Date of Award

8-17-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Stephen Grace

Abstract

Understanding the complex mechanisms of drought tolerance in wild relatives of important crop plants offers a promising strategy for developing more stress resistant commercial cultivars. However, progress is limited by our understanding of the complex pathways involved in drought-tolerant phenotypes. This dissertation investigates and compares the biochemical and physiological effects of water deficit in a drought-tolerant wild tomato species (Solanum habrochaites) and a drought-sensitive commercial cultivar (Solanum lycopersicum cv. M82). M82 shows marked wilting reflected in low relative water content and water potential of the leaves, whereas S. habrochaites retains turgor, leaf water content, and high water potential levels. S. habrochaites displayed a lower bulk elastic modulus value suggesting an alternative strategy for dealing with leaf desiccation compared to osmotic shifts in M82. S. habrochaites alleviates photooxidative stresses caused by drought-induced stomatal closure by utilizing cyclic electron transport in combination with alternative electron sinks. In comparison, M82 suffers from photoinhibition during the same conditions. S. habrochaites increases photorespiration after stomatal closure while showing a significant upregulation of an Fd-dependent oxidoreductase gene used in the re-assimilation of the ammonia produced in photorespiration. Additionally, S habrochaites showed increases in photorespiratory metabolites, gene expression levels, and rubisco oxygenation rates unlike M82. Cyclic electron transport supplements photorespiration and nitrogen assimilation by providing ATP without producing additional NADPH. This is evident by the increased expression of a PGRL5 gene in S. habrochaites, a gene believed to be the molecular “switch” to initiate cyclic electron movement. The glutamate produced from nitrogen re-assimilation may then be used as a carbon source in the TCA cycle. This is indicated by increases in glutamate, aspartate, fumaric acid, citric acid, and expression levels of the respective genes measured in S. habrochaites. Furthermore, S. habrochaites shows increased expression of genes involved in ascorbate and chlorogenic acid metabolism, abscisic acid signaling, metal ion binding, and aquaporin genes. These represent genes commonly observed to be regulated in drought-tolerant plants.

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