Abstract
Deep-shade plants have adapted to low-light conditions by varying morphology and physiology of cells and chloroplasts, but it still remains unclear, if prolonged periods of high-light or darkness induce additional modifications in chloroplasts' anatomy and pigment patterns. We studied giant chloroplasts (bizonoplasts) of the deep-shade lycopod Selaginella erythropus in epidermal cells of mature fully developed microphylls and subjected them to prolonged darkness and high-light conditions. Chloroplast size and ultrastructure were investigated by light and electron microscopy. Physiological traits were studied by pigment analyses, photosynthetic performance of photosystem II, and formation of reactive oxygen species. Results show that (a) thylakoid patterns and shape of mature bizonoplasts vary in response to light and dark conditions. (b) Prolonged darkness induces transitory formation of prolamellar bodies, which so far have not been described in mature chloroplasts. (c) Photosynthetic activity is linked to structural responses of chloroplasts. (d) Photosystem II is less active in the upper zone of bizonoplasts and more efficient in the grana region. (e) Formation of reactive oxygen species reflects the stress level caused by high-light. We conclude that during prolonged darkness, chlorophyll persists and even increases; prolamellar bodies form de novo in mature chloroplasts; bizonoplasts have spatial heterogeneity of photosynthetic performance.
Highlights
Plants from deep‐shade environments such as the understory of dense tropical forests have adapted to low‐light by developing distinct morphological traits
Considering the results obtained from the imaging Pulse amplitude modulated (PAM), and the ultrastructure from the upper zone of the BZs, we suggest that these thylakoid stacks can be interpreted as very extended grana containing photosystem II (PSII), but future studies on the supramolecular level are needed to confirm the presence of PSII in this region
In pictures from the literature, we found one similar appearance of chloroplasts, where small prolamellar bodies (PLBs) occur within established stroma and grana thylakoid networks (Lütz, 1996); in meristematic cells of the higher plant Eriophorum angustifolium differentiating chloroplasts were described after cold treatment, and the importance of temperature in addition to light for chloroplast development was pointed out
Summary
Plants from deep‐shade environments such as the understory of dense tropical forests have adapted to low‐light by developing distinct morphological traits. Sheue et al (2007) described the ultrastructure of the giant chloroplasts in S. erythropus and detected two different kinds of thylakoid arrangements: (a) a lower zone where thylakoids are organized in grana and stroma thylakoids similar to the structure of typical chloroplasts of higher plants and (b) an upper zone that forms a lens‐shaped cover of several stacks of three elongated thylakoids. We relate structural changes of the chloroplasts to changes of the photosynthetic performance and pigment patterns, and we discuss changes of chlorophyll and the formation of ROS as indicators for stress
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