Studies in Plant Physiology

Primary plant productivity sustains life on Earth and is a natural process for regulating atmospheric carbon dioxide (CO2) concentration. Many of the main features of plant growth and development can be modeled by treating plants as organisms composed of modular morphological subunits with physiological functionality, e.g., leaf, shoot and root. Each subunit performs a specialized function for the plant, thus it can be studied in isolation. The integration of the subunits to sustain the entire living organism is provided by physiological vascular and biochemical regulatory networks. The vascular networks provide the connectivity for distribution of nutrients (carbon, nitrogen and minerals) and other substances throughout the plant. The regulatory networks control the rates of biochemical processes and influence the allocation of substances. For this reason the most developed vascular networks are in the path between sugar producing sites and locations of high growth priority, i.e., sinks.

The overall growth rate and development of plants depend on the availability of exogenous resources. When under prevailing stressful environmental conditions, plants modify their growth and development patterns to increase acquisition of the limiting resources. Such morphological adjustments are mediated by responses of the regulatory network to environmental conditions. The mechanisms that sense environmental conditions and control the allocation of resources (e.g., carbon and nitrogen) in plants are poorly understood. The main goals of the research are:

1) To identify and measure the properties of shifts in the allocation of carbon (sugars) and nitrogen due to rapid changes in environmental conditions; and

2) To measure the physical parameters in existing plant physiology models and also develop new models for substance translocation and allocation, e.g., phloem loading and unloading.

Left: Photograph of PhytoBeta detector imager next to the spicebush (Lindera benzoin). The CO2 gas cuvette (A) is shown near PhytoBeta detector (B) prior to clamping the cuvette on the apical portion of a leaf. Right: Photograph of a leaf with the imaged distribution of 11C overlaid. The imager head abuts is next to the labeling cuvette on the leaf (not shown).

Concept and Experimental Setup

Our approach is to identify cause and effect relationships between changes in environmental conditions and physiological responses that result in shifts in the allocation of carbon and/or nitrogen. Once an effect is observed, the next step is to measure physical quantities associated with resource allocations, e.g., fractional distribution of carbon and/or nitrogen from a source to the sinks and translocation times, and to determine the time scale (e.g., seconds, minutes or hours) for the observed physiological response. Because the details of the responses to changes in environmental conditions depend on the development stage of plants and varies with species, we plan to study a variety of grass and tree species at different stages of development. In addition to learning about the particular species studied, this work should provide insights about growth resource control mechanisms that are applicable to a broad range of plant species.

The measurement technique used in this work is radiotracing with short-lived isotopes that decay by positron emission and direct positron imaging. The isotopes are produced in the tandem laboratory at TUNL and the labeling measurements are carried out at Phytotron facility, which is located about 100 m from the target area where the isotopes are produced. The Phytotron is a controlled environment facility for plant research that is operated by the Duke Biology Department. It has 45 growth chambers with environment controls, e.g., light intensity, atmospheric CO2 concentration, nutrient, rooting medium, and temperature. One chamber is dedicated to this project. We have demonstrated the capability of producing several isotopes in chemical form that can be used for plant studies. These include 11C tagged CO2, 13N tagged NO3 in aqueous solution, 19F ions in aqueous solution, and 15O tagged water.

Experimental Setup: The experimental setup comprises of production of radio-isotopes, the transport of these isotopes to the growth chamber, delivery to various parts of the plant, and imaging of the gamma rays from positron annihilation or direct positron imaging.