Herbicide Resistant Weeds

Effects of isoproturon on photosynthesis in susceptible and resistant biotypes of Phalaris minor and wheat.

Singh, S., R. C. Kirkwood, and G. Marshall. 1997. Effects of isoproturon on photosynthesis in susceptible and resistant biotypes of Phalaris minor and wheat. Weed Research. 37: 315-324.

Wheat (cv. WH-147) and five biotypes of Phalaris minor (KR-1, H-4, K-2, H-2 and J-1) were treated with isoproturon in controlled environmental conditions to assess their level of resistance. Resistance of P. minor to isoproturon was found in the order of KR-1 > H-4 > K-2 > H-2 = J-1. Compared with the susceptible (S) biotype (H-2), the resistant (R) biotypes (KR-1, H-4 and K-2) of P. minor required 13.0, 4.5 and 2.7 times higher concentrations of isoproturon for a 50% reduction in growth (GR50) and 2.4 times that of the S biotype (H-2) by wheat. The corresponding figures for KR-1, H-4, K-2 biotypes and wheat were 18, 4.1, 2.4 and 4.6 times based on dry weight reduction. The effect of isoproturon on photosynthesis was studied in vitro using five biotypes of P. minor and in vivo with wheat, KR-1 (R) and H-2 (S) biotypes of P. minor. Under in vitro treatment conditions, isoproturon inhibited the photosynthesis of all five P. minor biotypes, whereas in vivo the recovery was greater in the R biotype than in the wheat and the S biotype. Effects on chlorophyll fluorescence were also measured in wheat and the KR-1 (R) and H-2 (S) biotypes of P. minor. A 4-h treatment of excised leaves incubated in isoproturon solution (0.025 and 0.05 mM concentration) resulted in a decreased fluorescence coefficient (Fv/Fm ratio, in which Fv = variable fluorescence (Fm - Fo); Fm = the maximum fluorescence and Fo = initial fluorescence) in wheat and both biotypes of P. minor. The recovery was, however, greater in the R biotype than in wheat and it was completely recovered within 24 h. No recovery was recorded in the case of the S biotype of P. minor, and a greater recovery time was required for wheat than the R biotype. The higher concentration required for growth inhibition in the R biotype and rapid recovery of oxygen evolution and fluorescence coefficient under in vivo conditions, together with the absence of selectivity in vitro, suggests that the target site was unaffected. It can be conjectured that resistance to isoproturon is most probably because of enhanced metabolism or sequestration of isoproturon, resulting in decreased target site delivery.