Carbon and nitrogen assimilation in relation to yield: mechanisms are the key to understanding production systems

Improved understanding of crop production systems in relation to N-supply has come from a knowledge of basic plant biochemistry and physiology. Gene expression leads to protein synthesis and the forma- tion of metabolic systems; the ensuing metabolism determines the capacity for growth, development and yield production. This constitutes the genetic potential. These processes set the requirements for the supply of resources. The interactions between carbon dioxide (CO2) and nitrate (NO 3) assimilation and their dynamics are of key importance for crop production. In particular, an adequate supply of NO3, its assimilation to amino acids (for which photo- synthesized carbon compounds are required) and their availability for protein synthesis, are essential for metabolism. An adequate supply of NO3 stimu- lates leaf growth and photosynthesis, the former via cell growth and division, the latter by larger contents of components of the light reactions, and those of CO2 assimilation and related processes. If the supply of resources exceeds the demand set by the genetic potential then production is maximal, but if it is less then potential is not reached; matching resources to potential is the aim of agriculture. However, the connection between metabolism and yield is poorly quantified. Biochemical characteristics and simula- tion models must be better used and combined to improve fertilizer-N application, efficiency of N- use, and yields. Increasing N-uptake at inadequate N-supply by increasing rooting volume and density is feasible, increasing affinity is less so. It would increase biomass and NuC ratio. With adequate N, at full genetic potential, more C-assimilation per unit N would increase biomass, but energy would be limiting at full canopy. Increasing C-assimilation per unit N would increase biomass but decrease NuC at both large and small N-supply. Increasing produc- tion of all biochemical components would increase biomass and demand for N, and maintain NuC ratio. Changing C- or N-assimilation requires modifications to many processes to effect improvements in the whole system; genetic engineeringumolecular bio- logical alterations to single steps in the central metabolism are unlikely to achieve this, because targets are unclear, and also because of the complex interactions between processes and environment. Achievement of the long-term objectives of improv- ing crop N-use and yield with fewer inputs and less pollution, by agronomy, breeding or genetic engin- eering, requires a better understanding of the whole system, from genes via metabolism to yield.