Research Interests and Objectives

 David T. Tissue

My research interests are in the physiological, growth and reproductive responses of plants to changes in the availability of resources.  Research efforts in my lab are centered on two general areas: 1) a mechanistic understanding of developmental and physiological factors that influence whole plant response to global climate change factors such as elevated atmospheric CO₂, temperature, and water availability, and 2) environmental control of plant growth, community composition and ultimately ecosystem productivity.

 Research Projects

 Arid Land Response to Changes in Precipitation

 Significant changes in precipitation patterns are predicted as part of the global climate change scenario. Therefore, we have established a large scale precipitation manipulation experiment at Big Bend National Park in the Chihuahuan Desert in Texas, where we are manipulating both summer and winter precipitation. This research is being conducted in collaboration with similar research facilities in the Mojave Desert (Nevada), Sonoran Desert (Arizona), and Great Basin Desert (California) in an effort to develop a unified theory for arid land response to changes in precipitation. In particular, we are testing the impact of alterations in the magnitude and timing of precipitation events, as predicted by global climate models, on species composition, plant physiology and ecosystem carbon and water fluxes. We are working with plant physiologists, microbial ecologists and ecosystem modelers on this project in an effort to link above-ground and below-ground processes to ecosystem productivity in arid lands.

 Impact of Deficit Irrigation and Temperature Stress on Peanut Physiology and Yield

 An important issue in farming is the efficient use of resources such as water while producing a high quality crop. In West Texas, the primary groundwater source is the Ogallala Aquifer which is rapidly being depleted.  In order to conserve water, we are altering the quantity of irrigation during various stages of crop development to identify the best irrigation management strategy that can be utilized to optimize yield, maturity and flavor in peanuts. We are assessing the impact of these various irrigation strategies on plant physiology, leaf nutrient status, root growth, pod development, nitrogen fixation and ultimately yield, maturity and flavor. We are also using molecular techniques to assess changes in gene expression, biochemistry and enzyme function in response to changes in soil moisture.

 Impact of Elevated CO₂ on Physiology and Growth of Onion

 Elevated CO₂ and low levels of lighting are environmental conditions that must be tolerated by plants grown on spacecrafts.  Atmospheric CO₂ levels may reach super-elevated levels (>2000 ppm) in these environments, and at such high levels of CO₂, plant productivity may be reduced. In fact, some studies have indicated that plant response to very high CO₂ concentrations may not be predictable based on plant responses to approximately double ambient CO₂ and that growth at highly elevated CO₂ levels may not be beneficial for plants. In general, it is well known that plants sense and respond to changes in atmospheric CO₂ conditions, but the long-term response of plants to continuous exposure to highly elevated CO₂ levels is not well known. In this research, we examine the effects of low light and elevated and super-elevated CO₂ on plant carbon fluxes, enzyme activity, nutritional composition, and harvestable yields in onion (Allium).  Our goal is to develop a mechanistic understanding of plant response to environmental conditions that may be encountered during space flight so that the production of Allium with desired sensory attributes and phytochemical composition can be maximized. We are also very interested in the long-term impact of super-elevated CO₂ on photosynthesis, respiration and their relationship as mediated by source-sink relationships, carbohydrate content, and enzyme activity and content.

 Effects of Developmental Changes on Physiological Processes

 A central issue in plant ecology is how plant responses to environmental variation are regulated by interactions between developmental and physiological processes. The importance of this issue is exemplified by the difficulty in predicting how individual plants will respond to increasing atmospheric CO₂.  Although developmental processes are clearly key factors regulating physiological responses to the environment, few studies have manipulated development to analyze the effects of normal developmental processes on photosynthesis or respiration. In a series of experiments we are examining whether specific developmental events regulate photosynthetic and respiratory responses to elevated CO₂ and varying nutrient (N and P) levels. We have manipulated the onset of flowering in Xanthium strumarium (cocklebur) to produce plants of different ages and sizes at the same phenological stage (flowering) and plants of different phenological stages (flowering vs. vegetative) at the same age. Our research will address the regulatory mechanisms that control photosynthetic and respiratory responses to developmental processes, specifically the role of soluble sugars and amino acids in regulating key photosynthetic and respiratory enzymes.

 Mistletoe-Spruce Parasitic Interaction as Model System For Understanding the Integration of Development and Physiology Across Scales

 Modular organisms such as trees can die as a consequence of their responses to pathogens as much as from the direct effects of the pathogens. For example, infection by eastern dwarf mistletoe directly affects branch architecture, xylem anatomy and stomatal conductance of host white spruce. Host trees compensate for infection-induced reductions in xylem conductivity by reducing needle size. Reductions in needle size restore the balance between the demand for water and the stem’s capacity to deliver water, thus preserving needle function. However, this host response may actually serve to increase the overall negative impact of infection on whole-tree carbon gain by prolonging the survival of infected branches with poor or possibly even negative carbon balance and by allowing those branches to serve as a source of parasite seed for further infection. Thus, developmental compensation to infection at the branch and needle scale may prove maladaptive in white spruce, as it worsens the impact of infected branches on whole-tree carbon gain and ultimately causes tree death.  The goal of this research is to quantify the direct impacts of infection and the indirect compensatory responses to infection in order to understand how and why white spruce succumbs to mistletoe infection. In Maine, we will measure host water relations and carbon balance at scales ranging from needle physiology, to xylem anatomy and hydraulic properties, to branch and whole-tree physiology and growth.