Investigating the suitability of chlorophyll fluorescence imaging for determining the mode of action of herbicides, phytotoxicity, and the effect of adjuvants

Chlorophyll fluorescence is light re-emitted from a plant, to expel excess energy not being used for photochemistry. Chlorophyll fluorescence imaging (CFI) is a well-established method for the analysis of plant photosynthetic health and any perturbations. Herbicides, the most used type of pesticide, can have several possible modes of action (MOA), some of which affect the photosynthetic reactions of a plant, either directly or indirectly. As all plant processes ultimately depend on photosynthesis, any disruption to metabolisms will affect chlorophyll fluorescence. Therefore, the effects of herbicides should be detectable using CFI. With only a single new herbicidal MOA being introduced in the past 30-40 years, and with growing numbers of herbicidal resistant weeds and a need to increase food production for the growing global population, novel tools to screen for new MOAs are needed. The present research successfully showcased the potential utility of CFI as a tool in herbicidal research. Study 1 utilised Lemna minor to examine CFI as a high-throughput MOA screening tool. The results demonstrated that CFI was an extremely effective tool for detecting herbicides with a MOA targeting photosystem II (PSII), producing conclusive results within 24-hours. Study 2 utilised three important terrestrial species, to determine the efficacies of herbicides, and the effect of adjuvants, to gain a deeper understanding of the potential information that can be obtained from CFI. This study demonstrated that CFI can detect the effects of a range of MOAs on plants, not just those targeting PSII. Both studies found that CFI was able to detect the effects of adjuvants on the efficacies of herbicides. Future research should use CFI alongside traditional methods. The present work provides a key developmental step in producing a methods basis for the utilisation of CFI for MOA determination, with key benefits being rapid generation of results with minimal costs.