Photosynthetic characterisation of tropical and temperate rainforest species

Abstract

Rubisco catalyses a rate-limiting step in photosynthesis and is the largest nitrogen sink in leaves. The maximum rate of carboxylation of Rubisco, Vcmax, is routinely estimated from gas exchange using the Farquhar, von Caemmerer & Berry 1980 model of photosynthesis. As Vcmax allows mechanistic representation of photosynthesis, it has been incorporated into terrestrial biosphere models to estimate global primary productivity. However, doubts remain about previous estimates of Vcmax for globally important biomes, such as moist forests, both in tropical and temperate regions. In my thesis, I present a survey of Vcmax values – calculated assuming infinite mesophyll conductance - along a 3,300-meter elevation gradient from lowland western Amazon to the Andean tree line in Peru; this region is home to the largest moist forest on Earth. Large variations in Vcmax were found within and across the 18 field sites. As hypothesised, when estimated at a common measuring temperature (25°C), average Vcmax values of lowland Amazon trees were significantly lower than that of Andean trees. When data for the lowland Amazon and upland Andean trees were combined, the resultant mean tropical Vcmax value was lower than that of temperate trees reported in past studies. My analysis points to low Vcmax of Peruvian tropical trees being linked to limitations in phosphorus supply, and to a high proportion of Rubisco being inactive. The second part of my thesis investigated how mesophyll conductance influences the estimation of Vcmax for several Australian tropical (i.e. warm-adapted) and temperate (i.e. cool-adapted) moist-forest trees. Consistent with previous glasshouse studies, the selected tropical tree species exhibited significantly lower Vcmax values than their temperate counterparts. Importantly, I showed, for the first time, that the Vcmax estimated on the basis of intercellular CO2 partial pressure was equivalent to that on the basis of chloroplastic CO2 partial pressure, when using appropriate Michaelis-Menten constants for CO2 and O2. Thus, low mesophyll conductance in tropical moist forest is unlikely to account for the low estimates of Vcmax found in the Peruvian field work study. Finally, mechanisms underpinning development of photosynthesis in tropical moist forest trees, which include ontogenetic changes in leaf anatomy, and mesophyll and stomatal conductances, were examined. Key components of photosynthesis such as Vcmax, maximum electron transport rate and chlorophyll content increased synchronously during expansion, accompanied by development of leaf internal structures such as intercellular air spaces and mesophyll cells. The balance between photosynthetic carbon uptake and respiratory release changed dramatically during leaf development, reflecting a two-fold decline in area-based rates of respiration in expanding leaves as photosynthesis became fully functional. The dataset presented in my PhD thesis adds to the growing number of empirical estimates highly needed by the photosynthetic modelling communities, and validates the accuracy of Vcmax estimation using biochemical approaches. Collectively, my study is expected to contribute towards better understanding and representation of Vcmax in tropical forests.