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• 1. [Article] Approaches for Characterizing Plant Physiological Responses to Environmental Stress

  It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, ...

  Citation × Citation Citation Abstract Copy Title: Approaches for Characterizing Plant Physiological Responses to Environmental Stress Author: Marias, Danielle E. It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, and responses to stress vary by functional type and developmental stage. Thus, it is crucial to investigate physiological responses to heat and drought stress on multiple species, populations, and growth stages with diverse approaches. In this dissertation, I employ a suite of physiological and modeling methods to inform our knowledge of plant physiological responses to environmental stress in Coffea arabica saplings, Pseudotsuga mensizeii (PSME) and Pinus ponderosa (PIPO) seedlings, and old-growth PIPO. In Chapter 2, I evaluate the effect of leaf age and methodology on the thermotolerance or heat tolerance of C. arabica leaf discs using chlorophyll fluorescence and electrolyte leakage methods. I found that mature leaves were more heat tolerant than expanding leaves, longer time between temperature exposure and measurement yielded more accurate thermotolerance assessments, and photochemistry was more heat-sensitive than cell membranes. To complement the second chapter investigating heat stress responses on detached leaf discs, Chapter 3 examines the effect of leaf age and heat stress duration (45 min or 90 min) on whole-plant physiological responses and capacity to recover in C. arabica by monitoring chlorophyll fluorescence (F[subscript V]/F[subscript M]), gas exchange, and foliar non-structural carbohydrate (NSC) dynamics in situ in response to a simulated heat wave (49°C) in a growth chamber. I found that the 90 min treatment resulted in greater photosynthetic damage and slower recovery than the 45 min treatment, expanding leaves recovered more slowly than in mature leaves, and both heat treatments inhibited flowering. A leaf energy balance model demonstrated that heat stress would be exacerbated by drought-induced stomatal closure. Heat treatment duration significantly impacted NSC dynamics that were closely related to reproduction and repair. Because seedling establishment governs species’ distributions, and because seedlings are particularly threatened by high temperatures at the soil surface, in Chapter 4 I examined the thermotolerance and heat stress responses of PIPO and PSME seedling populations from contrasting climates. Unexpectedly, I found that PSME was more heat tolerant the PIPO. I also monitored physiological recovery after exposure to a simulated heat wave (45°C) by measuring photosynthesis, F[subscript V]/F[subscript M], foliar NSC, and carbon stable isotope ratios (proxy for intrinsic water use efficiency, iWUE). Heat stress responses were consistent with phenotypic plasticity and reflected the conditions under which the plants were grown, while iWUE, a measure of potential drought resistance, was consistent with ecotypic differentiation and the climates from which the seedlings originated. To investigate responses to environmental stress on larger temporal and spatial scales without the challenges of making repeated physiological measurements on old-growth trees, in Chapter 5 I used long-term trajectories of tree-ring growth and carbon and oxygen isotopes of tree-ring cellulose (δ¹³Ccell, and δ¹⁸Ocell) to successfully predict the stand characteristics of two sets (upland, riparian) of old-growth PIPO using the Physiological Principles in Predicting Growth (3-PG) model, the δ¹³Ccell submodel, and a δ¹⁸Ocell submodel added by me. The expanded model helped to explain physiological drivers underlying the different tree-ring growth, δ¹³Ccell, and δ¹⁸Ocell trajectories measured at the upland and riparian sites. The combination of both δ¹⁸O and δ¹³Ccell submodels provided a useful and novel way to constrain 3-PG. This dissertation demonstrates an innovative strategy of applying diverse approaches to understand the physiological mechanisms behind vegetation responses to environmental stress. Title: Approaches for Characterizing Plant Physiological Responses to Environmental Stress Author: Marias, Danielle E. It is uncertain how predicted changes in climate will impact vegetation responses and plant species’ distributions because the physiological mechanisms underlying thresholds for damage are not well understood, and responses to stress vary by functional type and developmental stage. Thus, it is crucial to investigate physiological responses to heat and drought stress on multiple species, populations, and growth stages with diverse approaches. In this dissertation, I employ a suite of physiological and modeling methods to inform our knowledge of plant physiological responses to environmental stress in Coffea arabica saplings, Pseudotsuga mensizeii (PSME) and Pinus ponderosa (PIPO) seedlings, and old-growth PIPO. In Chapter 2, I evaluate the effect of leaf age and methodology on the thermotolerance or heat tolerance of C. arabica leaf discs using chlorophyll fluorescence and electrolyte leakage methods. I found that mature leaves were more heat tolerant than expanding leaves, longer time between temperature exposure and measurement yielded more accurate thermotolerance assessments, and photochemistry was more heat-sensitive than cell membranes. To complement the second chapter investigating heat stress responses on detached leaf discs, Chapter 3 examines the effect of leaf age and heat stress duration (45 min or 90 min) on whole-plant physiological responses and capacity to recover in C. arabica by monitoring chlorophyll fluorescence (F[subscript V]/F[subscript M]), gas exchange, and foliar non-structural carbohydrate (NSC) dynamics in situ in response to a simulated heat wave (49°C) in a growth chamber. I found that the 90 min treatment resulted in greater photosynthetic damage and slower recovery than the 45 min treatment, expanding leaves recovered more slowly than in mature leaves, and both heat treatments inhibited flowering. A leaf energy balance model demonstrated that heat stress would be exacerbated by drought-induced stomatal closure. Heat treatment duration significantly impacted NSC dynamics that were closely related to reproduction and repair. Because seedling establishment governs species’ distributions, and because seedlings are particularly threatened by high temperatures at the soil surface, in Chapter 4 I examined the thermotolerance and heat stress responses of PIPO and PSME seedling populations from contrasting climates. Unexpectedly, I found that PSME was more heat tolerant the PIPO. I also monitored physiological recovery after exposure to a simulated heat wave (45°C) by measuring photosynthesis, F[subscript V]/F[subscript M], foliar NSC, and carbon stable isotope ratios (proxy for intrinsic water use efficiency, iWUE). Heat stress responses were consistent with phenotypic plasticity and reflected the conditions under which the plants were grown, while iWUE, a measure of potential drought resistance, was consistent with ecotypic differentiation and the climates from which the seedlings originated. To investigate responses to environmental stress on larger temporal and spatial scales without the challenges of making repeated physiological measurements on old-growth trees, in Chapter 5 I used long-term trajectories of tree-ring growth and carbon and oxygen isotopes of tree-ring cellulose (δ¹³Ccell, and δ¹⁸Ocell) to successfully predict the stand characteristics of two sets (upland, riparian) of old-growth PIPO using the Physiological Principles in Predicting Growth (3-PG) model, the δ¹³Ccell submodel, and a δ¹⁸Ocell submodel added by me. The expanded model helped to explain physiological drivers underlying the different tree-ring growth, δ¹³Ccell, and δ¹⁸Ocell trajectories measured at the upland and riparian sites. The combination of both δ¹⁸O and δ¹³Ccell submodels provided a useful and novel way to constrain 3-PG. This dissertation demonstrates an innovative strategy of applying diverse approaches to understand the physiological mechanisms behind vegetation responses to environmental stress. Close

• 2. [Article] Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress

  Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species ...

  Citation × Citation Citation Abstract Copy Title: Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress Author: Mosel, Jamie E. Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species dependent, and even within species different populations may demonstrate a spectrum of responses to drought, from susceptibility to resistance. As both loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii) have broad geographic ranges, they provide an opportunity to assess drought resistances of provenances associated with a variety of site climate conditions across their ranges. Furthermore, while studies of singular drought events have revealed important information about seedling stress responses, it has been shown that drought may impart effects even following the release from drought. In some cases, effects of drought exposure can be detrimental or lethal; however, exposure to non-lethal drought may also lead to drought acclimation, which could potentially enhance seedling functioning under subsequent droughts. This potential phenotypic plasticity, i.e. the extent to which a seedling can acclimate to drought conditions, may vary across species and across provenances within a species. This study used physiological (electron transport rate, fluorescence, and water potentials) and growth (biomass accumulation and height increment) responses for two goals: 1) to assess the drought resistances of three provenances of loblolly pine and three provenances of Douglas-fir associated with varying site climate conditions, and 2) to investigate whether previous exposure to a drought results in acclimation to a second drought. We hypothesized that: 1) drought responses would differ among the provenances in accordance with associated site climate conditions (i.e., provenances associated with mesic site climates as characterized by low climatic moisture deficit and high mean annual precipitation would be drought susceptible, and more xeric site climates as characterized by high climatic moisture deficit and low mean annual precipitation would be more drought resistant) as evidenced by physiological measures of electron transport rate, fluorescence, water potentials and growth; and 2) previous exposure to drought would result in acclimation to drought as evidenced by maintenance of physiological function (i.e., higher levels of electron transport and fluorescence) in previously drought exposed seedlings compared to previously unexposed seedlings. We also hypothesized that levels of acclimation would vary among provenances. The study yielded some evidence to support the first hypothesis regarding provenance differences in drought resistances in both species. Provenance drought resistances conformed largely to expectations, though differences were less than expected. In loblolly pine, although not statistically significant, during the second drought there was a pattern of lower maximum electron transport rates, which appeared sooner in the more mesic provenance than in the other two provenances. There were also provenance differences in seedling heights, with the most xeric of the three provenances being shorter at the beginning and the end of the study. In Douglas-fir, there were significant differences in provenance and in the interaction of treatment and provenance for maximum electron rates and fluorescence. Dark-adapted fluorescence was lower in the Coos Bay (mesic site climate) provenance during drought than in the Cascades (mesic site climate) and New Mexico (xeric site climate) provenances. The New Mexico provenance showed the least differences in fluorescence between droughted and watered treatments. The study also yielded some evidence to support the second hypothesis regarding drought acclimation in both species and among provenances. During the second drought, there was a pattern of higher maximum electron transport rates and fluorescence in previously drought exposed seedlings as compared to seedlings previously unexposed to drought. This difference was significant in the mesic provenance (North Carolina) of loblolly pine, with maximum electron transport rates significantly higher in the previously drought exposed treatment compared to the newly exposed treatment during and following the second drought. Patterns of lower electron transport rates in seedlings previously unexposed to drought compared to seedlings previously exposed to drought also appeared in Douglas-fir during the second drought, though not with statistical significance. However, the most xeric provenance showed the reverse pattern during and following the second drought, with lower maximum electron transport rates in the previously drought exposed treatment compared to the treatment previously unexposed to drought. Chlorophyll fluorescence values were significantly higher during the second drought in the previously droughted treatments compared to newly drought-exposed treatments in some provenances of loblolly pine and Douglas-fir. Lastly, seedlings exposed to an early drought had significantly lower final heights than seedlings unexposed to an early drought in both species, although the differences were greater in loblolly pine than Douglas-fir. It may be that growth acclimation, especially in the form of height reductions, influenced physiological responses during a second drought. Further studies are necessary to provide more conclusive evidence in support or against the two hypotheses. Nonetheless, this study provides valuable information on the drought responses of young, greenhouse-grown seedlings of two species that are widespread in North America and that are economically important throughout the world. Further studies in a wider range of age classes, incorporating field studies or more natural settings, may help better predict plant responses in the face of changing climate. Title: Physiological Responses of Loblolly Pine and Douglas-fir Seedlings from Various Provenances to Timing and Frequency of Drought Stress Author: Mosel, Jamie E. Drought is expected to increase in many parts of the world and has been shown to affect tree physiology and growth, with seedlings being particularly vulnerable. Seedling drought responses are often species dependent, and even within species different populations may demonstrate a spectrum of responses to drought, from susceptibility to resistance. As both loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii) have broad geographic ranges, they provide an opportunity to assess drought resistances of provenances associated with a variety of site climate conditions across their ranges. Furthermore, while studies of singular drought events have revealed important information about seedling stress responses, it has been shown that drought may impart effects even following the release from drought. In some cases, effects of drought exposure can be detrimental or lethal; however, exposure to non-lethal drought may also lead to drought acclimation, which could potentially enhance seedling functioning under subsequent droughts. This potential phenotypic plasticity, i.e. the extent to which a seedling can acclimate to drought conditions, may vary across species and across provenances within a species. This study used physiological (electron transport rate, fluorescence, and water potentials) and growth (biomass accumulation and height increment) responses for two goals: 1) to assess the drought resistances of three provenances of loblolly pine and three provenances of Douglas-fir associated with varying site climate conditions, and 2) to investigate whether previous exposure to a drought results in acclimation to a second drought. We hypothesized that: 1) drought responses would differ among the provenances in accordance with associated site climate conditions (i.e., provenances associated with mesic site climates as characterized by low climatic moisture deficit and high mean annual precipitation would be drought susceptible, and more xeric site climates as characterized by high climatic moisture deficit and low mean annual precipitation would be more drought resistant) as evidenced by physiological measures of electron transport rate, fluorescence, water potentials and growth; and 2) previous exposure to drought would result in acclimation to drought as evidenced by maintenance of physiological function (i.e., higher levels of electron transport and fluorescence) in previously drought exposed seedlings compared to previously unexposed seedlings. We also hypothesized that levels of acclimation would vary among provenances. The study yielded some evidence to support the first hypothesis regarding provenance differences in drought resistances in both species. Provenance drought resistances conformed largely to expectations, though differences were less than expected. In loblolly pine, although not statistically significant, during the second drought there was a pattern of lower maximum electron transport rates, which appeared sooner in the more mesic provenance than in the other two provenances. There were also provenance differences in seedling heights, with the most xeric of the three provenances being shorter at the beginning and the end of the study. In Douglas-fir, there were significant differences in provenance and in the interaction of treatment and provenance for maximum electron rates and fluorescence. Dark-adapted fluorescence was lower in the Coos Bay (mesic site climate) provenance during drought than in the Cascades (mesic site climate) and New Mexico (xeric site climate) provenances. The New Mexico provenance showed the least differences in fluorescence between droughted and watered treatments. The study also yielded some evidence to support the second hypothesis regarding drought acclimation in both species and among provenances. During the second drought, there was a pattern of higher maximum electron transport rates and fluorescence in previously drought exposed seedlings as compared to seedlings previously unexposed to drought. This difference was significant in the mesic provenance (North Carolina) of loblolly pine, with maximum electron transport rates significantly higher in the previously drought exposed treatment compared to the newly exposed treatment during and following the second drought. Patterns of lower electron transport rates in seedlings previously unexposed to drought compared to seedlings previously exposed to drought also appeared in Douglas-fir during the second drought, though not with statistical significance. However, the most xeric provenance showed the reverse pattern during and following the second drought, with lower maximum electron transport rates in the previously drought exposed treatment compared to the treatment previously unexposed to drought. Chlorophyll fluorescence values were significantly higher during the second drought in the previously droughted treatments compared to newly drought-exposed treatments in some provenances of loblolly pine and Douglas-fir. Lastly, seedlings exposed to an early drought had significantly lower final heights than seedlings unexposed to an early drought in both species, although the differences were greater in loblolly pine than Douglas-fir. It may be that growth acclimation, especially in the form of height reductions, influenced physiological responses during a second drought. Further studies are necessary to provide more conclusive evidence in support or against the two hypotheses. Nonetheless, this study provides valuable information on the drought responses of young, greenhouse-grown seedlings of two species that are widespread in North America and that are economically important throughout the world. Further studies in a wider range of age classes, incorporating field studies or more natural settings, may help better predict plant responses in the face of changing climate. Close