Cadmium accumulation in chloroplasts and its impact on chloroplastic processes in barley and maize.

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.

Lower air humidity reduced both the plant growth and activities of photosystems I and II under prolonged heat stress.

The warming is global problem. In natural environments, heat stress is usually accompanied by drought. Under drought conditions, water content decreases in both soil and air; yet,the effect of lower air humidity remains obscure. We supplied maize and barley plants with an unlimited source of water for the root uptake and studied the effect of relative air humidity under heat stress. Young plants were subjected for 48 h to several degrees of heat stress: moderate (37 °C), genuine (42 °C), and nearly lethal (46 °C). The conditions of lower air humidity decreased the photochemical activities of photosystem I and photosystem II. The small effect was revealed in the control (24 °C). Elevating temperature to 37 °C and 42 °C increased the relative activities of both photosystems; the photosystem II was activated more. Probably, this is why the effect of air humidity disappeared at 37 °C; the small inhibiting effect was observed at 42 °C. At 46 °C, lower air humidity substantially magnified the inhibitory effect of heat. As a result, the maximal and relative activities of both photosystems decreased in maize and barley; the photosystem II was inhibited more. Under the conditions of 46 °C at lower air humidity, the plant growth was greatly reduced. Maize plants increased water uptake by roots and survived; barley plants were unable to increase water uptake and died. Therefore, air humidity is an important component of environmental heat stress influencing activities of photosystem I and photosystem II and thereby plant growth and viability under severe stress conditions.

Function of plastoquinone in heat stress reactions of plants.

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.