XANTHOPHYLL CYCLE PATTERNS AND IN-VIVO PHOTOINHIBITION IN HERBICIDE-RESISTANT BIOTYPES OF CONYZA-CANADENSIS

The xanthophyll cycle and in vivo photoinhibition were investigated in the herbicide-susceptible (S), paraquat-resistant (PQ-R), atrazine-re sistant (Atr-R) and paraquat-atrazine co-resistant (PQAtr-R) biotypes of Conyza canadensis using both low-(LLG) and high-light-grown (HLG) p lants. HPLC revealed markedly reduced levels of zeaxanthin under photo inhibitory conditions in atrazine-resistant (Atr-R and PQAtr-R) biotyp es as compared with the S and PQ-R biotypes. The variable chlorophyll fluorescence (F-v) decrease and constant fluorescence (F-o) increase r evealed an enhanced susceptibility to in vivo photoinhibition in Atr-R and PQAtr-R biotypes. LLG plants were more susceptible than HLG plant s to in vivo photoinhibitory treatment. It is suggested that the incre ased susceptibility to photoinhibition in Atr-R and PQAtr-R biotypes o f C. canadensis is a consequence not only of D1 protein mutation, but also possibly of a lower rate of violaxanthin to zeaxanthin de-epoxida tion.