Optimizing Antenna Size to Maximize Photosynthetic Efficiency

Abstract

The theoretical upper limit for the operational efficiency of plant photosynthesis has been estimated from a detailed stepwise analysis of the biophysical and biochemical subprocesses to be about 4.6% for C 3 and 6.0% C 4 plants (Zhu et al., 2008(Zhu et al., , 2010. (These estimates assume a leaf temperature of 30°C and an atmospheric [CO 2 ] of 387 ppm and were calculated relative to the full solar spectrum at the earth's surface. These efficiencies would be slightly more than double if calculated relative to only the photosynthetically active radiation [i.e. 400-700 nm; Zhu et al., 2008].) While estimates of the theoretical upper limit of photosynthetic efficiency in microalgae have not been conducted as systematically, the same considerations apply as for plants, although microalgae may have a lower respiratory rate that would raise the upper limit for the efficiency of net photosynthesis accordingly (Melis, 2009). Microalgae expressing bicarbonate transporters will mimic the higher efficiency of C 4 plants due to suppression of photorespiration, while those that do not actively concentrate CO 2 would have upper efficiency limits similar to C 3 plants. The highest short-term efficiencies observed for plants in the field, assessed from maximum growth rates, are about 3.5% for C 3 and 4.3% for C 4 plants, and these drop further to 2.4% and 3.4% when calculated over a full growing season (Monteith, 1977;Piedade et al., 1991;Beale and Long, 1995). For crop plants, these highest full growing season efficiencies are those that define the yield potential (Fischer and Edmeades, 2010) or record yield of the crop and are at least twice greater than photosynthetic efficiencies observed under most commercial farming conditions.The primary reason why the highest observed photosynthetic efficiencies are 30% or more lower than theoretical efficiencies is light saturation of photosynthesis. Photosynthesis responds nonlinearly to increases in insolation. For example, C 3 leaf photosynthesis is saturated by approximately 25% of maximum full sunlight, and light intercepted above this amount will lower photosynthetic efficiency in proportion to the excess light absorbed (Fig.

Redesigning photosynthesis to sustainably meet global food and bioenergy demand

“…total canopy photosynthetic rate (10). This increase would be achieved both through increasing quantum yield of PSII via decreasing photoprotective energy loss and improving light energy distribution inside the canopy.…”

“…Plants have a variety of mechanisms for safely diverting excess absorbed energy, principally as thermal dissipation (9), but they represent a significant loss in efficiency. If plants had fewer light-harvesting pigments (e.g., chlorophylls and carotenoids) per photosystem (10) and fewer photosystems in their uppermost leaves, light might be absorbed more judiciously, and a greater proportion of absorbed photons converted to biomass. This avenue has been pursued for some years in algae and cyanobacteria, often with notable success (20,21), but not yet convincingly in crop plants.…”

“…The slow catalytic rate of the carboxylation enzyme Rubisco is a major barrier to increasing the rate of carbon assimilation at the leaf level. Although current Rubisco properties may be an insurmountable barrier to increasing photosynthetic rate to make full use of direct midday sunlight at the level of an individual leaf, limitations due to Rubisco might be substantially mitigated by improved sharing of light within a crop canopy (10), by reengineering Rubisco (11), or by actively concentrating CO 2 at the active site of Rubisco (ref. 12 and as discussed in Targets of Opportunity).…”

A Novel Mechanism, Linked to Cell Density, Largely Controls Cell Division in Synechocystis

“…Considering that the metabolism does not control the rate of cell division, we tested whether already known factors were responsible, such as (1) nutrient starvation (Lazazzera, 2000;Berla and Pakrasi, 2012), (2) quorum sensing (Sharif et al, 2008), and (3) reduction of light quality and intensity available to the cells due to high cell density (self-shading; Raven and Kübler, 2002;Ort and Melis, 2011;Lea-Smith et al, 2014).…”

The functions of WHIRLY1 and REDOX-RESPONSIVE TRANSCRIPTION FACTOR 1 in cross tolerance responses in plants: a hypothesis

“…The photosynthetic electron transport chain is a major sensor of the light environment, facilitating short-term and longterm adjustments to the photosynthetic machinery in order to optimize photosynthetic efficiency [30,31]. Within this context, the redox states of the plastoquinone (PQ) pool and the cytochrome b 6 ,f complex are important in the short-term control of electron transport and in the longer-term adjustments of membrane protein content and composition through the regulation of gene expression [5,31].…”