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.