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

Birch (Betula pendula Roth.) seedlings were kept for two growing seasons under ambient (similar to 350 mu mol mol(-1)) and elevated (similar to 700 mu mol mol(-1)) [CO2]. The present study was designed to examine the effects of [CO2] and pot size on growth and carton 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 a:ere not affected by either [CO2] or pot treatments when the saplings mere 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 thr main effect of elevated [CO2] on tree growth.