Resource allocation in the Pseudoviviparous alpine meadow grass (Poa alpine L.)

Abstract

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 such pseudoviviparous plantlets are capable of photosynthesis, the source-sink characteristics of these synflorescence systems are unknown, including the degree to which plantlets are capable of providing for their own carbon requirements, or contributing to parental sinks. An initial anatomical investigation of the culm revealed that transpiration flow, as delimited by Lucifer Yellow tracer dye, was maintained despite advanced senescence (as evidenced by loss of chlorophyll and chloroplasts), with plantlet leaves driving transpiration flow. Transpiration flow was not hindered by cavitation or tylosis in older culms, the low frequencies of these processes beingbypassed via nodal plexi. Despite this, water content of plantlets declined over time and visual indications of water stress became apparent, suggesting that water supply via the determinate culm was not sufficient for the increasing transpirational demand of indeterminate plantlets. Photosynthetic rates within the paracladial zone, as determined by infrared gas analysis (IRGA), exceeded respiratory rates by 3-4 fold, indicating that plantlets were sources of carbon. 14C tracer studies determined that the paracladial zone was not only as efficient at fixing carbon as the youngest fully expanded leaf, but that both organs exported carbon basipetally (c.f acropetal export from this leaf in seminiferous grasses). Distal plantlets fixed approx. 20% more 14C than proximal plantlets, by virtue of greater dry weight. Manipulative growth analysis of the paracladial zone suggests the operation of a system of apicaldominance, with distal plantlets becoming dominant over proximal plantlets. At dehiscence, distal plantlets were more likely to become established, and possessed relative growth rates more than ten times those of proximal plantlets. Paracladial heterogeneity was also apparent as an increased proportion of aborted spikelets on proximal paracladia. Data indicate that this abortion was, at least in part, a result of constraint imposed by the pseudostem on the developing synflorescence. When grown in conditions of differing resource availability (altered nutrient supply and atmospheric C02 concentration), low nutrient availability in concert with elevated C02 concentration induced particularly low photosynthetic nitrogen and phosphorus use efficiencies inboth parent and plantlet tissues. This occurred in concert with acclimatory loss of photosynthetic capacity leading to a decreased reproductive response of the plant; a product of the number of tillers in flower and the subsequent growth of attached plantlets. lt is predicted that in future climatic conditions Poa alpina will decline in habitats that include species which exhibit less acclimatory loss, no change, or an increase in photosynthetic capacity. These experiments alsorule out resource availability as a cause of heterogeneity within the paracladial zone. A direct study of the phytohormonal characteristics of the pseudoviviparous system is therefore proposed in order to elucidate the mechanism of control within the paracladial zone.