Yield Potential, Plant Assimilatory Capacity, and Metabolic Efficiencies

ABSTRACT The structure, control, and efficiency of photosynthetic and respiratory systems are examined. Genetic control is complex and highly conserved. While many features are still unresolved, basic efficiency seems little altered by domestication and breeding of crops. Rubisco, the carboxylase–oxygenase enzyme central to photosynthesis and photorespiration, remains a weak point but may be amenable to improvement. However, the actual radiation‐use efficiency of crops is generally less than the potential with present rubisco kinetics, leaving considerable room for improvement without change in rubisco. Good opportunities for progress lie in definition of optimal canopies of leaves having suitable acclimation and photoprotection. The efficiency of the respiratory system also seems unaffected by plant breeding. Precise evaluation of the roles and efficiencies of the glycolytic pathway and the tricarboxylic acid cycle in production is difficult because, in addition to being sources of energy carriers and reductant, those systems also supply carbon skeletons for biosyntheses. How those systems are controlled and balanced for such diversions is largely unknown. The alternative oxidase found in mitochondria may be involved in that balance but its true role(s) is also unknown. Distinguishing two components of respiration, one related to maintenance and the other to growth, remains a powerful tool in theoretical studies. In such work, the respiratory system appears efficient, but proving that in experiments remains elusive.