COMPARISON OF FLASH-INDUCED LIGHT-SCATTERING TRANSIENTS AND PROTON EFFLUX FROM AUXINIC-HERBICIDE RESISTANT AND SUSCEPTIBLE WILD MUSTARD PROTOPLASTS - A POSSIBLE ROLE FOR CALCIUM IN MEDIATING AUXINIC HERBICIDE RESISTANCE

Comparison of flash-induced light-scattering signals from protoplasts of auxinic herbicide-resistant ('R') and -susceptible ('S') wild mustard biotypes suggest that ATP-dependent activity manifested as a conformational change differs between the biotypes. The signal obtained from R protoplasts is unaffected by 50 mu M picloram, an auxin analogue. In comparison, the signal amplitude from the S protoplasts decreases 40% in 20 mu M picloram and is abolished in 50 mu M picloram. Signal amplitude from the R protoplasts can be decreased by simultaneous incubation with 50 mu M picloram and 50 mu M verapamil, a calcium channel blocker. Incubation of the S protoplasts with the calcium ionophore A23187 in the presence of 0.5 mM calcium reduced the inhibitory effect of picloram. Absorbance spectrophotometry using acridine orange as a pH indicator suggests that picloram affects the proton flux in the S biotype only, Based on these results, we hypothesize that picloram affects the calcium and hydrogen dynamics in the S biotype. Because the scattering characteristics of S protoplasts incubated with picloram can be nearly reproduced in the R protoplasts by simultaneous incubation with picloram and verapamil, calcium may have a significant role in modulating auxinic herbicide resistance. To our knowledge, this is the first report to implicate such a role for calcium.