CARBON-ISOTOPE DISCRIMINATION AND PHOTOSYNTHETIC GAS-EXCHANGE IN COFFEE HEDGEROWS DURING CANOPY DEVELOPMENT

In evergreen species, leaf carbon isotope discrimination (DELTA) integrates phenological rhythms in gas exchange as well as seasonal changes in environmental conditions. However, few reports on long term variations in DELTA of woody plants are available. We measured DELTA, gas exchange, nitrogen content, and photosynthetic nitrogen-use efficiency (PNUE) in coffee hedgerows at different stages of canopy development encompassing a range of leaf area index (LAI) from 0.7 to 7.5. Assimilation was highest in sun leaves, but stomatal conductance was highest in shaded leaves. This resulted in a high correlation between assimilation and stomatal conductance in sun, but not in shaded leaves. DELTA was about 2 parts per thousand lower in sun than in shaded leaves, and varied by 2.3 parts per thousand among leaves at different positions along two-year-old branches. These differences in DELTA were the result of changes in carbon isotope composition that occurred in mature, fully expanded leaves as they became shaded during subsequent canopy growth. Results from a mass balance model based on leaf gas exchange characteristics and measured foliar DELTA values suggested that about 50% of the carbon originally fixed during leaf development in the sun may have subsequently been turned over in the shade. DELTA of sun leaves from the upper canopy decreased by about 2 parts per thousand with increasing LAI, indicating that intrinsic water-use efficiency (WUE) of this canopy layer increased during canopy development. In contrast, instantaneous WUE, estimated as assimilation divided by canopy transpiration obtained from sap flow measurements, seemed to decrease with increasing LAI. PNUE of upper canopy sun leaves decreased with increasing LAI, suggesting a physiological compromise between WUE and PNUE mediated by stomatal conductance, which also decreased with increasing LAI. A strong negative correlation obtained between leaf DELTA and N content was consistent with a trade-off between intrinsic water- and N-use efficiency.