Abstract
The present work studies the effects of cold on photosynthesis, as well as the involvement in the chilling stress of chlororespiratory enzymes and ferredoxin-mediated cyclic electron flow, in illuminated plants of Hibiscus rosa-sinensis. Plants were sensitive to cold stress, as indicated by a reduction in the photochemistry efficiency of PSII and in the capacity for electron transport. However, the susceptibility of leaves to cold may be modified by root temperature. When the stem, but not roots, was chilled, the quantum yield of PSII and the relative electron transport rates were much lower than when the whole plant, root and stem, was chilled at 10°C. Additionally, when the whole plant was cooled, both the activity of electron donation by NADPH and ferredoxin to plastoquinone and the amount of PGR5 polypeptide, an essential component of the cyclic electron flow around PSI, increased, suggesting that in these conditions cyclic electron flow helps protect photosystems. However, when the stem, but not the root, was cooled cyclic electron flow did not increase and PSII was damaged as a result of insufficient dissipation of the excess light energy. In contrast, the chlororespiratory enzymes (NDH complex and PTOX) remained similar to control when the whole plant was cooled, but increased when only the stem was cooled, suggesting the involvement of chlororespiration in the response to chilling stress when other pathways, such as cyclic electron flow around PSI, are insufficient to protect PSII.
Highlights
Plants are frequently exposed to environmental stress, both in natural and horticultural conditions, and as a result growth may be affected
The relative electron transport rate fell below the values predicted by the optimum line when the PSII quantum yield decreased because the light became excessive
A saturated rate was reached, which represents the photosynthetic electron transport capacity. This capacity, as well as the PSII quantum yields and the relative electron transport rates, were lower in plants exposed to cold temperatures than in control plants, the differences with respect to the control values were greater in plants exposed to 10°C/24°C than in those exposed to 10°C/10°C
Summary
Plants are frequently exposed to environmental stress, both in natural and horticultural conditions, and as a result growth may be affected. Understanding the physiological processes that underlie stress injury and the tolerance mechanisms of plants to environmental stress is of immense importance for both horticulture and the environment. Tolerance to stress results from integrated events occurring at all organization levels, from the anatomical and morphological to the cellular, biochemical and molecular levels. Plants alter their metabolism in various ways to accommodate environmental stress, photosynthesis being one of these ways. Hibiscus rosa-sinensis is a flowering plant species widely grown as an ornamental throughout the tropics, subtropics and in temperate regions. Many plant species of tropical and PLOS ONE | DOI:10.1371/journal.pone.0137472. Many plant species of tropical and PLOS ONE | DOI:10.1371/journal.pone.0137472 September 11, 2015
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