Architectural and physiological heterogeneity within the synflorescence ofthe pseudoviviparous grass Poa alpina var. vivipara L.

Many biotypes of the northern-hemisphere Arctic-Alpine grass Poa alpina L, reproduce asexually via prolification of the spikelet axis to produce dehis cing shoots. Although capable of photosynthesis, the source-sink characteri stics of these synflorescence systems are unknown, including the degree to which plantlets from different regions of the synflorescence are capable of providing for their own carbon requirements, or contributing to other sink s. Photosynthetic rates within the paracladial zone, as determined by infra red gas analysis (IRGA), exceeded respiratory rates by 3-4-fold. (CO2)-C-14 tracer studies determined that the paracladial zone was not only as effici ent at fixing carbon as the youngest fully expanded leaf (per unit dry weig ht), but that both organs exported carbon mainly basipetally (cf, extensive acropetal export from this leaf in seminiferous grasses). Distal plantlets of the paracladial zone fixed approximately 20% more (CO2)-C-14 than did p roximal plantlets, This was by virtue of their greater dry weight. At dehis cence, 'distal' plantlets were more likely to become established, and posse ssed relative growth rates more than 10 times those of 'proximal' plantlets , Paracladial heterogeneity was also apparent as an increased proportion of aborted spikelets on proximal paracladia, The possible causes of this hete rogeneity are discussed.