Stomatal acclimation to dynamic light: implications for photosynthesis and water use efficiency

Although stomata typically occupy only a small portion of the leaf surface (0.3-5%), stomata control approximately 95% of all gas exchange between the leaf interior and external environment.Therefore, stomatal behaviour has major consequences for photosynthetic CO2fixation and water loss from leaf to canopy levels, influencing carbon and hydrological cycles at global scales. Plant acclimation to growth light environment has been studied extensively; however, the majority of these studies have focused on constant light intensity and photo-acclimation, with few studies exploring the impact of dynamic growth light on stomatal acclimation and behaviour. Initially, in this thesis natural variation in the response of stomatal conductance (gs) to light was assessed in the model tree species Populus nigra. Dynamic growth light regimes (varying in intensity and pattern) were subsequently used, to explore how stomatal acclimation to growth light impacts stomatal behaviour, photosynthesis (A) and water use efficiency (Wi). The rate, magnitude and diurnal behaviour of the response of gs to light varied significantly between genotypes and growth light treatments, which promoted differences in A and therefore Wiover the course of the day. The findings in this study illustrate the impact of growing plants in dynamic light regimes, similar to those experienced by plants in the natural environment, on the physiology and performance of model species Populus nigraand Arabidopsis thaliana. Furthermore, it emphasizes that growing plants under laboratory conditions and square-wave illumination does not accurately represent plant acclimation anddevelopment under a natural environment. Highlighting the need to potentially rethink how we grow plants as a community if we are to infer results from the lab to the field. Finally, this study highlights the importance of considering plant acclimation togrowth light, and the impact this has on the functional response of stomata, when attempting to model the response of gsacross leaf to ecosystem and global scales

Investigating the Dynamics and Density Evolution of Returning Plasma Blobs from the 2011 June 7 Eruption

This work examines infalling matter following an enormous Coronal Mass Ejection (CME) on 2011 June 7. The material formed discrete concentrations, or blobs, in the corona and fell back to the surface, appearing as dark clouds against the bright corona. In this work we examined the density and dynamic evolution of these blobs in order to formally assess the intriguing morphology displayed throughout their descent. The blobs were studied in five wavelengths (94, 131, 171, 193 and 211 \AA) using the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA), comparing background emission to attenuated emission as a function of wavelength to calculate column densities across the descent of four separate blobs. We found the material to have a column density of hydrogen of approximately 2 $\times$ 10$^{19}$ cm$^{-2}$, which is comparable with typical pre-eruption filament column densities. Repeated splitting of the returning material is seen in a manner consistent with the Rayleigh-Taylor instability. Furthermore, the observed distribution of density and its evolution are also a signature of this instability. By approximating the three-dimensional geometry (with data from STEREO-A), volumetric densities were found to be approximately 2 $\times$ 10$^{-14}$ g cm$^{-3}$, and this, along with observed dominant length-scales of the instability, was used to infer a magnetic field of the order 1 G associated with the descending blobs.Comment: 9 pages, 13 figures, accepted for publication in Ap

Acclimation to fluctuating light impacts the rapidity and diurnal rhythm of stomatal conductance.

Plant acclimation to growth light environment has been studied extensively, however, the majority of these studies have focused on light intensity and photo-acclimation, with few studies exploring the impact of dynamic growth light on stomatal acclimation and behavior. In order to assess the impact of growth light regime on stomatal acclimation, we grew plants in three different lighting regimes (with the same average daily intensity); fluctuating with a fixed pattern of light, fluctuating with a randomized pattern of light (sinusoidal), and non-fluctuating (square wave), to assess the effect of light regime dynamics on gas exchange. We demonstrated that gs acclimation is influenced by both intensity and light pattern, modifying the stomatal kinetics at different times of the day resulting in differences in the rapidity and magnitude of the gs response. We also describe and quantify response to an internal signal that uncouples variation in A and gs over the majority of the diurnal period, and represents 25% of the total diurnal gs. This gs response can be characterized by a Gaussian element and when incorporated into the widely used Ball-Berry Model greatly improved the prediction of gs in a dynamic environment. From these findings we conclude that acclimation of gs to growth light could be an important strategy for maintaining carbon fixation and overall plant water status, and should be considered when inferring responses in the field from laboratory based experiments

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

Stomatal control of transpiration is critical for maintaining important processes, such as plant water status, leaf temperature, as well as permitting sufficient CO2 diffusion into the leaf to maintain photosynthetic rates (A). Stomatal conductance often closely correlates with A and is thought to control the balance between water loss and carbon gain. It has been suggested that a mesophyll-driven signal coordinates A and stomatal conductance responses to maintain this relationship; however, the signal has yet to be fully elucidated. Despite this correlation under stable environmental conditions, the responses of both parameters vary spatially and temporally and are dependent on species, environment, and plant water status. Most current models neglect these aspects of gas exchange, although it is clear that they play a vital role in the balance of carbon fixation and water loss. Future efforts should consider the dynamic nature of whole-plant gas exchange and how it represents much more than the sum of its individual leaf-level components, and they should take into consideration the long-term effect on gas exchange over time