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

Small birch plants (Betula pendula Roth) were cultivated in a hydroponic sp ray solution where the relative growth rare (R-G; day(-1)) was controlled b y the relative supply rate of zinc (R-Zn; day(-1)). After an adjustment pha se 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 addit ional treatment, there was free access to all nutrients, including zinc (FA treatment). The pH of the nutrient solution was approximately 4.5 and cond uctivity was 100 mu S 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)). T he plants showed persistent and typical zinc-deficiency symptoms at steady- stare growth when RG was 0.05 and 0.125 day(-1) whereas there were few symp toms when RG was 0.2 day(-1). The Zn concentration of the plants ranged fro m 8 (at R-Zn = 0.05 day(-1)) to 21 mu g g(DM)(-1) (DM = dry mass) (at R-Zn = 0.2 day(-1)) and was approximately 42 mu g g(DM)(-1) in the FA treatment. Uptake rates of Zn, calculated per root growth rate (mu mol g(DM,root)(-1) ), were about 2.8 times higher at R-Zn= 0.20 day(-1) than at R-Zn = 0.05 da y(-1). The root and stem biomass fractions were approximately constant at a ll 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-Z n = 0.05 day(-1). Leaf area ratio and specific leaf area were smaller at lo w supply rates of zinc than at high supply rates.