The first objective of the study was to investigate whether measurement of leaf fluorescence can predict the inhibition of plant growth by the herbicide metoxuron. The second objective was to investigate whether fluorescence can be used as a tool for optimizing adjuvants for metoxuron. Metoxuron inhibits photosynthesis and as a consequence increases leaf fluorescence. Fluorescence was used to monitor metoxuron activity shortly after foliar application to common lambsquarters (Chenopodium album L.), black nightshade (Solatium nigrum L), and pea (Pisum sativum L.).

Varying the dose of commercially formulated metoxuron (wettable powder) demonstrated that fluorescence, measured 3 h (for common lambsquarters and pea) or 24 h (all species) after treatment, correlated well with the reduction of fresh weight determined 8 to 14 days after treatment. In order to optimize an emulsifiable rapeseed oil for metoxuron, we varied the concentration of emulsifier (polyoxyethylene C₁₃/C₁₅ oxo alcohol) in the oil (2.5, 10, and 40% w/v) and the concentration of emulsifiable oil in the spray solution (0.001, 0.01, 0.1, and 1% v/v). Generally, the concentration of the oil adjuvant in the spray solution was much more relevant to metoxuron activity than the emulsifier content of the oil. In common lambsquarters and in black nightshade the lowest concentration of oil tested (0.001%) was effective. In pea, the oil was effective at 0.01% and above. The fluorescence measurements with the metoxuron-adjuvant combinations correlated well with the influence of these combinations on the reduction of fresh weight of the species.

We conclude that fluorescence can be a rapid, accurate, and time-saving tool for the comparison of adjuvants or formulations for photosynthesis-inhibiting herbicides.