How photosynthetic light phase of spiny cocklebur (Xanthium spinosum L.) changes during times after herbicide application

Introduction

Among all herbicides which are used in crop protection, nearly 50% affect the chloroplast. Several herbicides such as benzothiadiazinones are known as inhibitors of photosystem II (PSII) activity (Cobb and Read 2010). Bentazon (Basagran, SL48%) which belongs to the benzothiadiazinones is a very strong inhibitor (Grumbach 1982). This herbicide is used for the control of broadleaf weeds in crops such as corn and soybean (Cobb and Read 2010). Apart from inhibiting electron transport, many herbicides have more than one side of action. Photosystem II inhibitor herbicides like bentazon also alter chloroplast ultrastructure and pigment composition (Grumbach 1982). Nicosulfuron (Cruz, SC4%) is a sulfonylurea herbicide being considered for registration for post-emergence weed control in crops such as corn (Williams and Gilham 1990). Sulfonylureas are a class of herbicides that inhibit the activity of acetohydroxyacid synthase/acetolactate synthase and decrease the synthesis of valine, leucine and isoleucine as the branched- chain amino acid (Ray 1984; Cobb and Read 2010).

Many stresses such as herbicide application on plants can affect the photosynthetic activity of leaves and change the ChlF parameters )Oukarroum et al. 2007; Kalaji and Guo 2008; Hassannejad et al. 2020). ChlF analysis as a rapid, non-intrusive  and  inexpensive  method,  provides

detailed information on the energy flow in the photosynthetic apparatus (Oukarroum et al. 2007). This method can be used for understanding the stressful effects of herbicides on weeds (Hassannejad et al. 2020). Thus, the purpose of this experiment was to evaluate the photosynthetic light phase by JIP-test of chlorophyll a fluorescence in spiny cocklebur (Xanthium spinosum L.) plants as a noxious weed species present in corn (Zea mays L.), soybean (Glycine max L.), and some other summer crops after post-emergence application of nicosulfuron and bentazon herbicides.

Materials and Methods

Plant Materials and Growth Conditions

A pot experiment using a randomized complete block design with three replications was conducted in the glass greenhouse conditions under natural light (27-32 °C) in the University of Tabriz in 2017 to assess the effect of nicosulfuron and bentazon herbicides on spiny cocklebur (X. spinosum L.) chlorophyll a florescence (ChlF) parameters and PSII activity. Fifteen seeds of spiny cocklebur were sown in each plastic pot (20 × 20 cm) containing 1.0 kg of perlite at a depth of 1 cm and then tap water (0.6 dS m^-1) was added to achieve 100% field capacity. Plants were thinned to 5 plants per pot, after seedling establishment. The losses of pots were made up with Hoagland solution (Electrical conductivity =1.3 dS m^-1 and pH= 6.5-7) and every 20 days, water was added to prevent further increase in the electrical conductivity. Herbicides were applied at the 4-5 leaf stage of spiny cocklebur at the recommended dose in the field as 2 L.ha^-1.

      ChlF parameters from the upper surface of spiny cocklebur leaves were monitored with a handy-PEA portable fluorometer (Hansatech, UK) after 12, 36, 60 and 84 hours of herbicide application. This device has software for calculation, numerical presentation and memorization of chlorophyll a fluorescence parameters (Table 1).

      All the data were analyzed based on the experimental design, using SAS 9.1 software. The means of each trait were compared using Duncan’s multiple range test at p≤ 0.05.

Results

Application of nicosulfuron and bentazon increased initial fluorescence (F0) of plants. Bentazon had a higher effect on F0 at all times after application as compared with nicosulfuron. However, in most cases, especially at 12 and 84 hours after application of nicosulfuron, the effect of this herbicide on F0 was similar to the control treatment. Maximum F0 was observed at 36 hours after bentaznon application (Figure 1a). Maximum fluorescence (Fm) was only affected by bentazon and the effect of nicosulfuron on Fm was not significant. Fm significantly declined with increasing the time after bentazon application, so that the lowest amount of this variable was observed at 84 hours (Figure 1b).

Table 1. ChlF parameters measured on spiny cocklebur after herbicides application.

Figure 1. Changes in F0 (a) and F0 (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

Fv/Fm was not affected by nicosulfuron, while significantly decreased by bentazon. As the time after bentazon application increased, the amount of Fv/Fm declined, so that the lowest Fv/Fm was obtained at 84 hours after bentazon treatment (Figure 2a). Bentazon harmed Fv/F0, but nicosulfuron only slightly influenced this variable. Treated plants by nicosulfuron had lower Fv/F0 than that of control plants and this reduction started at 36 hours after the treatment. However, Fv/F0 decreased significantly with increasing the time after bentazon application. Thus, the minimum amount of Fv/F0 was observed at 84 h after bentazon application (Figure 2b).

Variable fluorescence (Fv) was reduced after  

Figure 2. Changes in Fv/Fm (a) and Fv/F0 (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

bentazon application, and this reduction continued with increasing the time after the herbicide application (Figure 3a). Area slightly increased by the application of nicosulfuron, however, this increase was not significant at 12 and 60 hours after herbicide application. But, at 36 hours after bentazon application, Area decreased substantially (Figure 3b).

Sm and Sm/Tfm were significantly increased with the nicosufuron application. But, bentazon reduced Sm and it was not recovered after bentazon application (Figure 4a, b). Sm declined after 36 hours of the bentazon treatment and then remained unchanged (Figure 4a). However, the effect of bentazon on Sm/Tfm was not significant (Figure 4b).

The time taken to reach F_m (Tfm) was only affected by bentazon. Tfm in the bentazon treated plants at 12 hours after application was higher than that of other treatments; this variable was strongly reduced at 36 hours after herbicide application and then remained constant (Figure 5a). Photosynthesis relative vitality (PI) as an important chlorophyll a fluorescence parameter was strongly decreased by betnazon. PI in treated plants at 12 hours after bentazon application was close to zero and at 36 hours after this herbicide application had no activity. The negative effect of

Figure 3. Changes in Fv (a) and Area (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

Figure 4. Changes in Sm (a) and Sm/Tfm (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

nicosulfuron on PI was observed at 36 hours after

the herbicide application and after that time, this variable was not recovered significantly (Figure 5b).

Application of nicosulfuron after 12 h, caused a slight increase in K_P but, after 36 h, it decreased K_P in plants. However, application of bentazon after 12 h strongly decreased K_P, and this reduction was more evident with increasing the time after bentazon application (Figure 6a). Application of nicosulfuron did not affect K_N, however, bentazon strongly increased K_N after 12 h of application and this enhancement was greater at 84 h after the bentazon treatment (Figure 6b).

The effect of both nicosulfuron and bentazon herbicides on other ChlF parameters was presented as a spider plot graph in Figure 7. Bentazon increased Fv at 2 ms (Vj) and especially increased the absorption of ABS/RC, and with increasing the time after bentazon application, these effects were more pronounced. In contrast, this herbicide decreased ET₀/RC, ѱ_E0, φ_E0 and φ_R0.

Figure 5. Changes in Tfm (a) and PI (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

Figure 6. Changes in K_P (a) and K_N (b) of spiny cocklebur (Xanthium spinosum L.) after 12, 36, 60 and 84 hours of nicosulfuron and bentazon herbicides application.

However,   the   application   of  nicosulfuron did not affect the JIP parameters.

The lethal effects of an herbicide in higher plants depend on the particular side at which a physiological reaction is inhibited in a plant cell and its compartments. In the chloroplast, there are two main target sides of the herbicide action. One target is represented by the electron transport chain with its electron carriers and enzymes which are involved in phosphorylation and NADP photo-reduction. Another main target of the herbicide action is the biosynthesis of chlorophylls and carotenoids that are contained in the light-harvesting complex and the antennae of the photosynthetic reaction centers (Hassannejad et al. 2020; Baghbani et al. 2019). In our research application of bentazon in comparison with nicosulfuron strongly influenced PSII activity of spiny cocklebur plants. Bentazon enhanced F0 but decreased  Fm  of   plants   Figure 1). F0    is    the

Figure 7.Spider plot presenting the JIP-test parameters calculated from nicosulfuron (a) and bentazon (b) treated spiny cocklebur (Xanthium spinosum L.) plants.

fluorescence    level    when    plastoquinone  (PQ)

electron acceptor pool is fully oxidized and it may change upon exposure to stresses (Fracheboud et al. 2004). An increase in F0 can be interpreted as a reduction of the rate constant of energy trapping by PSII centers, which could be the result of a physical dissociation of light-harvesting complex from PSII core observed in several plant species under environmental stresses (Rong-hua et al. 2006). Reduction in Fm under bentazon application (Figure 1b) may have caused by the inhibition of electron transport at the donor side of the PSII (Hassannejad et al. 2020), which resulted in the Tfm decrease (Figure 5a).

Application of bentazon destructed the reaction centers of PSII in the treated plants (photo-chemically active), thus electron transport capacity in PSII and the number of quanta absorbed per unit time decreased. Bentazon treatment not only decreased protein kinase activity but also destructed D₁ protein level and correspondingly, Fv/Fm, electron transfer rate (ETR) and photosynthetic rate were reduced (Figure 2a). Fv/F0 is the most sensitive component in the photosynthetic electron transport chain (Hassannejad et al. 2020). The decrease in Fv/F0 (Figure 2b) as a consequence of bentazon application, results from photosynthetic electron transport destruction, which affects the Sm/Tfm parameter in plants (Figure 4b). Bentazon inhibits the electron transfer rate at the donor side of PSII and the electron transportation from reaction centers to the plastoquinone pool and decreased Area (Figure 3b). Decreasing in Sm (Figure 4a) was the consequence of a decrease in Area (Figure 3b) and Fv (Figure 3a) under application of bentazon. Application of bentazon impaired both light and dark reactions of photosynthesis as a result of the reduction in PI (Figure 5b). Decreasing  in PI  may  be  related  to