Date of Award

Fall 2023

Project Type


Program or Major

Biological Sciences

Degree Name

Doctor of Philosophy

First Advisor

Subhash Minocha

Second Advisor

Rakesh Minocha

Third Advisor

Thomas Foxall


ABSTRACTThe first objective of my Ph.D. research was focused on understanding the reasoning for changes observed in the physiology and metabolism of the poplar plants in response to foliar application of nitrogen. Increased use of nitrogen in the soil in agriculture, in the form of nitrogen fertilizers, has helped to promote high-yield agriculture over the past decades. However, a large fraction of nitrogen applied to croplands in the form of fertilizer and manure ends up entering the freshwater system causing degradation of the water quality and eutrophication of groundwater, rivers, lakes, and coastal and marine ecosystems. Chemical fertilization is also associated with a number of problems, such as low fertilizer utilization rates, soil acidification, and soil salinization. Foliar fertilizer application is an effective method to increase the contents of trace elements in crops and crop yield and to improve the soil environment. While many studies have focused on the effects of foliar application of nutrients, understanding of the metabolic effects of foliar application of nitrogen is scarce. The second objective of my Ph.D. research was focused on evaluating and understanding the effects of soil application of lead the physiology and metabolism of the NM6 poplar plants in response to soil application of lead in order to determine if this species has tolerance to lead toxicity. Soil pollution and contamination due to non-degradable heavy metals such as lead, has become a worldwide concern. Lead in soil can enter the food chain and eventually accumulate in the human body. Lead directly or indirectly affects photosynthesis, plant’s nutrient uptake, seedling growth, enzyme activities, water imbalance, and membrane permeability. Phytoremediation techniques exploit natural hyperaccumulators or transgenic plants that can uptake and accumulate toxic heavy metals such as lead. Phytoremediation is a plant-based technology employed on either wild-type or genetically modified plant species including poplar for restoring contaminated land and water sources. The third objective of my Ph.D. research aimed at understanding the effectiveness of enhanced production of Put via genetic manipulation using the mouse ornithine decarboxylase (mODC) gene in plants on stress tolerance in this species. This gene has been very effective in enhancing putrescine production in cell cultures of poplar NM6, tobacco, carrot, and Arabidopsis thaliana plants. The goal was to produce transgenic plants of poplar NM6 that could be grown in pots in the greenhouse and tested for stress tolerance. The first introduction gives an overview of the economical values of poplar plants and a general introduction to the nitrogen cycle. My first chapter investigates the efficacy of foliar application of different forms of nitrogen on the growth and nitrogen metabolism in young poplar plants. I found that different forms of nitrogen are metabolized differently in plants. Additionally, the level of nitrogen starvation achieved did affect the outcome significantly. My second chapter investigates the changes in the metabolism in response to lead stress in poplar plants. Here we concluded that single application of lead to large plants was not sufficient to induce a strong metabolic response. I found that foliar putrescine could alleviate some of the mild effects of lead observed in the present study. Further investigations will be needed to understand the tolerance of poplar NM6 to lead stress. My third chapter aimed to produce and metabolically characterize transgenic plants of hybrid poplar NM6 expressing a mODC gene that regulates polyamine biosynthesis constitutively and inducibly. Altogether, this research provides a better understanding of the changes in the plant nitrogen metabolism in response to different stress conditions.