Export of carbon from leaf blades of Poa alpina L-at elevated CO2 and two nutrient regimes

The hypothesis was tested that, in plants of the alpine! meadow grass (Poa alpina L,) exposed to elevated CO2, net photosynthesis and export from source leaves is; reduced as a result of feedback from sinks. Nutrient supply was used as one way of reducing photosynthesis and export. Single plants were grown in sand culture under specified controlled environmental conditions for a period of 50 d at two levels of nitrogen and phosphorus ('low': 0.2 mol m(-3) N, 0.04 mol m(-3) P;'high': 2.5 mol m(-3) N, 0.5 mol m(-3) P). Compartmentation within, and export of carbon from, individual youngest fully expanded leaves of acclimated plants was determined using C-14 feeding and efflux plus mass balance calculations of carbohydrate export. Independent of treatment, the bulk of soluble carbohydrate (65-75%) was present as fructan, with most of the remainder being sucrose. Depending on nutrient supply, CO2 could alter export from source leaves either by a reduction in the amount of sucrose present in a readily available pool for transport, or by altering the rate constant describing phloem loading.