Based on new experimental data of linear and nonlinear spectroscopic investigations of LHC2 complexes, both the energy transfer and energy relaxation of excitons were studied in the limiting cases of strong pigment-pigment and pigment-protein interactions, respectively. Consequences of the structural basis and the dominance of strong pigment-pigment interactions in monomers were independently established by different optical measurements. In this model, the energy relaxation of excitons was determined by dynamical processes of exciton interactions with the radiation fíeld and the long-wave protein vibrations, respectively, in particular with the mode having the wave number of 43 cm'f
We investigated the physiological effect of night chilling (CN) on potted seedlings of two tropical tree species, Calophyllum polyanthum and Linociera insignis, in Xishuangbanna, southwest China. Seedlings grown under 8, 25, and 50 % daylight for five months were moved to a 4-6 °C cold storage house for three consecutive nights, and returned to the original shaded sites during the day. CN resulted in strong suppression of photosynthesis and stomatal conductance for L. insignis, and reduced photorespiration rates, carboxylation efficiency, and maximum photochemical efficiency of photosystem 2 (PS2) at dawn and midday for both species. CN increased dawn and midday rates of non-photochemical quenching, and the contents of malondialdehyde and H2O2 for both species. CN also induced inactivation or destruction of PS2 reaction centres. The impacts of CN on tropical seedlings increased with the number of CN. Shading could significantly mitigate the adverse effects of CN for both species. After 3-d-recovery, gas exchange and fluorescence parameters for both species returned to pre-treatment levels in most cases. Thus CN induced mainly stomatal limitation of photosynthesis for L. insignis, and non-stomatal limitation for C. polyanthum. C. polyanthum was more susceptible to CN than L. insignis. Fog, which often occurs in Xishuangbanna, could be beneficial to chilling sensitive tropical seedlings in this area through alleviating photoinhibition or photodamage by reducing sunlight. and Y.-L. Feng, K.-F. Cao.
Progressive microwave power saturation (P1/2) measurements have been performed on the tyrosine D radical (YD*) of photosystem II (PSII) in order to examine its relaxation enhancement by the oxygen-evolving complex (OEC) poised to the reduced S-1 and S-2 oxidation states by NO treatment. Analysis of the power saturation curves showed that the S-1 oxidation state of the OEC does not enhance the relaxation of YD*: it therefore possesses a diamagnetic ground state. In contrast, the Mn(II)-Mn(III) multiline electron paramagnetic resonance (EPR) signal characteristic of the S-2 oxidation state of the OEC was shown to provide a relaxation enhancement pathway for YD*, however less efficient relative to the one provided by the S2-state multiline EPR signal. We also examined the YD * relaxation enhancement characteristics of the EPR-silent oxidation state produced after brief (1-5 min) dark incubation at 0°C of a PSII sample poised to the EPRactive S-2 state. This EPR-silent oxidation state denoted as "0°C incubation" state was shown to possess remarkably similar P1/2 values with the EPR-active S-2 state in the overall examined temperature range (6-20 K). In addition, these values remained unchanged after successive cycles of the OEC between the EPR-active S-2 state and the "0°C incubation" state. The data presented in this work point to the conclusion that the "0°C incubation" state is indeed an S-2 oxidation state with half-integer spin.
Twelve-day-old barley seedlings were supplied with 23 μM methyl jasmonate (MeJA) or 10 μM paraquat (Pq) via the transpiration stream and kept in the dark for 24 h. Then they were exposed to 100 μmol m-2 s-1 PAR and samples were taken 1, 2, 3, and 6 h after irradiation. Treatment of seedlings with MeJA alone resulted in decreased content of chlorophyll (Chl), and net photosynthetic (PN) and transpiration rates. Pq treatment led to a decrease in Chl content and to a very strong inhibition of PN, the effects were manifested by 1 h of irradiation. Pq treatment did not affect the activity of ribulose-1,5 bisphosphate carboxylase (RuBPC, EC 4.1.1.39) but increased the activity of the photorespiratory enzymes phosphoglycolate phosphatase (PGP, EC 3.1.3.18), glycolate oxidase (GO, EC 1.1.3.1), and catalase (EC 1.11.1.6). Pre-treatment of seedlings with MeJA before exposure to Pq fully blocked the inhibitory effect of Pq on photosynthesis and protected against subsequent Pq-induced oxidative damage. and V. A. Hristova, L. P. Popova.