Kirschbaum, M.U.F. (2011). Does enhanced photosynthesis enhance growth? Lessons learnt from CO₂ enrichments studies. Plant Physiology 155: 117-124.

Plants typically convert only 2-4% of the available energy in radiation into new plant growth. This low efficiency has provided an impetus for trying to genetically manipulate plants in order to achieve greater efficiencies. But to what extent can increased photosynthesis be expected to increase plant growth? This question is addressed by treating plant responses to elevated CO₂ as an analogue to increasing photosynthesis through plant breeding or genetic manipulations. For plants grown optimal growth conditions and elevated CO₂, photosynthetic rates can be more than 50% higher than for plants grown under normal CO₂ concentrations. This reduces to 40% higher for plants grown under the average of optimal and sub-optimal conditions, and over the course of a full day, average photosynthetic enhancements under elevated CO₂ are estimated to be about 30%. The 30% enhancement in photosynthesis is reported to increase relative growth rate by only about 10%. This discrepancy is probably due to enhanced carbohydrate availability exceeding many plants’ ability to fully utilise it due to nutrient or inherent internal growth limitations. Consequently, growth responses to elevated CO₂ increase with plants’ sink capacity and nutrient status. However, even a 10% enhancement in relative growth rate can translate into absolute growth enhancements of up to 50% during the exponential growth phase of plants. When space constraints and self-shading force an end to exponential growth, on-going growth enhancements are likely to be closer to the enhancement of relative growth rate. The growth response to elevated CO₂ suggests that increases in photosynthesis almost invariably increase growth, but that growth response is numerically much smaller than the initial photosynthetic enhancement. This lends partial support to the usefulness of breeding plants with greater photosynthetic capacity, but dramatic growth stimulation should not be expected. The usefulness of increasing photosynthetic capacity can be maximised through changes in management practices and manipulation of other genetic traits to optimise the conditions under which increased photosynthesis can lead to maximal growth increases.

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