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

[sup 13]C enrichment in emerging leaves and its effect on carbon isotopic composition (δ[sup 13]C) 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 δ[sup 13]C, gas exchange, nitrogen, and leaf mass per area. Low intercellular to atmospheric carbon dioxide (CO[sub 2]) partial pressure (Pi : Pa); and high δ[sup 13]C of CO[sub 2] in air, intrinsic water-use efficiency, internal resistance, and carboxylation capacity were discounted as causes of [sup 13]C enrichment. [sup 13]C enrichment might occur when leaf growth is from imported organic carbon but might not reflect δ[sup 13]C values at the leaf producing the carbon. Results support hypotheses that: de novo sucrose synthesis causes [sup 13]C enrichment of mobile sugars after export from source leaves; and high ratios of PEP carboxylase (PEPc): PEPc + Rubisco cause emerging leaves to be [sup 13]C enriched relative to their growth substrate. Carbon contributions of [sup 13]C-enriched early growth could yield inaccurate Pi : Pa estimates from δ[sup 13]C 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, δ[sup 13]C analyses provide valuable information about age-related source—sink relations in leaves. [ABSTRACT FROM AUTHOR]