radiolabelled herbicide in com (Zea Mays), velvetleaf (Abutilon theophrasti), and soybean (Glycine max). Absorption and translocation were not factors contributing to carfentrazone selectivity. Metabolic half-lifes of carfentrazone correlated well with species sensitivity. Half-life of carfentrazone in com was less than 2 h, less than 8 h in soybean, and greater than 24 h in velvetleaf. Field research utilizing shade cloth tents and furrow irrigation was conducted to determine the influence of light intensity and plant water status on carfentrazone crop [com, soybean, and wheat (Triticum aestivum)] response. Com was relatively insensitive to either factor. Carfentrazone caused more injury in shaded, high-moisture soybean and wheat versus unshaded, dryland treatments. The complex mechanism of action of carfentrazone introduces other means that affect a plant's ability to tolerate carfentrazone. Research was conducted to determine the influence of water and nutrient stress on the porphyrin biosynthesis pathway. Protochlorophyllide was measured in com and wheat placed in darkness and protoporphyrin IX was measured in light-pulsed plants following treatment with carfentrazone. Low nutrient treatments reduced protochlorophyllide production, but protochlorophyllide was not influence by water status. Protochlorophyllide levels correlated with visual crop injury. Differences in carfentrazone metabolism appear to explain selectivity among species. Environmental factors affecting flux through the porphyrin pathway can influence the carfentrazone sensitivity within a species.