CARBON ISOTOPE COMPOSITION IN RELATION TO LEAF GAS-EXCHANGE AND ENVIRONMENTAL-CONDITIONS IN HAWAIIAN METROSIDEROS-POLYMORPHA POPULATIONS

Carbon isotope composition, photosynthetic gas exchange, and nitrogen content were measured in leaves of three varieties of Metrosideros polymorpha growing in sites presenting a variety of precipitation, temperature and edaphic regimes. The eight populations studied could be divided into two groups on the basis of their mean foliar delta-C-13 values, one group consisting of three populations with mean delta-C-13 values ca. -26 parts per thousand and another group with delta-C-13 values ca. -28 parts per thousand. Less negative delta-C-13 values appeared to be associated with reduced physiological availability of soil moisture resulting from hypoxic conditions at a poorly drained high elevation bog site and from low precipitation at a well-drained, low elevation leeward site. Gas exchange measurements indicated that foliar delta-C-13 and intrinsic water-use efficiency were positively correlated. Maximum photosynthetic rates were nearly constant while maximum stomatal conductance varied substantially in individuals with foliar delta-C-13 ranging from -29 to -24 parts per thousand. In contrast with the patterns of delta-C-13 observed, leaf nitrogen content appeared to be genetically determined and independent of site characteristics. Photosynthetic nitrogen-use efficiency was nearly constant over the range of delta-C-13 observed, suggesting that a compromise between intrinsic water- and N-use efficiency did not occur. In one population variations in foliar delta-C-13 and gas exchange with leaf cohort age, caused the ratio of intercellular to atmospheric partial pressure of CO2 predicted from gas exchange and that calculated from delta-C-13 to be in close agreement only in the two youngest cohorts of fully expanded leaves. The results indicated that with suitable precautions concerning measurement protocol, foliar delta-C-13 and gas exchange measurements were reliable indicators of potential resource use efficiency by M. polymorpha along environmental gradients.