Photosynthetic characterisation of tropical and temperate rainforest species
Date
2016
Authors
Abdul Bahar, Nur Hazwani
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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.
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Photosynthesis, tropical trees, temperate trees, mesophyll conductance
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Thesis (PhD)
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