Growth and nutrition of Betula pendula at different relative supply rates of zinc

Abstract

Small birch plants (Betula pendula Roth) were cultivated in a hydroponic spray solution where the relative growth rate (R(G); day-1) was controlled by the relative supply rate of zinc (R(Zn); day-1). After an adjustment phase to steady-state growth, R(G) equaled R(Zn). The R(Zn) treatments were 0.05, 0.125 and 0.20 day-1 with free access to all other nutrients. In an additional treatment, there was free access to all nutrients, including zinc (FA treatment). The pH of the nutrient solution was approximately 4.5 and conductivity was 100 μS cm-1. The duration of each treatment depended on R(Zn) and ranged from 4 (FA treatment) to 10 weeks (at R(Zn) = 0.05 day-1). The plants showed persistent and typical zinc-deficiency symptoms at steady-state growth when R(G) was 0.05 and 0.125 day-1, whereas there were few symptoms when R(G) was 0.2 day-1. The Zn concentration of the plants ranged from 8 (at R(Zn) = 0.05 day-1) to 21 μg g(DM)-1 (DM = dry mass) (at R(Zn) = 0.2 day-1) and was approximately 42 μg g(DM)-1 in the FA treatment. Uptake rates of Zn, calculated per root growth rate (μmol g(DM, root)-1), were about 2.8 times higher at R(Zn) = 0.20 day-1 than at R(Zn) = 0.05 day-1. The root and stem biomass fractions were approximately constant at all supply rates of Zn, whereas the leaf biomass fraction tended to increase with increasing supply rate of Zn. Net assimilation rate was constant from FA to an R(Zn) of 0.125 day-1 but decreased by a factor of about 2 at R(Zn) = 0.05 day-1 (Leaf area ratio and specific leaf area were smaller at low supply rates of zinc than at high supply rates.