Intrinsic water-use efficiency and heterotrophic investment in tropical leaf growth of two Neotropical pioneer tree species as estimated from δ13C values

Abstract

• 13C enrichment in emerging leaves and its effect on carbon isotopic composition (δ13C) of mature leaves were investigated in the neotropical pioneer tree species, Cecropia longipes and Urera caracasana, in Panama. • Leaves of all ages were analysed for δ13C, gas exchange, nitrogen, and leaf mass per area. Low intercellular to atmospheric carbon dioxide (CO2) partial pressure (pi:pa); and high δ13C of CO2 in air, intrinsic water-use efficiency, internal resistance, and carboxylation capacity were discounted as causes of 13C enrichment. • 13C enrichment might occur when leaf growth is from imported organic carbon but might not reflect δ13C values at the leaf producing the carbon. Results support hypotheses that: de novo sucrose synthesis causes 13C enrichment of mobile sugars after export from source leaves; and high ratios of PEP carboxylase (PEPc): PEPc + Rubisco cause emerging leaves to be 13C enriched relative to their growth substrate. Carbon contributions of 13C-enriched early growth could yield inaccurate pi:pa estimates from δ13C in mature leaves. • A model estimated investment of imported organic carbon to leaf growth and improved estimates of pi:pa from mature leaves. With such adjustment, δ13C analyses provide valuable information about age-related source-sink relations in leaves. © New Phytologist (2001).