the decrease in the number of open PSII reaction centers and the
efficiency of energy capture by these open centers. The model of
PSII function proposed by Genty et al. [19] distinguishes between
the Fv/Fm and qP of these centers. The former is a measure of the
supply of energy reaching the PSII reaction centers, and the latter
is a measure of the redox state of PSII. The decrease in PSII is
the product of qP and Fv/Fm. In the experiment, the data clearly
show that a decline in PSII was accompanied by similar decreases
in both qP and Fv/Fm (Fig. 4D–F). This finding confirms that linear
electron passage through PSII was reduced. A rapid downregulation
of PSII photochemical activity plays an indispensable photoprotective
role under LNT in response to the inhibition of photosynthetic
carbon metabolism. Therefore, changes in PSII under LNT are the
consequence rather than the cause of the partial loss of the photosynthetic
capacity. Consequently, the absorbed energy may become
excessive because of the lower energy demand for carbon fixation.
At the same time, the remarkable increase in NPQ indicated
that excessive energy was partly dissipated as heat and that the
photoprotective process of PSII took place (Fig. 4G). The decline
in Fv/Fm is a manifestation of the nonphotochemical quenching
of PSII excitation. These data support the suggestion that the regulation
of PSII efficiency by the nonphotochemical quenching of
excitation energy can operate to a substantial degree to reduce the
excitation energy that reaches the reaction centers of PSII. Garstka
et al. [43] pointed out that subsequent photoactivation can decrease
the photochemical rate in dark-chilled pea and tomato leaves and
that the prolonged photoactivation can cause a partial recovery of
photochemical activity.
A limited number of studies regarding PSI have assessed isolated
thylakoid membranes using artificial electron donors or