Source-sink relations affect growth but not the allocation pattern of birch (Betula pendula Roth) seedlings under elevated [CO2]

ABSTRACT

Birch (Betula pendula Roth.) seedlings were kept for two growing seasons under ambient (～350 µmol mol^-1) and elevated (～700 µmol mol^-1) CO₂]. The present study was designed to examine the effects of CO₂] and pot size on growth and carbon allocation under conditions of non-limiting water and nutrient supply, in order to separate the effects of source-sink interaction from the effects of nutrient deficiency. The manipulation of the source-sink relations had a strong influence on the growth response to elevated CO₂]. When the rooting volume was inadequate, it resulted in a source-sink imbalance which constrained growth under elevated CO₂]. When root exploration was unconstrained, total dry mass was significantly increased (by about 24%) under elevated CO₂]. However, the allometric relationships in allocation pattern and in morphogenetic development were not affected by either CO₂] or pot treatments when the saplings were of the same size. Thus, by constraining dry mass production, small sinks affected the magnitude of the growth responses to elevated CO₂], but did not affect the plant allocation pattern and allometric relationships when nutrient supply was non-limiting. However, by slowing down growth, sink restrictions counteract the speed-up of ontogeny which is the main effect of elevated CO₂] on tree growth.