Data on cadmium accumulation in chloroplasts of terrestrial plants are scarce and contradictory. We introduced CdSO4 in hydroponic media to the final concentrations 80 and 250 μM and studied the accumulation of Cd in chloroplasts of Hordeum vulgare and Zea mays. Barley accumulated more Cd in the chloroplasts as compared to maize, whereas in the leaves cadmium accumulation was higher in maize. The cadmium content in the chloroplasts of two species varied from 49 to 171 ng Cd/mg chlorophyll, which corresponds to one Cd atom per 728-2,540 chlorophyll molecules. Therefore, Mg(2+) can be substituted by Cd(2+) in a negligible amount of antenna chlorophylls only. The percentage of chloroplastic cadmium can be estimated as 0.21-1.32 % of all the Cd in a leaf. Photochemistry (F v/F m, ΦPSII, qP) was not influenced by Cd. Non-photochemical quenching of chlorophyll-excited state (NPQ) was greatly reduced in barley but not in maize. The decrease in NPQ was due to its fast relaxing component; the slow relaxing component rose slightly. In chloroplasts, Cd did not affect mRNA levels, but content of some photosynthetic proteins was reduced: slightly in the leaves of barley and heavily in the leaves of maize. In all analyzed C3-species, the effect of Cd on the content of photosynthetic proteins was mild or absent. This is most likely the first evidence of severe reduction of photosynthetic proteins in leaves of a Cd-treated C4-plant.

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The effect of high temperature treatment (40 degrees C, 3 h, illumination at 100 micromol m(-2) s(-1)) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of Q(A)(-) reoxidation by Tyr(z)(ox) in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following Q(A)(-) reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with Q(A)(-) reoxidized through recombination with the S(2)/S(3) state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from Q(A)(-) to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.

It has been shown that removal of manganese from the water-oxidizing complex (WOC) of photosystem II (PSII) leads to flash-induced oxygen consumption (FIOC) which is activated by low concentration of Mn(2+) (Yanykin et al., Biochim Biophys Acta 1797:516-523, 2010). In the present work, we examined the effect of transition and non-transition divalent metal ions on FIOC in Mn-depleted PSII (apo-WOC-PSII) preparations. It was shown that only Mn(2+) ions are able to activate FIOC while other transition metal ions (Fe(2+), V(2+) and Cr(2+)) capable of electron donation to the apo-WOC-PSII suppressed the photoconsumption of O2. Co(2+) ions with a high redox potential (E (0) for Co(2+)/Co(3+) is 1.8 V) showed no effect. Non-transition metal ions Ca(2+) by Mg(2+) did not stimulate FIOC. However, Ca(2+) (in contrast to Mg(2+)) showed an additional activation effect in the presence of exogenic Mn(2+). The Ca(2+) effect depended on the concentration of both Mn(2+) and Ca(2+). The Ca effect was only observed when: (1) the activation of FIOC induced by Mn(2+) did not reach its maximum, (2) the concentration of Ca(2+) did not exceed 40 μM; at higher concentrations Ca(2+) inhibited the Mn(2+)-activated O2 photoconsumption. Replacement of Ca(2+) by Mg(2+) led to a suppression of Mn(2+)-activated O2 photoconsumption; while, addition of Ca(2+) resulted in elimination of the Mg(2+) inhibitory effect and activation of FIOC. Thus, only Mn(2+) and Ca(2+) (which are constituents of the WOC) have specific effects of activation of FIOC in apo-WOC-PSII preparations. Possible reactions involving Mn(2+) and Ca(2+) which could lead to the activation of FIOC in the apo-WOC-PSII are discussed.