Identifying candidate loci for drought tolerance in Douglas-fir

Baesen, Kailey (2023) Identifying candidate loci for drought tolerance in Douglas-fir. Masters thesis, Northern Arizona University.

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Abstract

Douglas-fir (Pseudotsuga menziesii) is an economically and ecologically important conifer species native to North America that is currently experiencing an increase in drought conditions due to climate change. Two varieties of Douglas-fir are widely recognized: coastal (var.menziesii) and interior (var. glauca). These two varieties hybridize to produce coastal x interior inter-varietal hybrids. This work aims to utilize newly developed genetic resources to analyze population structure and understand the genetic basis of drought tolerance in natural populations of Douglas-fir. This study uses an annotated reference genome, genome-wide single nucleotide polymorphisms (SNPs) molecular markers, and physiological measurements in greenhouse seedlings to gain insight into population structure, and to test for associations between phenotypic and environmental data. This work explores the genetic basis of drought tolerance by identifying candidate genes that are significantly associated with water-use efficiency (WUE) (inferred from carbon isotope ratios), photosynthetic rates (inferred from % nitrogen), osmolality, stomatal density, stomatal rows, and heights at 4, 25, 37 and 48 months. Further, population structure was analyzed to confirm genetic varieties and to understand how candidate genes are distributed across the species’ geographic range. Lastly, a drought experiment was implemented on Douglas-fir seedlings to understand the physiological response to drought stress and expand upon previous work that was only conducted on well-watered seedlings. The results of our population structure analysis indicate four genetic clusters: coastal, hybrid, interior north, and interior south. Our results identified 402 unique SNPs associated with drought tolerance that map to 337 unique genes. Due to the absence of a chromosome-scale reference genome and linkage map in Douglas-fir, candidate loci were located only in scaffolds and not in chromosomes. A trade-off was found between tree height at 48 months old and percent nitrogen in well-watered conditions. Percent nitrogen is indicative of photosynthetic capacity; therefore, our results indicate a trade-off between tree height and photosynthetic capacity. This likely arises from the nitrogen budget where plants can either use their nitrogen for the production of cell walls or for photosynthesis. Results from the drought experiment indicates increased WUE in hybrids compared to the pure varieties when placed under drought stress due to an increase in δ13C. Plants will often avoid drought stress by closing their stomata to avoid transpiration. Hybrid individuals, having an increased δ13C under drought conditions, are likely closing their stomata and dealing with the 13C that accumulates as they use 12C for photosynthesis. Drought stress and the use of 13C for photosynthesis can produce reactive oxygen species (ROS). Therefore, hybrid individuals may have a better molecular and physiological response for dealing with drought conditions and ROS. Further, plants will store solutes as a method of holding onto the available water within their cells, this is referred to as osmotic potential. However, there was no significant difference in osmotic potential under well-watered or drought conditions in this study. Future studies should increase the sample size of osmotic potential under well-watered and drought conditions to further understand physiological and genetic responses to drought.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Douglas fir; Seelings; Drought stress; Climate change
Subjects:	S Agriculture > SD Forestry
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Forestry
Date Deposited:	23 Oct 2025 21:22
Last Modified:	23 Oct 2025 21:22
URI:	https://openknowledge.nau.edu/id/eprint/6271

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