Resource allocation in the pseudoviviparous
alpine meadow grass (Poa alpina l.)

Abstract

Many biotypes of the northem-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 being
bypassed 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 apical
dominance, 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 in
both 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 also
rule 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.