Architectural and physiological heterogeneity within the synflorescence of the 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 dehiscing shoots. Although capable of photosynthesis, the source-sink characteristics 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 sinks. Photosynthetic rates within the paracladial zone, as determined by infrared gas analysis (IRGA), exceeded respiratory rates by 3-4-fold. 14CO2 tracer studies determined that the paracladial zone was not only as efficient at fixing carbon as the youngest fully expanded leaf (per unit dry weight), 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 14CO2 than did proximal plantlets. This was by virtue of their greater dry weight. At dehiscence, 'distal' plantlets were more likely to become established, and possessed 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 heterogeneity are discussed.