Date of Graduation
Eberly College of Arts and Sciences
Jennifer S. Hawkins
Decreases in land quality and quantity threaten the efficient production of agriculturally and economically pivotal crops. Such reductions in arable lands are a consequence of population growth and urbanization, and often result in the introduction of various abiotic stresses. The most common abiotic stressors include water limitation (drought), water logging (over watering), poor water quality (salinity), and extreme temperatures (cold, frost, heat). Each of these stressors negatively impact plant growth, development, and yield. Soil salinity, specifically, is a considerable constraint affecting lands used in agriculture. Salts in the soil rise both naturally and through anthropogenic factors making the abundance a threat. Salt toxicity affects plants in two phases, the first of which is osmotic stress. Similar to drought stress, salinity imposed osmotic stress is when soil water potential is lower than the osmotic potential of the plant, therefore limiting water and nutrient extraction. Following osmotic stress, plants accumulate salt ions (e.g. Na+, Cl-, SO42-, NO3-) that can increase to toxic concentrations and disrupt normal metabolic processes. Such toxicity results in reduction of important traits such as root biomass, live aboveground biomass, height, and grain yield. The goals of my dissertation work involved dissecting the various morphological, physiological, and genetic underpinnings of salinity tolerance in Sorghum bicolor. Since research to date demonstrates a significant amount of underlying genetic variation, I designed various projects to investigate the genetic controls associated with phenotypic responses to salinity stress among a diverse group of Sorghum genotypes. In my first research chapter, I screened 21 sorghum accessions and interpreted tolerance as the ability to maintain biomass, similar to plants in control conditions, in response to a long-term treatment of 75 mM NaCl. Findings from this research chapter, when combined with published phylogenetic relationships, suggest that the greater salinity tolerance observed in some accessions of Sorghum bicolor, specifically in the landrace durra, are a byproduct of drought tolerance acquired during domestication. To further expand on these findings, I selected two accessions that showed significant variation in tolerance and used qPCR to investigate expression variation of genes associated with salt detoxification. During the secondary phase of salinity stress (referred to as ion toxicity) Na+ and Cl- ions enter the plant and disrupt normal metabolic processes. If the ions are not effectively managed, the primary evident effect is increased leaf senescence. Tolerant genotypes, however, are able to overcome ion toxicity if they can 1) continue production of new growth and 2) efficiently and effectively exclude, extrude, sequester, and transport ions. Results from this chapter indicate that the tolerant accession experienced an earlier onset of osmotic adjustment, promoting the efficient uptake and transport of water. Additionally, the sensitive accession experienced an earlier onset of ionic stress, suggesting poor exclusion at the root level. These findings suggest that the tolerant genotype has better control over osmotic adjustment and ion transport systems, therefore requiring fewer resources to be diverted for the stress response, providing more available energy that can be utilized for new growth and development. Finally, to further investigate the variation in genetic response to salt exposure, I evaluated the salinity tolerance that existed within a recombinant inbred line (RIL) population produced from a cross between Sorghum bicolor and Sorghum propinquum, two species that differ in response to salt exposure. In this study, I used a high-density genetic map to identify genetic markers correlated with salinity tolerance. I identified 146 candidate genes within the18 QTL intervals. QTL containing candidate genes that aid in the alleviation of osmotic stress (i.e. water acquisition, osmotic adjustment) were significantly associated with live aboveground biomass, and QTL containing candidate genes that aid in ionic detoxification (i.e. sensing, signaling, transporting) were significantly associated with an increase in dead aboveground biomass. Given the QTL and their associated phenotypes observed in the study, I suspect that the increased tolerance observed in S. bicolor is a result of early osmotic adjustment followed by effective sensing and signaling during the ionic phase of salinity response. In summary my dissertation work suggests that salinity stress in sorghum triggers a complex network of tightly regulated response elements, where the accumulation of ions, if properly transported and sequestered, aid in osmotic adjustment and ionic alleviation. Further, given Sorghum bicolor’s domestication history, it appears that increased salinity tolerance arose as a byproduct of the drought tolerance acquired during domestication, therefore aiding in an early osmotic adjustment and subsequent water acquisition.
Henderson, Ashley N., "The morphological, physiological, and genetic underpinnings of intraspecific salinity tolerance in Sorghum bicolor" (2020). Graduate Theses, Dissertations, and Problem Reports. 7524.