Adaptive responses of mature giant chloroplasts in the deep‐shade lycopod Selaginella erythropus to prolonged light and dark periods

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.

Characterization of chloroplast iridescence in Selaginella erythropus

“…In general, and considering the complexity of the real system, the agreement is outstanding. Contrary to previous reports [11,13], our observations and measurements of the reflected light from individual chloroplasts have enabled us to demonstrate that blue structural colour is produced by the bizonoplasts. This emphasizes the need for suitable techniques to characterize structural colour in intracellular organelles; structural colour can be subtle and inconspicuous to the human eye with no necessity to be visually detectable in the leaf.…”

“…In fact, it is this particularly plastic development of chloroplast morphology which has probably led to the dismissal of structural colour present in S. erythropus. In our experience, optimum growth conditions (with respect to the rate of new microphyll production) agree with that of Ghaffar et al [13] at a light level of 15 mmol m 22 s 21 ; however, light levels below this (1-5 mmol m 22 s 21 ) still obtain substantial microphyll production while the development of blue coloration is more obvious.…”

Light-Dependent Morphological Changes Can Tune Light Absorption in Iridescent Plant Chloroplasts: A Numerical Study Using Biologically Realistic Data

“…References used to build Table 1 state that the experimental results were measured under high light with an intensity ≳ 500 μmol photons m -2 s -1 (12,21) whilst low light conditions were achieved with intensities between 15-30 μmol photons m -2 s -1 (22). Interestingly, the lowest intensities applied in the laboratory are higher than those found under the forest canopy (16 μmol photons m -2 s -1 (5)), where those plant species showing iridoplast organelles are usually found.…”

“…Their photonic properties were recently characterized for the first time (32). A schematic of their structure is represented in Figure 4 a and it behaves as a one-dimensional photonic crystal which reflects strongly in the blue, around 450 nm (21,22,32,33).…”

“…that grow iridoplasts, the development of highly ordered thylakoid arrangements with intrinsic photonic properties in iridoplasts is highly plastic during the early stages of leaf growth. For a specimen grown under high light conditions, iridoplasts present morphologies similar to the thylakoid arrangement in a non-photonic chloroplast while specimens grown under low light or deep-shade light conditions, the iridoplast develops thylakoid and granum arrangements with photonic crystal properties (21,22). Herein, we focus on the latter case where the plant has been grown under low light conditions.…”