Theoretical plant growth models postulate an important role for growth substrates such as sugars and amino acids. To test this experimentally, spinach plants were grown under controlled conditions and with nitrogen added daily, following different exponential addition schemes. Plants were harvested during exponential growth. Free amino acid levels or free sugar levels were only weakly correlated with growth and biomass partitioning. Factor analysis showed however that the product of free sugar concentration and amino acid concentration yielded a parameter adequately reflecting the plant's nutritional state.
It is concluded that growth and biomass partitioning under limiting N conditions cannot be modelled solely based on N substrate levels.