4.1 Chlorophyll fluorescence parameters
An increase in the minimum fluorescence of chlorophyll (F0) in both cucumber and wheat plants was observed in this study. This effect could be an indication of the reaction centers degradation in the photosystem II, alteration in structure and content of photosynthetic pigments under stress conditions. Decline in quinone A (QA) capacity and its lower oxidation rate due to the slow electron flow along the electron transport chain and eventually the inactivation of photosystem II can be another reason. When quinone molecules form, the first electron acceptors in photosystem II, are in the oxidized state, the system has the lowest fluorescence. As the quinone reduces, the reaction centers in photosystem II are closed and no more electrons transfer to photosystem I, which gradually leads to an increase in fluorescence. In this enhanced fluorescence mode, the photosystem II centers are in the state of maximum chlorophyll fluorescence (Fm). Increasing F0 under stress conditions will allow the system to reach Fm faster and reduce its efficiency for quinone reduction and electron transfer (Soheili Movahhed et al., 2017). In this study, maximum chlorophyll fluorescence in cucumber increased but in wheat remained constant. Moreover, increasing Fm improved electron transfer state in cucumber. It seems that changes in beta-carotene concentration in cucumber and wheat plants were involved in this observation. Chlorophyll variable fluorescence (Fv) is an indicator to show complete quinone reduction and its increase in cucumber plant confirms the improvement of its photosynthetic status. Reduction of maximum quantum efficiency of photosystem II (Fv/Fm) in wheat plant reveals disruption of electron transfer in photosynthetic electron transfer chain of this plant. The oxidized quinone B (QB) accumulation under such conditions indicates no electron transfer from the reduced QA to the QB. Its reason is not clear well with the available data in the literature and the present investigation, but it seems that the decreasing CO2assimilation due to the stomata closure in wheat plant leads to non-consumption of products derived from photosynthetic electron transfer chains (NADPH/H+ and ATP) and the increasing of the reduced feredoxin. As a result of the accumulation of reduced ferredoxin, free radicals production is stimulated and leads to destruction of thylakoid membrane proteins, which are involved in the photosynthetic electron transport chain. This event reduces the rate of electron transfer, increases the chlorophyll fluorescence, and reduces the function of photosystem II and ultimately destroys the D1 subunit of this photosystem. Excessive opening of cucumber stomata (although is not beneficial for plant in the long period of time because of higher water transpiration) causes hypersensitivity to the redroot pigweed’s allelochemicals. However, stomata opening leading to higher utilization of NADPH/H+ and ATP in the chloroplasts for CO2 fixation, declines the amount of reduced ferredoxin, controls the level of chlorophyll fluorescence, and finally prevents or reduces the production of free radicals. Increasing non-photochemical quenching of chlorophyll (NPQ and SV0) in both plants after treatment by amaranth leachate indicates that the system is under challenge and both plants try to decline the reduced ferredoxin content and consequently the production of free radicals by heat dissipation. According to the obtained results of this study, the rate of non-cyclic electron transfer (ETR) decreased in wheat plant. Therefore, it seems that in this plant the production of reduced ferredoxin and free radicals is enhanced due to closing of the stomata. Then, by increasing the amount of free radicals in the photosynthetic space, D1 subunit of photosystem II is destroyed. As a result, electron transfers to the ferredoxin and its reduction is prevented. Due to the inability to utilize this strategy, the cucumber plant, offers excessive opening of stomata and continues carbon fixation to decline the reduced ferredoxin.