Plant overwintering strategies

1. Plant overwintering strategies Jason Zarnowski

2. Over-wintering Success • Plants, not just animals, have adapted for success in cold climates. • As with many evolutionary traits, there is not “best answer”. • Common stressors: • Low temperatures (as low as -65°C in Siberia) • Desiccation • Mechanical Stress Success of plants depends on overcoming these two factors

3. - One strategy is to simply die. - Surviving winter is an energy expensive process. - Annuals produce hearty seeds that remain dormant until conditions become optimal for germination. Herbaceous Annuals

4. - Perennials regenerate year after year. - Root stock stays protected below ground. - New plant produced from existing roots or bulbs. Herbaceous Perennials

5. Woody-stemmed plants • Acclimation to the cold. • “…process by which plants each year become tolerant to subfreezing temperatures without sustaining injury,” Marchand. • Not the process of producing “anti-freeze”. • Plants tissues can undergo “super cooling”. • Process by which tissues are colder than freezing point without forming ice. • Starts chain reaction of flash freezing that released latent heat. • This causes marked rise in temperature around plant stem. • Cytoplasmic water migrates from area of greater energy (cytoplasm) to area of lesser energy (ice forming in extracellular space).

6. Woody-stemmed plants • Causes intercellular solute concentration to increase. • Lowers freezing point. • Formation of ice, not low temperatures, causes cell injury and death. • Even non hardy plants can survive -196°C in liquid nitrogen. • Water is solidified without being oriented into ice crystals “vitrification”.

7. Shows ice formation in extracellular space without perforating cell wall. Woody-stemmed Plants

8. Woody-stemmed plants • Slow cooling key to even heartiest plant species. • Rapid cooling which occurs occasionally in nature, can cause water within cells to freeze resulting in cell death. • Water is trapped in the cell. • Water expands nearly 7% when it becomes ice.

9. Acquiring Freeze Tolerance • Two step process that begins in late summer, early fall. • First stage linked to end of growing season. • Translocation of biochemical compounds. • Simple sugar • Hormone • Second stage linked to first “hard frost”.

10. Acquiring Freeze Tolerance • Alteration of plant membrane also vital. • Membrane structure and permeability altered. • Lipid content increased for freeze tolerance. • Decreased saturation of membrane lipids. • Crystallization point of membrane depressed. • Lipids exhibit higher flexibility when unsaturated. • Increased permeability of membrane offers little resistance to water exiting the cell. • Freeze tolerance exerts strong selective pressure for many species. • Tolerance often within a few degrees of average minimum temperature in species northern limits.

11. Desiccation • Dry winter winds often cause severe desiccation to plants exposed above snowpack at timberline. • Greatest water loss in most plants is on calm, sunny days. • Water loss directly proportional to water vapor in outside air. • Increased leaf temperature leads to increased evapotranspiration rate.

12. Desiccation • Leaf temperatures can be as high as 20°C above ambient temperature due to insolation. • Stomates, cuticle, and boundary layer of air offer resistance to water vapor loss. • Wind removes this layer of air increasing water vapor loss potential, but also lowers leaf temperature thus counteraction loss of air layer.

13. Mechanical Stress • Snow accumulated on branches can be substantial. • As much as 3000 kg in spruce uplands of Finland. • High winds carrying ice particles have affect of a ‘sand blaster’. • Can abrade bark accelerating water loss.

14. Thermogenic Plants • Members of the Araceae family. • Bloom in late February, early March. • Inflorescence can be 15°-35°C above ambient temperature. • Large starch reserves and high metabolic rate create heat as a by-product.

15. Conclusion • Strategy for surviving winter not a “one size fits all” approach. • For example, deciduous vs. evergreens. • Different strategies work equally well for different species. • Adapting to cold climates is an active process.