Growth and nutrition of birch seedlings at varied relative addition rates of magnesium.

Growth and nutrition of hydroponically cultivated birch seedlings (Betula pendula Roth.) were investigated at various magnesium (Mg) availabilities. Suboptimum Mg conditions were created by adding Mg once per hour in exponentially increasing amounts at one of four relative addition rates (R(Mg)): 0.05, 0.10, 0.15 or 0.20 day(-1). Seedlings given free access to Mg were used as controls. After an acclimation period, the relative growth rate of the seedlings attained the same value as the corresponding relative rate of Mg addition. In all suboptimum Mg treatments, deficiency symptoms in the form of chloroses and necroses developed in the older leaves, both during and after the phase of growth acclimation. The severity of these symptoms was correlated with the availability of Mg. The relative growth rate of seedlings was linearly correlated with plant Mg status. The root fraction of the total biomass decreased from 22% in control plants to 8% in plants receiving the lowest rate of Mg addition. A shift in Mg availability from free access to R(Mg) = 0.05 day(-1) decreased the photosynthetically active leaf area per plant weight, despite a concomitant increase in the leaf weight ratio (leaf dry weight/plant dry weight) from 0.61 to 0.75. The loss in assimilating leaf area was mainly a consequence of enhanced leaf mortality and formation of necroses, and to a minor extent attributable to increased carbon costs for leaf area production. A decrease in starch concentration was observed in leaves showing Mg-deficiency symptoms, whereas the starch concentration in healthy leaves was unaffected by Mg availability. It was concluded that shortage of carbohydrates constituted the major growth constraint, particularly for roots, under Mg-limiting conditions.