THE ANALYSIS OF PHOTOSYNTHETIC PERFORMANCE IN LEAVES UNDER FIELD CONDITIONS - A CASE-STUDY USING BRUGUIERA MANGROVES

In this report, we analyze the photosynthetic capacity and performance of leaves under field conditions with a case study based on the mangroves Bruguiera parviflora and B. gymnorrhiza. Using a tower thorugh a closed canopy at a field sight in North Queensland and portable infra-red gas analyzers, a large data set was collected over a period of 11 days early in the growing season. The set was used to analyze the relationship between net photosynthesis (P(net)) and light, leaf temperature, stomatal conductance and intracellular CO2 (C(i)). There are three objectives of this report: (1) to determine photosynthetic potential as indicated by the in situ responses of P(net) to light and stomatal conductance, (2) to determine the extent to which photosynthetic performance may be reduced from that potential, and (3) to explore the basis for and physiological significance of the reduction. The results indicate that even under harsh tropical conditions, the mangrove photosynthetic machinery is capable of operating efficiently at low light and with maximal rates of more than 15-mu-mol CO2 m-2 s-1. Though stomata were more often limiting than light, in any single measurement the average reduction of P(net) from the maximum value predicted by light or conductance responses was 35%. Analysis of single leaf light and CO2 responses indicated that photosynthetic performance was under direct photosynthetic, or non-stomatal, control at all light and conductance levels. Capacity was adjustable rapidly from a maximum value to essentially nil such that C(i) varied inversely with P(net) from ca. 150-mu-L L-1 at the highest rates of CO2 exchange to ambient at the lowest.