Water Deficit and Heat Affect the Tolerance to High Illumination in Hibiscus Plants

Romualdo Muñoz,María Quiles

doi:10.3390/ijms14035432

Romualdo Muñoz, María Quiles

Open Access

https://doi.org/10.3390/ijms14035432

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Abstract

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.

Highlights

Environmental stresses, such as high irradiance, high temperature and drought, negatively affect photosynthetic processes [1,2,3]
The involvement of the NADH dehydrogenase (NDH)-mediated cyclic electron pathways around PS I in supplying the extra adenosine triphosphate (ATP) required in conditions of mild water stress has been suggested [10]. These results suggested that the NDH complex and plastid-localized terminal oxidase (PTOX) are heat and water stress dependent
Plant water status was estimated by measuring the relative water content (RWC) of leaves in plants at the start of the experiment and immediately after exposure to three photoperiods with reduced irrigation and heat (Figure 1)

Summary

Introduction

Environmental stresses, such as high irradiance, high temperature and drought, negatively affect photosynthetic processes [1,2,3]. Plants are frequently exposed to stress, both in natural and agricultural conditions, and such exposure plays a major role in determining the distribution of plant species across different types of environment and in limiting crop productivity. Understanding the physiological processes that underlie stress injury and the tolerance mechanisms of plants to environmental stress is of immense importance to both agriculture and the environment. Tolerance to stress results from integrated events occurring at all organization levels, from anatomical and morphological, to cellular, biochemical and molecular levels. Plants alter their metabolism in various ways to accommodate environmental stress, and photosynthesis is one of these ways. Photosynthesis in chloroplasts involves a vectorial electron transfer from water in the lumen to nicotinamide adenine dinucleotide phosphate (NADP+) in the stroma, by means of redox carriers

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