Physiological influences on carbon isotope discrimination in huon pine (Lagarostrobos franklinii)

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Abstract
Measurements of the light environment and stomatal and photosynthetic behavior are reported for huon pine (L. franklinii, family Podocarpaceae) in western Tasmanian rain forest. For a variety of microenvironments, these are used in an analysis of stable carbon isotope measurements in the air, and in branch and leaf material, using a model for carbon isotope fractionation in leaves (Farquhar et al. 1982). The major features of .delta.13C variations with respect to branch position can be explained in terms of the direct influence of light level acting via the rate of CO2 assimilation. A relatively constant .delta.13C gradient of .apprx. 2.6.permill. between leaf tip and branch wood also is observed. Alternative explanations are advanced for the tip-to-wood gradient in .delta.13C. If the .delta.13C of leaf tips is taken to represent the value for photosynthate, maintenance respiration is proposed as a mechanism for the further fractionation; a significant 13C depletion in respired CO2 is implied which is not supported by indirect measurements of atmospheric isotope ratio. An assumption of significant sampling errors (e.g., related to humidity effects on assimilation) is required to obtain good quantitative prediction of the light influence. If the branch wood .delta.13C is taken to represent that of the photosynthate, the tip-to-wood gradient may find an explanation, via the model, in terms of tip tissue comprising C from immature cells. Translocation of photosynthate from exposed to shaded branches is then proposed as a means of obtaining quantitative agreement with the predicted light influence. The support provided for the applicability of the Farquhar et al. (1982) model in the field is discussed in the context of the problem of obtaining past global atmospheric CO2 levels from .delta.13C in tree-rings.