Abstract
The hypothesis was tested that, in plants of the alpine meadow grass (Poa alpina L.) exposed to elevated CO 2 , 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 14 C 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, CO 2 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.
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