GROWTH AND NUTRITION OF BIRCH SEEDLINGS AT VARIED RELATIVE ADDITION RATES OF MAGNESIUM

Growth and nutrition of hydroponically cultivated birch seedlings (Bet ula pendula Roth.) were investigated at various magnesium (Mg) availab ilities. Suboptimum Mg conditions were created by adding Mg once per h our in exponentially increasing amounts at one of four relative additi on rates (RM(g)): 0.05, 0.10, 0.15 or 0.20 day(-1). Seedlings given fr ee access to Mg were used as controls. After an acclimation period, th e relative growth rate of the seedlings attained the same value as the corresponding relative rate of Mg addition. In all suboptimum Mg trea tments, deficiency symptoms in the form of chloroses and necroses deve loped in the older leaves, both during and after the phase of growth a cclimation. The severity of these symptoms was correlated with the ava ilability of Mg. The relative growth rate of seedlings was linearly co rrelated with plant Mg status. The root fraction of the total biomass decreased from 22% in control plants to 8% in plants receiving the low est rate of Mg addition. A shift in Mg availability from free access t o R(Mg) = 0.05 day(-1) decreased the photosynthetically active leaf ar ea 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 mo rtality and formation of necroses, and to a minor extent attributable to increased carbon costs for leaf area production. A decrease in star ch 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 cons tituted the major growth constraint, particularly for roots, under Mg- limiting conditions.