Response of plant respiration to past and future climates

Abstract

Since the industrial revolution, atmospheric CO{u2082} concentrations have increased from below 300 {u00B5}L L{u207B}{u00B9} to the current 387 {u00B5}L L{u207B}{u00B9} , and are likely to rise to 700 {u00B5}L L{u207B}{u00B9} in the coming decades. Associated with the rise in atmospheric [CO{u2082}] have been an increase in global temperatures and the frequency of severe droughts. To predict the impact of climate change on the biosphere, one must understand the effects of a range of abiotic (particularly increasing atmospheric [CO{u2082}], growth temperatures and drought) and biotic (e.g. symbioses with fungi) factors on plant respiration (R) rates. While much is known about effect of climate variables on rates of photosynthesis and plant productivity, little is known about the climate responses of leaf and root R. In this thesis, I sought to understand how a wide environmental envelope that encompasses past and future climate scenarios affects rates of plant R, with particular focus being given to the effects of subambient to elevated atmospheric CO{u2082} concentrations. Additional studies were conducted to assess the main and interactive effects of atmospheric [CO{u2082}] with water availability, differences in growth temperature, and colonization of roots by mycorrhizal fungi. In seeking to understand how past and future climates, particularly atmospheric CO{u2082} concentrations impact on the carbon economy of plants, I conducted two experiments using contrasting plant species: soybean (Glycine max L.) and Sydney Blue Gum Eucalyptus saligna Sm. For soybean, plants were grown in pots and developed in growth cabinets differing in CO{u2082} concentrations. In this study, I combined a functional growth analysis with measurements of photosynthetic and respiratory processes. Crucially, I quantified the impacts of growth under 290, 400 and 700 {u00B5}L L{u207B}{u00B9} atmospheric [CO{u2082}] on root and leaf R (both in light (Rlight; determined using the Kok method) and dark (Rdark), with short-term changes in measurement [CO{u2082}] and [O{u2082}] being used to further explore the relationship between light inhibition of leaf R and photorespiratory flux. My data showed that elevated [CO{u2082}] resulted in higher relative growth rates (RGR). Moreover, in contrast to the inhibitory effect of low [CO{u2082}] on light-saturated photosynthesis, growth [CO{u2082}] had no significant effect on rates of R in both roots and leaves. To further explore the impact of atmospheric [CO{u2082}] on leaf and root R of soybean, I conducted an additional experiment using mycorrhizal and nonmycorrhizal plants under two growth CO{u2082} concentrations [ambient (400 {u00B5}L L{u207B}{u00B9}) and elevated (700 {u00B5}L L{u207B}{u00B9} ) ] . Here, I tested the hypothesis that plants whose roots are colonized by arbuscular mycorrhiza (AM) fungi would exhibit higher rates of photosynthesis and root R than plants whose roots are not colonized by AM fungi.. Surprisingly, there was no significant difference in the relative growth rate (RGR), rate of photosynthesis, leaf or roots R rates, across the four treatment combinations. To establish the effect of past and future climates on leaf R rates on Eucalyptus, I conducted an experiment under glasshouse conditions, which was CO{u2082} and temperature controlled. Plants were raised in pots. Well-watered and drought-treated plants were grown under conditions differing in growth [CO{u2082}] (280,400 and 640 {u00B5}L L{u207B}{u00B9}) and temperature (26 and 30{u00B0}C). Rates of leaf R in both darkness (Rdark) and in the light (Rlight) were measured. Growth [CO{u2082}] and temperature had little impact on area-based rates of Rdark or Rlighb with the latter indicating little thermal acclimation to the 4{u00B0}C difference in growth temperature (it was short term temperature subjection). However, sustained drought resulted in reduced rates of Rdark, Rlight and light-saturated photosynthesis (Asat) , with the inhibitory effect of drought on Asat and Rlight being greater than on Rdark. Finally, my study from both species provided strong evidence that rates of leaf Rlight were lower than those of Rdark, (with light inhibiting leaf R by 17-47%) and that variations in Rlight can be predicted with a high degree of accuracy from knowledge of the underlying rates of Rdark and associated rates of photosynthetic capacity. Collectively, my research findings highlight the main and interactive effects of several important abiotic factors on plant R, with the results providing a novel framework via which rates of Rlight can be predicted using readily available gas exchange parameters.