Young birch(Betula pendulaRoth) seedlings were grown in hydroponic cultures to which nutrients were added in amounts that increased exponentially over time. The nutrient additions were adjusted to give three different suboptimal, but stable, relative growth rates (RGR). Levels of glucose, fructose, sucrose and starch and the hormones 3‐Indolyl acetic acid (IAA) and abscisic acid (ABA) were determined in immature (sink) and mature (source) leaves and related to the measured RGR of the seedlings. The results show that ABA increased and IAA decreased in the sink leaves as the RGR of the plants decreased. This occurred in concert with a decrease in soluble sugar levels and starch accumulation in the source leaves. These observations suggest that ABA and IAA may be involved in source‐sink communication in the seedlings, although such causal relationships remain to be proven.