Growth and nutrition of small Betula pendula plants at different relative addition rates of iron

Summary Small birch plants (Betula pendula Roth.) were cultivated in a hydroponic spray solution where the relative addition rate of iron (R_Fe; g g^–1 day^–1), was the growth-controlling variable. All other elements were added in free access. An additional treatment was performed where all nutrients, including iron, were in free access (FA). The plants showed deficiency symptoms at steady-state growth and severe limitation of iron, R_Fe 0.05 and 0.10 day^–1. There were few symptoms at R_Fe of 0.15 or above. Plant relative growth rate (R_G; g g^–1 day^–1), equalled the relative rate of increase in iron supply, R_Fe. Internal iron concentration of the plants ranged from 40 to 70 g g^–1 dry weight (DW) over the range for which iron supply was limiting growth. At FA, the internal concentration was approximately 200 g g^–1 DW without further increase in R_G, demonstrating that iron may be taken up in excess without affecting growth. Internal concentrations of macronutrients were stable at the different R_Fe, except for Ca and Mg in shoots which were higher at low iron supply. Uptake rates of iron, calculated per root growth rate (mol g^–1 root DW), were approximately twice as high at R_Fe 0.20 as at 0.05 day^–1. The effect of iron limitation on dry matter allocation to leaves was small, with increases in the root fraction being largely at the expense of the stem. Leaf area ratio was constant regardless of R_Fe and the specific leaf area tended to increase with increasing iron limitation. Net assimilation rate decreased by a factor of 6 from free access to severe iron limitation, largely accounting for the differences in plant R_G.