The Effect of PSII Inhibitors on Kautsky Curve and Chlorophyll Fluorescence in Common Lambsquarters (Chenopodium album L.) and Common Purslane (Portulaca oleracea L.)

Desmedipham phenmedipham ethofumesate, phenylcarbamates benzofuranyl alkanesulfonate herbicides, is widely used for post-emergence broad-leaved weed control in sugar beet. Chloridazon, a pyridazinone herbicide, is used as a pre- and post- emergence herbicide in sugar beet. Desmedipham, phenmedipham and chloridazon, are photosystem II (PSII) inhibitors, their translocation via xylem are slow, mostly absorbed not only by roots, but also by foliage. Their mode of action is through the blocking of electron transfer between the primary and secondary quinones (QA and QB) of PSII by binding to the QB-binding site and accepting electrons from QA in the chloroplasts. Measures of changes to the chlorophyll fluorescence induction curve (Kautsky curve), is a rapid, non-invasive and simple method for monitoring the physiological status of the photosynthetic apparatus in the plant. There are three phases found on the O, J, I and P steps. These phases primarily point out photochemical events relevant to PSII. The three phases are described as follows: at the O-J phase complete reduction of the primary electron acceptor QA of PSII takes place from 50 μs to 2 ms, the J-I phase corresponds to electron transfer from QA to QB happens between 2 to 30 ms and the I-P phase corresponds to the release of fluorescence quenching by the oxidized plastoquinone pool taking place within 30-500 ms.
In order to determine how exposure affects the fluorescence induction curve (Kautsky curve) and its parameters, two dose-response experiments carried out for chlorophyll fluorescence measuring. The treatments involved desmedipham phenmedipham ethofumesate at 0, 51.38, 102.75, 205.5, 308.25, 411, 616.5 and 822 g a.i. ha-1 and chloridazon at 0, 81.25, 162.5, 325, 650, 1300, 1950 and 2600 g a.i. ha-1 on common lambsquarters (Chenopodium album L.) and common purslane (Portulaca oleracea L.) at the research glasshouse of Agricultural Faculty of Ferdowsi University of Mashhad, Iran. Spraying was performed by overhead trolley sprayer (Matabi 121030 Super Agro 20 litre sprayer), 8002 flat-fan nozzle at 300 kPa and a spray volume of 200 Lha-1. The plants were treated at 21 days (at the four- to six-true leaf stage) after planting. Chlorophyll fluorescence measurements were carried out on dark-adapted leaves at the same stages of development among pots. Fluorescence emissions were measured using a portable chlorophyll fluorometer (Handy-PEA), which emits light of 650 nm wavelength with an intensity of 3000 µmol photons m−2 s−1 for 10s. Leaves were dark adapted for a minimum of 30 min prior to measurement. The fluorescence measurements were taken 4 hours after spraying (HAS) for common lambsquarters and common purslane and again at 24, 48, 72 and 168 HAS. The Kautsky curves were visually examined for the effects of time and dose by the BIOLYZER program with OJIP steps as fix points.
Results The results showed that the parameters of measured had different sensitivity to the herbicides application, So that, four hours after desmedipham phenmedipham ethofumesate application on weeds mentioned, maximum quantum efficiency of PSII (Fv/Fm), did not show any change whereas the relative change in J stage fluorescence (Fvj) and the area between the curve and Kautsky (Area) Fm strongly reduced during this period, but chloridazon usage was not changes on Kautsky curve and their parameters during this period due to lower solubility, and sediment deposition in the sprayer tank mix. Common purslane was more affected by desmedipham phenmedipham ethofumesate application because of sponge leaves and stems while common lambsquarters had lesser sensitivity to both of herbicides application due to powdery and white cover over leaves and stems. Among chlorophyll fluorescence parameters measured, Fvj was appropriate parameter and quicker to detect the herbicides effects on both of experiments plants.
In conclusion, various fluorescence parameters can be used to describe the shape and change of Kautsky curves in different plant species. In this paper we focused on common fluorescence parameters for two tested herbicides. The parameter Fv/Fm seems to be less sensitive to detecting changes than are Fvj and area. Since chlorophyll fluorescence parameters were affected after herbicide application immediately; therefore, it could be used as a practical tool for assessing the efficacy of herbicides in the early hours after spraying herbicides in the greenhouse and field experiments.