Abstract
One of the major constraints limiting biomass production in autotrophs is the low thermodynamic efficiency of photosynthesis, ranging from 1 to 4%. Given the absorption spectrum of photosynthetic pigments and the spectral distribution of sunlight, photosynthetic efficiencies as high as 11% are possible. It is well-recognized that the greatest thermodynamic inefficiencies in photosynthesis are associated with light absorption and conversion of excited states into chemical energy. This is due to the fact that photosynthesis light saturates at one quarter full sunlight intensity in plants resulting in the dissipation of excess energy as heat, fluorescence and through the production of damaging reactive oxygen species. Recently, it has been demonstrated that it is possible to adjust the size of the light harvesting antenna over a broad range of optical cross sections through targeted reductions in chlorophyll b content, selectively resulting in reductions of the peripheral light harvesting antenna size, especially in the content of Lhcb3 and Lhcb6. We have examined the impact of alterations in light harvesting antenna size on the amplitude of photoprotective activity and the evolutionary fitness or seed production in Camelina grown at super-saturating and sub-saturating light intensities to gain an understanding of the driving forces that lead to the selection for light harvesting antenna sizes best fit for a range of light intensities. We demonstrate that plants having light harvesting antenna sizes engineered for the greatest photosynthetic efficiency also have the greatest capacity to mitigate high light stress through non-photochemical quenching and reduction of reactive oxygen associated damage. Under sub-saturating growth light intensities, we demonstrate that the optimal light harvesting antenna size for photosynthesis and seed production is larger than that for plants grown at super-saturating light intensities and is more similar to the antenna size of wild-type plants. These results suggest that the light harvesting antenna size of plants is designed to maximize fitness under low light conditions such as occurs in shaded environments and in light competition with other plants.
Highlights
In nature, photosynthetic organisms grow under constantly varying light intensities ranging from full sunlight intensities to sub-saturating light intensities
To determine the sensitivity of plants having different light harvesting antenna sizes to high light (HL) stress, we analyzed the impact of HL stress (1,000 μmol photons m−2 s−1 for 24 h) on photosystem II (PSII) photochemical efficiency as determined by the Chl fluorescence
To gain greater insights into the biophysical basis for these differences in light stress sensitivity associated with different light harvesting antenna sizes, we compared the relative levels of darkadapted non-photochemical quenching (NPQ) activity of plants having a range of light harvesting antenna sizes (Figure 1B)
Summary
Photosynthetic organisms grow under constantly varying light intensities ranging from full sunlight intensities to sub-saturating light intensities. Leaves at the top of the canopy experience higher light intensities than those at the bottom of the canopy This raises the question why do virtually all plants have light harvesting antenna sizes that capture photons at rates. Plants mitigate high light (HL) stress through non-productive energy dissipation pathways including heat and fluorescence and the production of damaging reactive oxygen species (Perrine et al, 2012; Friedland et al, 2019). This raises the question why have plants evolved large fixed size light harvesting antenna sizes that light saturate at one quarter full sunlight intensity
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