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

In evergreen species, leaf carbon isotope discrimination (DELTA) integ rates phenological rhythms in gas exchange as well as seasonal changes in environmental conditions. However, few reports on long term variat ions in DELTA of woody plants are available. We measured DELTA, gas ex change, nitrogen content, and photosynthetic nitrogen-use efficiency ( PNUE) in coffee hedgerows at different stages of canopy development en compassing a range of leaf area index (LAI) from 0.7 to 7.5. Assimilat ion was highest in sun leaves, but stomatal conductance was highest in shaded leaves. This resulted in a high correlation between assimilati on and stomatal conductance in sun, but not in shaded leaves. DELTA wa s about 2 parts per thousand lower in sun than in shaded leaves, and v aried by 2.3 parts per thousand among leaves at different positions al ong two-year-old branches. These differences in DELTA were the result of changes in carbon isotope composition that occurred in mature, full y expanded leaves as they became shaded during subsequent canopy growt h. Results from a mass balance model based on leaf gas exchange charac teristics and measured foliar DELTA values suggested that about 50% of the carbon originally fixed during leaf development in the sun may ha ve subsequently been turned over in the shade. DELTA of sun leaves fro m the upper canopy decreased by about 2 parts per thousand with increa sing LAI, indicating that intrinsic water-use efficiency (WUE) of this canopy layer increased during canopy development. In contrast, instan taneous WUE, estimated as assimilation divided by canopy transpiration obtained from sap flow measurements, seemed to decrease with increasi ng LAI. PNUE of upper canopy sun leaves decreased with increasing LAI, suggesting a physiological compromise between WUE and PNUE mediated b y stomatal conductance, which also decreased with increasing LAI. A st rong negative correlation obtained between leaf DELTA and N content wa s consistent with a trade-off between intrinsic water- and N-use effic iency.