Photosynthesis Research 29: 11-22, 1991. © 1991 KluwerAcademic Publishers. Printedin the Netherlands. Regular paper The analysis of photosynthetic performance in leaves under field conditions: A case study using Bruguiera mangroves J.M. Cheeseman 1, B.F. Clough 2, D.R. Carter I, C.E. Lovelock 2, Ong Jin Eong 3 & R.G. Sim 2 1Department of Plant Biology, University of Illinois, 505 S. Goodwin Ave., Urbana, IL 61801, USA; 2Australian Institute of Marine Science, PMB 3, Townsville, Qld. 4810, Australia; 3School of Biological Science, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 25 January 1991; accepted in revised form 14 May 1991 Key words: convexity, gas exchange, photosynthetic capacity, photosynthetic control Abstract 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 through 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 (Pact) and light, leaf temperature, stomatal conductance and intracellular CO 2 (Ci). There are three objectives of this report: (1) to determine photosynthetic potential as indicated by the in situ responses of Pnct 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 machin- ery is capable of operating efficiently at low light and with maximal rates of more than 15/zmol CO 2 m -2 s -1. Though stomata were more often limiting than light, in any single measurement the average reduction of Pa~t from the maximum value predicted by light or conductance responses was 35%. Analysis of single leaf light and CO 2 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 Pn~t from ca. 150 /~L L -1 at the highest rates of CO 2 exchange to ambient at the lowest. Introduction Despite an increasingly sophisticated under- standing of photosynthetic processes at the chlo- roplast, biochemical and molecular levels, and careful characterizations of gas exchange in leaves under controlled conditions, our under- standing of photosynthetic capacity and per- formance under natural conditions is, in general, rudimentary. Clearly, a major factor contribut- ing to the problem is the extreme variability which characterizes measurements under field conditions. It is difficult to decide whether that variability is simply noise, whether it reflects simple differences between otherwise well- behaved leaves, or whether it reflects a more dynamic variability or control of the integrated photosynthetic processes. In this paper, we will explore those differences with a case study based on two species of man- groves. Mangroves have long been interesting to physiologists and ecologists for the 'characteris-