BIO 112 Notes: Adaptations of Plants and Fungi

What role do Fungi play in plant nutrition and what are the two main types of associations the Fungi have with plant roots? How does each benefit from this relationship?

Fungi play an essential role in plant nutrition by forming ==mutualistic associations== with plant roots. These associations are called ==mycorrhizae,== and they enhance the ability of plants to absorb nutrients, particularly phosphorus, from the soil.

  • The two main types of mycorrhizal associations are ectomycorrhizae and arbuscular mycorrhizae. Ectomycorrhizal fungi form a sheath around the root cells of the plant, while arbuscular mycorrhizal fungi penetrate the cells of the root itself.

In both types of mycorrhizae, the fungi form a symbiotic relationship with the plant. The plant provides the fungi with carbohydrates produced by photosynthesis, while the fungi provide the plant with increased access to nutrients, particularly phosphorus, which is often limiting in soil. The fungi have a large surface area that enables them to absorb and transport nutrients more effectively than plant roots alone.

  • ==Ectomycorrhizae==: commonly found in woody plants such as trees, and the fungi in these associations often produce fruiting bodies, such as mushrooms. In addition to enhancing nutrient uptake, ectomycorrhizal fungi can also protect plants from pathogens and environmental stress.
  • ==Arbuscular mycorrhizae==: more common in herbaceous plants, and the fungi in these associations do not produce fruiting bodies. Arbuscular mycorrhizae are particularly important for crop plants, as they have been shown to improve crop yields and reduce the need for fertilizer.

Overall, the mutualistic associations between plants and fungi play a critical role in plant nutrition and ecosystem function. By ==increasing nutrient uptake and providing other benefits, such as protection from stress and pathogens==, these associations help to promote plant growth and health.

Describe some of the unusual adaptions that plants made to obtain the nutrition they need.

Fossil evidence suggests that mycorrhizae were an early evolutionary adaptation that helped plants colonize the land. When the earliest plants, which evolved from green algae, invaded the land, they encountered harsh conditions.

  • The early plants lacked the ability to extract essential nutrients from the soil, while the fungi were unable to manufacture carbohydrates.
  • By forming ==mycorrhizal associations==, both groups of organisms were able to succeed.
Describe the forces that drive the transport of water and minerals from the roots to the shoots via the xylem?
  • Transpiration ==drives the transport of water and minerals== from roots to shoots via the xylem. It provides the pull for the ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots.
    • Hence, xylem sap is normally under negative pressure, or tension.
  • Since transpiration is a “pulling” process, our exploration of the rise of xylem sap by the cohesion-tension mechanism begins with the leaves, where the driving force for transpirational pull begins.
What role do stomata have in regulating transpiration? What internal and external conditions regulate the stomata activity?

Stomata on a leaf’s surface lead to a network of internal air spaces that expose the mesophyll cells to the CO2 required for photosynthesis. The air in these spaces is saturated with water vapor.

  • Water vapor in the air spaces of a leaf diffuses down its water potential gradient and exits the leaf via the stomata.
  • Transpiration refers to this loss of water vapor from plants by diffusion and evaporation.
  • Stomatal opening at dawn is triggered by at least three cues: ==Light, CO2 depletion, and an internal “clock” in guard cells.==
What are sugar sources and sinks? How are sugars moved between the two?
  • Sugar sources: parts of the plant that ==produce and export== sugars. Examples of sugar sources include leaves, where photosynthesis takes place, and storage tissues, such as roots and tubers.
  • Sugar sinks: parts of the plant that ==import and consume== sugars. Examples of sugar sinks include developing seeds and fruits, growing shoot and root tips, and tissues undergoing repair.

Sugars are moved between sources and sinks through the process of ==phloem transport.== The phloem is a specialized tissue in plants that contains a network of cells that transport sugars and other nutrients throughout the plant.

  • Sugar movement in the phloem is driven by a ==pressure gradient.== Sugar is loaded into the phloem at the source, creating a higher concentration of sugars in the phloem there. This causes water to move into the phloem through osmosis, creating a high pressure region that drives the flow of sugar and other nutrients towards the sink.
  • Once sugars reach the sink, they can be used for ==energy production or storage==. Some sugars are converted into starch for long-term storage in roots, tubers, or seeds, while others are used immediately for growth and other metabolic processes.
  • The movement of sugars within a plant can be regulated by a variety of factors, including ==hormonal signals and environmental cues.==
    • For example, sugar movement to developing fruits can be enhanced by the hormone auxin, while exposure to cold temperatures can slow down sugar transport in some plants.