From sunlight to phytomass: on the potential efficiency of converting solar radiation to phyto‐energy

CONTENTS: Summary 939 I. Introduction 940 II. Approach 940 III. Solar radiation absorption 942 IV. Quantum requirement for CO₂ assimilation 943 V. Respiration 946 VI. Photosynthate mobilization and translocation 948 VII. Maintenance 949 VIII. Substrate requirement for growth 949 IX. From sunlight to phyto‐energy: potential overall efficiency 953 X. Assessment 955 Acknowledgements 955 References 955 SUMMARY: The relationship between solar radiation capture and potential plant growth is of theoretical and practical importance. The key processes constraining the transduction of solar radiation into phyto‐energy (i.e. free energy in phytomass) were reviewed to estimate potential solar‐energy‐use efficiency. Specifically, the out‐put : input stoichiometries of photosynthesis and photorespiration in C₃ and C₄ systems, mobilization and translocation of photosynthate, and biosynthesis of major plant biochemical constituents were evaluated. The maintenance requirement, an area of important uncertainty, was also considered. For a hypothetical C₃ grain crop with a full canopy at 30°C and 350 ppm atmospheric [CO₂], theoretically potential efficiencies (based on extant plant metabolic reactions and pathways) were estimated at c. 0.041 J J⁻¹ incident total solar radiation, and c. 0.092 J J⁻¹ absorbed photosynthetically active radiation (PAR). At 20°C, the calculated potential efficiencies increased to 0.053 and 0.118 J J⁻¹ (incident total radiation and absorbed PAR, respectively). Estimates for a hypothetical C₄ cereal were c. 0.051 and c. 0.114 J J⁻¹, respectively. These values, which cannot be considered as precise, are less than some previous estimates, and the reasons for the differences are considered. Field‐based data indicate that exceptional crops may attain a significant fraction of potential efficiency.