Is partitioning of dry weight and leaf area within Dactylis glomerata affected by N and CO2 enrichment?

We examined changes in dry weight and leaf area within Dactylis glomerata L . plants using allometric analysis to determine whether observed patterns w ere truly affected by [CO2] and N supply or merely reflect ontogenetic drif t. Plants were grown hydroponically at four concentrations of NO3- in contr olled environment cabinets at ambient (360 mu l 1(-1)) or elevated (680 mu l 1(-1)) atmospheric [CO2]. Both CO2 and N enrichment stimulated net dry ma tter production. Allometric analyses revealed that [CO2] did not affect par titioning of dry matter between shoot and root at high N supply. However, a t low N supply there was a transient increase in dry matter partitioning in to the shoot at elevated compared to ambient [CO2] during early stages of g rowth, which is inconsistent with predictions based on optimal partitioning theory. In contrast, dry matter partitioning was affected by N supply thro ughout ontogeny, such that at low N supply dry matter was preferentially al located to roots, which is in agreement with optimal partitioning theory. I ndependent of N supply, atmospheric CO2 enrichment resulted in a reduction in leaf area ratio (LAR), solely due to a decrease in specific leaf area (S LA), when plants of the same age were compared. However, [CO2] did not affe ct allometric coefficients relating dry weight and leaf area, and effects o f elevated [CO2] on LAR and SLA were the result of an early, transient stim ulation of whole plant and leaf dry weight, compared to leaf area productio n. We conclude that elevated [CO2], in contrast to N supply, changes alloca tion patterns only transiently during early stages of growth, if at all. (C ) 2000 Annals of Botany Company.