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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Henry, Hugh A.L.

Abstract

Both freezing and drought cause cellular dehydration and elicit similar physiological responses in plants, where similar protective compounds are upregulated, including soluble sugars and proteins that help reduce cellular damage. Drought-freeze cross acclimation occurs when drought exposure enhances the freezing tolerance of plants. However, few studies had investigated the reciprocal effects of freezing on drought tolerance, and it was unknown if these interactions could impact plant productivity. Therefore, I examined the effects of freezing on the drought tolerance of individual species and assessed the implications of freeze-drought interactions in an old field community. Poa pratensis was exposed to fall or spring freezing in chambers and a 3-week drought in a greenhouse. Spring-frozen plants had lower reductions in biomass after a summer drought compared to drought-only plants, but this response did not appear to be correlated with the retention of soluble sugars after freezing. Subsequently, I examined freeze-drought responses for a range of herbaceous species: 6 graminoids (Arrhenatherum elatius, Agrostis stolonifera, Bromus inermis, Lolium perenne, Festuca rubra, Poa compressa) and 2 forbs (Securigera varia and Plantago lanceolata). Exposure to a freezing event enhanced the drought tolerance of Agrostis stolonifera, Bromus inermis, Lolium perenne, Poa compressa and Plantago lanceolata, where declines in biomass were lower for freeze-drought plants compared to drought-only plants. These interactions were not correlated to changes in either leaf soluble sugar or protein content before or during the drought. I then collected intact plant-soil mesocosms from an old field and subjected them to spring freezing in chambers and summer drought in the greenhouse. At the functional group level, cross acclimation was observed for legumes; however, for total biomass and nitrogen content in aboveground tissue, cross acclimation was not observed because interactions and additive effects of freezing and drought varied amongst functional groups. Overall, my results highlight the importance of considering plant stress history in experiments, because stressors in different seasons may interact with one another. Nevertheless, cross acclimation responses at the species level may not always scale up to the community or ecosystem scales.

Summary for Lay Audience

Snow provides insulation for many plants during the winter, but rising air temperatures may reduce snow cover and expose these plants to increased freezing overnight or during cold spells. Additionally, less rainfall during the summer will increase the severity of drought in many regions. Exposure to one stress may modify a plant’s ability to tolerate a second, different stress (this is known as cross acclimation). For instance, drought can increase tolerance to freezing, because both cause water loss from plant cells and elicit similar stress responses. However, it is currently unknown whether freezing can result in higher drought tolerance and how the combination of these stresses affects plant growth. I examined the effects of freezing on summer drought tolerance for a range of non-woody plant species. I first exposed Kentucky bluegrass, a common turfgrass, to fall or spring freezing in temperature-controlled chambers, and then to summer drought in a greenhouse. Spring freezing enhanced the drought tolerance of Kentucky bluegrass, and freeze-drought plants survived more and were larger than drought-only plants. I then exposed eight additional non-woody species to spring freezing and assessed the effects on summer drought tolerance. Spring freezing enhanced the drought tolerance of Canada bluegrass, creeping bentgrass, perennial ryegrass, smooth brome and narrowleaf plantain. Additionally, blocks of soil containing plants were removed from an old field and exposed to spring freezing in chambers, and a 3-week summer drought in a greenhouse. Freezing did not increase drought tolerance in terms of total aboveground biomass and concentrations of shoot nitrogen, an important soil nutrient. Cross acclimation was observed for legumes, but not for the other species. Overall, these results suggest that the exposure of plants to spring freezing may help reduce damage for some species during summer drought. These interactions do not appear to affect total plant growth measured across all species but may alter which species are most abundant. Ultimately, freezing may modify the summer stress tolerance of plants and could potentially impact plant growth and the relative abundances of different species.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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