Effects of Different CO2Environments on the Photosynthesis-Yield Relationship and the Carbon and Water Balance of a White Clover (Trifolium repensL. cv. Blanca) Sward

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Abstract
Effects of atmospheric CO2 enrichment to a level above 600 parts 10−6 on leaf and canopy gas exchange characteristics were investigated in Trifolium repens, using an open system for gas exchange measurement. The cuvettes of the system served as growth chambers, allowing continuous measurement in a semi-controlled environment of ±350 and ±600 parts 10−6 CO2, respectively. Carbon balance data were compared with crop yield and effects on the canopy level were compared with measured leaf responses of photosynthesis and stomatal behaviour. Photosynthetic stimulation by high CO2 was stronger at the canopy level (103% on average) than for leaves (90% in full light), as a consequence of accelerated foliage area development. The latter increased absolute water consumption by 16%, despite strong stomatal closure. The overall result was a 63% improvement in canopy water use efficiency (WUE), while leaf WVE increased almost 3-fold in saturating light. The stomatal response was such that, while the internal CO2 concentration in the leaf, ch increased with rising atmospherical CO2 concentration, ca, ci/ca was somewhat decreased. Total canopy resistance, Rc, was generally lower at high CO2 levels, despite higher leaf resistance. Higher canopy CO2 loss at night and faster light extinction in a larger-sized high CO2 canopy were major drawbacks which prevented a further increase in dry matter production (the harvest index was increased by a factor 1.83).