9 - Environmental and Physiological Influences on Carbon Isotope Composition of Gap and Understory Plants in a Lowland Tropical Forest

Abstract

The stable carbon isotope composition of plant tissue is determined by the isotopic composition of the CO 2 source and discrimination against the heavier isotope 13 C during photosynthetic CO 2 fixation. In C 3 plants, this discrimination has two main components: one associated with diffusion of CO 2 through the stomata and one because of the discrimination against 13 C by the primary carboxylating enzyme RuBisCO. The balance between carboxylation and stomatal limitation of CO 2 diffusion is reflected in the ratio of intercellular to atmospheric partial pressure of CO 2 ( p i / p a ). In a study discussed in this chapter, the foliar δ 13 C and photosynthetic gas exchange of high light-requiring species growing in gaps and of shade-tolerant shrub species growing in the understory and in gaps in a lowland tropical forest were examined. The study compared the leaf gas exchange responses of shade-tolerant shrub species growing in the understory and the same species growing in gaps and comparing gas exchange responses of shade-tolerant and high light-requiring species both growing in gaps. The chapter determines the extent to which the variation in foliar δ 13 C observed could be attributed to differences in internal physiological features or to differences in the isotopic composition of the source CO 2 . The δ 13 C of the air was estimated from the ambient CO 2 concentration.