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

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