Nitrogen Stress in Birch Seedlings

An account is given of experiments with birch ( Betula verrucosa Ehrh.), in which experiments there was optimum availability of all other nutrients, while nitrogen was added in amounts corresponding to optimum as well as various non‐optimum consumption rates. At the beginning of the growth period the nitrogen concentration in the nutrient solutions was very low or zero in the sub optimum treatments. In the optimum and supra–optimum treatments the nitrogen concentration in the solution was varied up to high levels. The nutrient solutions were not changed but the nutrients taken up were replaced by frequent additions. The sub‐optimum nitrogen additions were carried out every second hour in amounts increasing exponentially day by day, and with a varied exponent. During a lag phase the seedlings in the sub optimum treatments had rapidly diminishing growth rates, deficiency symptoms, and a relatively high root growth rate with formation of long, thin roots. When the growth rate later on had stabilized, a period was recognized during which measurements and sampling were carried out. At the end of the lag phase, or in the beginning of the experimental period, the nitrogen deficiency symptoms of the leaves disappeared and the various parts of the seedlings grew with the same rate, and in a good linear correlation with the rate of nitrogen supply. The results indicate that nitrogen primarily influences the leaf area growth rate, which responds rapidly to a change in nitrogen availability. The following general growth response demonstrates the ability of the plants to adapt growth according to nitrogen supply, so as to create stable internal physiological conditions. Thus, there is a close agreement between the rate of nitrogen supply and consumption on one hand and growth rate on the other. The characteristic nitrogen deficiency symptoms of the seedlings during the lag phase are very similar to those reported in the literature. During the stable period that follows, they disappear from the leaves in agreement with the fact that natural vegetation generally is green also when nitrogen is strongly growth limiting. It is concluded that the exponentially increasing amounts of nitrogen added to lie nutrient solution, also when the exponent is far below optimum, makes nitrogen available to the seedlings in a similar way as does the exponentially growing root system in. a solid medium. Birch seedlings are able to adapt growth rate to low nitrogen supply and thereby show a healthy appearance without acute deficiency symptoms. Some ecological and practical implications are discussed.