Documentation, characterization, and proposed mechanism of diquat resistance in Landoltia punctata (G. Meyer) D. H. Les and D. J. Crawford

Trials conducted with an accession of landoltia [Landoltia punctata (G. Meyer) D.H. Les and D.J. Crawford] collected from a canal in Lake County, FL, showed developed resistance to diquat [resistance factor (Rf) of 50], and cross resistance to paraquat (Rf 29). Diquat resistance in landoltia was independent of photosynthesis, and the response to diquat was delayed compared to that of a biotype with no history of herbicide treatment (S). Less diquat was likely transported into the protoplast. Whole plant uptake of 14C-diquat was also inhibited and reduced in resistant (R) compared to the S biotype in both light and dark conditions, although differences in uptake could not explain the 50-fold resistance. Data indicate that less diquat was absorbed by the R biotype in light treatments, although the percentage difference between the two biotypes was minimal (<20%) and not consistent across all experiments, possibly due to differential diquat accumulation in the cytoplasm. Copper applied in combination with diquat overcame the diquat resistance in landoltia, and increased susceptibility of the R biotype to that of the S biotype. Copper probably increased permeability of the cell membrane to diquat by causing the formation of non-specific ion channels, as electrolyte leakage from copper exposure alone was rapid (<1 hr). Therefore, copper may alter the transport mechanism for diquat across the plasmalemma or open a secondary site for transport. Copper reportedly reacts with calcium (Ca2+) channels in the plant cell membrane creating non-specific ion channels. Diquat activity was reduced on the S biotype of landoltia preconditioned with the Ca2+ channel blocker lanthanum chloride (Rf = 86). This reduction of activity suggests that lanthanum was effective at inhibiting transport of diquat to the site of action, although lanthanum could also alter membrane permeability and evolution of radical oxygen. The relationship among copper, diquat, calcium channels, and lanthanum warrants further study as it relates to diquat transport and potential resistance mechanisms. These studies have documented the first aquatic plant to become resistant to the bipyridylium herbicides, and suggest that the resistance mechanism is related to reduced herbicide transport across cell membranes.