Photoacoustic characteristics of leaves of atrazine-resistant weed mutants

The photosynthetic characteristics of leaves of atrazine-resistant and-susceptible biotypes of several weed species (Solanum nigrum, Senecio vulgaris, Epilobium ciliatum and Chenopodium album) were compared using the photoacoustic method. Analysis of the dependence of the photoacoustic signal of the modulation frequency indicated that, in Solanum, Epilobium and Senecio, the relative quantum yield of O₂ evolution (estimated by the ratio of the amplitude of the O₂ signal, A_OX, to that of the photothermal signal, A_PT) was substantially reduced in the atrazine-resistant mutant, without any changes in the O₂ diffusion characteristics of the leaves. In contrast, in Chenopodium, atrazine-resistance was associated with a concomitant change in and in the leaf diffusion parameters. This latter change suggests that the leaf internal anatomy was modified in the resistant Chenopodium. Measurements of the Emerson enhancement indicated that the reduction of observed in the atrazine-resistant mutants was caused by a marked decrease in the photochemical potential of PS II (). The study of the light intensity dependence of the A_OX/A_PT ratio showed that saturation of O₂ evolution occurred at the same light level (around 2000 mol m^-2 s^-1) in both types of plants. However, the relative maximal rate of O₂ evolution was slightly lower (-10%) in the atrazine-resistant biotype as compared to the wild type. Reduced and light-saturated rate of O₂ evolution were also measured in atrazine-resistant weed biotypes using a conventional Clark-type O₂ electrode.Abbreviations A_OX modulated O₂ evolution component of the photoacoustic signal - A_PT photothermal component of the photoacoustic signal - Atrazine 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine - E Emerson enhancement - PS II and PS I photosystems II and I, respectively - Q_A primary electron acceptor of PS II - Q_B secondary electron acceptor of PS II - quantum yield of O₂ evolution