Abstract
Photosynthetic electron transport rates in higher plants and green algae are light-saturated at approximately one quarter of full sunlight intensity. This is due to the large optical cross section of plant light harvesting antenna complexes which capture photons at a rate nearly 10-fold faster than the rate-limiting step in electron transport. As a result, 75% of the light captured at full sunlight intensities is reradiated as heat or fluorescence. Previously, it has been demonstrated that reductions in the optical cross-section of the light-harvesting antenna can lead to substantial improvements in algal photosynthetic rates and biomass yield. By surveying a range of light harvesting antenna sizes achieved by reduction in chlorophyll b levels, we have determined that there is an optimal light-harvesting antenna size that results in the greatest whole plant photosynthetic performance. We also uncover a sharp transition point where further reductions or increases in antenna size reduce photosynthetic efficiency, tolerance to light stress, and impact thylakoid membrane architecture. Plants with optimized antenna sizes are shown to perform well not only in controlled greenhouse conditions, but also in the field achieving a 40% increase in biomass yield.
Highlights
Plants have evolved highly efficient photosynthetic light harvesting antenna that operate with varying energy conversion efficiencies over a wide variety of natural light conditions This flexibility allows for adaptability to both high and low light conditions as well as facilitates competition for light in mixed species consortia
This approach has been successfully implemented in algal cultures whose self-shading increases temporally as cultures grow Optimization of light harvesting antenna in algae resulted in a 40% increase in overall biomass yield[4,5]
One means to reduce light harvesting antenna size is to reduce the accumulation of chlorophyll b (Chl b)
Summary
Plants have evolved highly efficient photosynthetic light harvesting antenna that operate with varying energy conversion efficiencies over a wide variety of natural light conditions This flexibility allows for adaptability to both high and low light conditions as well as facilitates competition for light in mixed species consortia. Light-harvesting antenna sizes are restricted, in their ability to maximize photosynthetic efficiency in varying light regimes In plant canopies this leads to a situation in which the rate of photosynthetic electron transport in upper leaves is saturated at high light intensities (full sunlight) resulting in the dissipation of excess captured energy as heat and fluorescence. One strategy to better couple the rate of light capture to the downstream electron transfer processes while improving light penetration throughout the canopy is to reduce the apparent optical cross-section of the light harvesting antenna complex[3] This approach has been successfully implemented in algal cultures whose self-shading increases temporally as cultures grow Optimization of light harvesting antenna in algae resulted in a 40% increase in overall biomass yield[4,5]. These results indicate that there is a tipping point in antenna size where larger or smaller antenna sizes impacts light capture efficiency but light stress tolerance and overall electron transport rates
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