Thermal plasticity of leaf energy metabolism: ecological, physiological and biochemical linkages

Abstract

Climate change is warming terrestrial ecosystems across the Earth’s surface. Along with increases in mean daily temperatures, heat waves are predicted to become more frequent, higher intensity and longer duration across the globe in future decades. Under these scenarios, it is vital to understand spatial and temporal variations in the thermal responses of metabolic processes across wide climatic gradients. Understanding the relative contributions of plasticity and evolutionary adaptation in thermal regulation of both photosynthesis and respiration in leaves is critical if we are to better predict future carbon fluxes and vegetation dynamics. This thesis research applied two widely used physiological measurements: temperature responses of leaf dark respiration (R-T) and temperature responses of dark-adapted minimal fluorescence (Fo-T). For R-T curves, R25 (leaf respiration at 25°C) and Tmax (i.e. high temperature at which rates of respiration are maximal were quantified, with the latter providing an estimate of respiratory heat tolerance (RHT). Fo-T curves were used to quantify the temperature (Tcrit) at which Fo rises rapidly as leaves are heated; this was used as a measure of photosynthetic heat tolerance (PHT). This thesis first investigated the components of thermal acclimation (plasticity) and inherent differences (evolutionary adaptation) in respiration and photosynthesis by combing field surveys and controlled environment studies. The thesis also explored mechanisms underlying variation in heat response of PHT. The above traits were quantified at six field sites representing five thermally contrasting biomes across Australia, and in temperature-controlled glasshouses using species sourced from four thermally contrasting origins. For the field study, measurements were made in summer and winter. The first major finding was that thermal acclimation of R25 was evident in the glasshouse study but not when comparing summer and winter values in the field. Second, both Tmax and Tcrit showed consistent acclimation both in the field and glasshouse, with both parameters increased as growth temperature increased. Tcrit differed inherently among species origins, whereas Tmax did not; ca. 40% of the variation in Tcrit could Abstract III be explained by variations in fatty acid composition of cellular membranes. These results imply both acclimation and inherent differences contribute to the contemporary patterns of PHT while only acclimation contributed to the patterns of RHT. The third major finding was that the dynamic responses of Tcrit to heat stress in a tropical tree species were closely related to dynamic changes in heat shock proteins (HSPs) abundance and membrane fatty acid composition, indicating HSPs and membranes are playing significant roles in the adjustments of PHT. Collectively, this thesis enhances our understanding on the ecological patterns of plant metabolic temperature responses. It also provides insights into the biochemical linkages underlying thermal responses of both respiration and photosynthesis. The findings point to more future studies in the plant field in linking ecological, physiological, biochemical and molecular perspectives of heat tolerance response.