Photosynthetic gas exchange properties and carbon isotope ratios of some mangroves in North Queensland

Abstract

The gas exchange properties of fully expanded leaves on mature trees of Rhizophora stylosa and R. apiculata at Hinchinbrook Island, North Queensland were studied using a measurement system based on infrared gas analysers and dew point hygrometers. In some cases, CO₂ and water vapour exchange were followed in diurnal ‘tracking’ experiments under natural illumination. Leaf temperature and the vapour pressure deficit inside the leaf chamber were maintained under changing conditions which simulated those experienced by other leaves in the canopy at the time. These experiments revealed that leaf temperature was a key factor controlling assimilation under natural conditions.

In other experiments, the response of gas exchange parameters to variation in CO₂ partial pressure, light flux and leaf temperature under otherwise constant conditions were studied using artificial illumination. The stomata were found to be very sensitive to changes in environmental conditions, irrespective of whether these changed when simulating canopy conditions or in response to experimental manipulation. In both cases there appeared to be parallel changes in stomatal conductance and assimilation rate so that the calculated CO₂ partial pressure in the intercellular spaces was maintained more or less constant. The only case when this nexus between conductance and assimilation rate was not complete occurred when CO₂ partial pressures were manipulated. Maximum rates of assimilation in normal air were about 8 micromol m^–2 s^–1 (12.6 mg dm^–2 hr^–1) and occurred at a leaf temperature of about 30° C and light quantum fluxes above 700 micromol m^–2 s^–1.

The photosynthetic response of Rhizophora leaves to light, CO₂ partial pressure and temperature were typical of the C₃ photosynthetic pathway. Additional evidence for the operation of the C₃ pathway in mangroves was obtained from an analysis of the isotopic discrimination against ¹³C by 16 species from 14 genera of North Queensland mangroves. The degree of discrimination against ¹³C, as represented by the δ¹³C value, ranged from –32.2‰ to –24.6‰. These values are typical of C₃ plants and clearly more negative than the range for C₄ plants. These data lead to the conclusion that, in common with most trees, mangroves photosynthesize via the C₃ pathway.