Uptake of carbon and nitrogen at decreased nutrient availability in small birch (Betula pendula Roth.) plants.

Uptake of carbon and nitrogen in relation to mineral nutrient supply was investigated in small birch (Betula pendula Roth.) plants grown in solution culture. A step-decrease in nutrient availability (from near-optimal to a low rate of supply) caused an immediate and temporary cessation in net uptake of nitrogen to leaves and a sharp reduction in rate of leaf expansion. Both net uptake of nitrogen to leaves and leaf area expansion subsequently resumed but at a rate matching the decreased nutrient supply. Whole plant- and root-relative growth rates also decreased with mineral nutrient supply, although root growth rate decreased more slowly than that of the whole plant, resulting in an increased fraction of plant dry matter in the roots. During the lag phase, net shoot photosynthesis exceeded carbon use in structural growth and respiration as indicated by high starch concentrations in the whole plant. Over a wide range of mineral nutrient availabilities, the stable ratios of leaf area and plant growth rate to plant nitrogen content remained unchanged. Starch concentrations were higher at the lower nutrient availabilities, suggesting that carbon uptake was not regulated solely by the rate of carbon use in structural growth and respiration. It was calculated that net photosynthetic and transpiration rates were such that the intercellular-CO(2) concentration of leaves was largely independent of plant nutrient supply.