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This work studies the effects of water deficit and heat, as well as the involvement of chlororespiration and the ferredoxin-mediated cyclic pathway, on the tolerance of photosynthesis to high light intensity in Hibiscus rosa-sinensis plants. Drought and heat resulted in the down-regulation of photosynthetic linear electron transport in the leaves, although only a slight decrease in variable fluorescence (Fv)/maximal fluorescence (Fm) was observed, indicating that the chloroplast was protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. The incubation of leaves from unstressed plants under high light intensity resulted in an increase of the activity of electron donation by nicotinamide adenine dinucleotide phosphate (NADPH) and ferredoxin to plastoquinone, but no increase was observed in plants exposed to water deficit, suggesting that cyclic electron transport was stimulated by high light only in control plants. In contrast, the activities of the chlororespiration enzymes (NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX)) increased after incubation under high light intensity in leaves of the water deficit plants, but not in control plants, suggesting that chlororespiration was stimulated in stressed plants. The results indicate that the relative importance of chlororespiration and the cyclic electron pathway in the tolerance of photosynthesis to high illumination differs under stress conditions. When plants were not subjected to stress, the contribution of chlororespiration to photosynthetic electron flow regulation was not relevant, and another pathway, such as the ferredoxin-mediated cyclic pathway, was more important. However, when plants were subjected to water deficit and heat, chlororespiration was probably essential.

RESUMEN

Sun (Chrysanthemum morifolium) and shade (Spathiphyllum wallisii) plants were used to study the effects of drought, heat and high illumination. The stress conditions caused a greater accumulation of hydrogen peroxide in Chrysanthemum morifolium than in Spathiphyllum wallisii leaves. They also resulted in down-regulation of linear electron transport in the leaves of both species, as indicated by a gradual reduction in the photochemistry efficiency of PS II, which was associated with an increase in the non-photochemical quenching of fluorescence. Only a slight decrease in F(v)/F(m) was observed under stress conditions in either plant species, suggesting that the chloroplast is protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. In addition to the effects on photosynthetic activity, changes were also observed by immunoblot analysis in the plastidial NADH DH complex, PTOX and PGR5. The quantities of the PTOX and NDH-H subunit of the thylakoidal NADH DH complex, and the NADH DH activity in the thylakoid membranes were similar in control plants of both species and increased in stressed plants, particularly in Spathiphyllum wallisii. The level of PGR5 polypeptide was higher in Chrysanthemum morifolium than in Spathiphyllum wallisii control plants, while after stress, the quantity of PGR5 increased significantly in Chrysanthemum morifolium and remained constant in Spathiphyllum wallisii. These results indicate that the relative importance of chlororespiration and the cyclic electron pathways in the tolerance to drought, heat and high illumination differs in sun and shade plants, indicating different adaptive mechanisms to the environment. In the conditions studied, the PGR5-dependent cyclic pathway is more active in Chrysanthemum morifolium, a sun species, whereas in Spathiphyllum wallisii, a shade species, other ways involving the NADH DH complex and PTOX are stimulated in response to stress, which results in lower levels of ROS accumulation in the leaves.

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Two species of Brassica were used to study their acclimation to heat and high illumination during the first stages of development. One, Brassica fruticulosa, is a wild species from south-east Spain and is adapted to both heat and high light intensity in its natural habitat, while the other, Brassica oleracea, is an agricultural species that is widely cultivated throughout the world. Growing Brassica plants under high irradiance and moderate heat was seen to affect the growth parameters and the functioning of the photosynthetic apparatus. The photosystem II (PSII) quantum yields and the capacity of photosynthetic electron transport, which were lower in B. fruticulosa than in B. oleracea, decreased in B. oleracea plants when grown under stress conditions, indicating inhibition of PSII. However, in B. fruticulosa, the values of these parameters were similar to the values of control plants. Photosystem I (PSI) activity was higher in B. fruticulosa than in B. oleracea, and in both species this activity increased in plants exposed to heat and high illumination. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed a higher amount of both proteins in B. fruticulosa than in B. oleracea. In addition, PTOX activity in plastoquinone oxidation, and NADH DH activity in thylakoid membranes were higher in the wild species (B. fruticulosa) than in the agricultural species (B. oleracea). The results indicate that tolerance to high illumination and heat of the photosynthetic activity was higher in the wild species than in the agricultural species, suggesting that plant adaptation to these stresses in natural conditions favours subsequent acclimation, and that the chlororespiration process is involved in adaptation to heat and high illumination in Brassica.

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Calcium oxalate (CaOx) crystals in plants may serve as a sink for the absorption of excess calcium, and they could play an important role in heavy metal detoxification. In this study, the effect of heavy metals and different calcium concentrations on the growth of calcium oxalate crystals in leaves of Phaseolus vulgaris was investigated. Different analytical techniques were used to determine the influence of exogenous lead and zinc on CaOx deposition and to detect a presence of these metals in CaOx crystals. We found a positive correlation between the calcium concentration in the nutrient medium and the production of calcium oxalate crystals in leaves of hydroponically grown plants. On the other hand, addition of the heavy metals to the nutrient medium decreased the number of crystals. Energy dispersive X-ray spectrometry did not detect the inclusion of heavy metals inside the CaOx crystals. Our investigation suggests that CaOx crystals do not play a major role in heavy metal detoxification in P. vulgaris but do play an important role in bulk calcium regulation.

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Total soluble phenols, soluble flavanols, (+)-catechin, ferulic acid and 1-O-feruloyl-beta-d-glucose were analyzed during the development of a strawberry (Fragariaxananassa, cv. Chandler) callus culture. The time-course changes of the different phenols assayed were well correlated with callus growth and morphology. The changes in polyphenol oxidase (EC 1.10.3.1-2) and beta-glucosidase (EC 3.2.1.21) activities in the callus were also examined. The total phenol, soluble flavanols and (+)-catechin contents were high during the preexponential and exponential phases of growth. The subsequent decrease in (+)-catechin concentration coincided with high levels of polyphenol oxidase activity. The 1-O-feruloyl-beta-d-glucose content was highest as callus growth ceased, and its subsequent decrease was accompanied by the increased production of ferulic acid. This increase in ferulic acid was accompanied by an increase in beta-glucosidase activity. The ferulic acid content decreased at the end of culture, when callus growth had stopped and showed clear symptoms of senescence. This decrease in the ferulic acid concentration was accompanied by an increase in the levels of ferulic acid bound to cell wall components.