Growth and growth substrate levels in spinach under non-steady state conditions of nitrogen nutrition and light

Spinach plants ( Spinacia oleracea L. cv. Subito) were grown in a complete nutrient solution under ample light intensity (14 h day^?1 at 660 μmol m^?2 s^?1) before being transferred either to a minus-N solution (experiment 1), or to limiting light conditions (6 h day^?1 at 220 μmol m^?2 s^?1; experiment 2). Shoot growth in experiment 1 decreased significantly from 0.24 day^?1 to 0.07 day^?1 after the fourth day of transfer. Root relative growth rate increased after 1 day from 0.25 to 0.31 day^?1, but decreased on the fifth day after transfer to 0.11 day^?1. Shoot growth in experiment 2 decreased significantly from 0.25 to 0.17 day^?1 after the fourth day of transfer, while root growth decreased to half of its original level (0.25 day^?1) already on the second day. Growth substrate levels in the plants (free sugars, free amino acids) and starch levels depended on the plant age, the moment in the diurnal cycle, and the imposed treatment. Fluctuations in shoot growth or root growth resulting from the light or N limitation could not be explained by a correspondent increase or decrease in the levels of growth substrates. The hypotheses underlying the functional equilibrium theory, assuming shoot and root growth to be controlled by N- and C-containing substrates respectively, and several other growth and partitioning models are therefore questioned. A neglect of the osmotic role of the free sugars in these models might be the explanation for this.