8.7 Chapter 8 Summary

Plant Organs and Tissues

There are two main organ systems in plants:

1. Shoot System: This system generally grows above ground and consists of the non-reproductive parts (like leaves and stems) and the reproductive parts (like flowers and fruits). Its main function is to absorb light for photosynthesis.
2. Root System: This system is usually underground. Its function is to support/anchor the plant and absorb water and minerals.There are two main organ systems in plants:

Water’s Cohesive and Adhesive Properties

• The polarity of water molecules allows them to form hydrogen bonds with other molecules, giving water unique properties critical for life. The properties of cohesion and adhesion are essential for the transport of water from roots to leaves in plants.
• Cohesion: The attraction of water molecules to other water molecules (via hydrogen bonding).
  • Cohesion is responsible for surface tension, the capacity of water’s surface to resist rupture.
• Adhesion: The attraction of water molecules to different molecules (e.g., the walls of a glass tube or plant xylem).
• Capillary Action: The upward movement of water against gravity, which results from the combined forces of cohesion and adhesion.

Roots to Shoots (water movement in plants)

• Water enters the plant through root hairs by osmosis and as a result of active transport of ions from soil (higher water potential) into the plant into the root cells (lower water potential).
• The movement of water through the plant is driven by a difference in water potential, which is influenced by solute potential and pressure potential.
• Transpiration-Cohesion-Tension Mechanism: The main driving force for water movement.
  • Water loss via transpiration from leaves creates a negative pressure (tension) in the xylem.
  • This tension “pulls” the column of water up the plant.
  • The cohesion of water molecules (sticking to each other) maintains the continuous column in the xylem.
  • Adhesion of water to the xylem walls also aids in fighting gravity and holding the water column in place, especially when transpiration stops at night.

Source to Sink (sucrose movement in plants)

• Sections of the phloem near tissue where sucrose is produced have a lower water potential (more solutes)
• Water moves from the xylem into the phloem pushing sucrose away from the source cells.
• Sections of the phloem near tissue where sucrose is stored or used (e.g. fruit, roots, growing leaves) have a lower water potential (less solutes)
• Water moves from the phloem into the phloem, pulling sucrose in the phloem cells toward the sink cells.
• This differential pressure in the phloem drives sucrose from source to sink and may change over the lifetime of the plant.

Stomatal Conductance and Leaves

• Stomata are pores, typically on leaves, that regulate the exchange of gases and water vapor with the atmosphere.
• Stomatal Conductance measures the rate of water vapor loss via transpiration, which relates to gas exchange between CO₂ and O₂ in the leaf.
• Guard cells surrounding the stoma open and close the pore to balance the need for CO₂ uptake (for photosynthesis) against the risk of excessive water loss (transpiration).

Impact of Environmental Change on Plant Physiology

• C3 plants (like rice and wheat) struggle in hot, arid environments because when their stomata close, high levels of O2 compete with CO2 for binding to Rubisco and reduce the efficiency of G3P formation in the Calvin Cycle.

Two key photosynthetic adaptations have helped plants maintain their photosynthetic efficiency in hot, arid environments:

• CAM Plants: Desert plants (cacti, pineapples) that separate gas exchange in time. They open their stomata at night to take in and store CO2​ (preventing water loss during the hot day), and then perform photosynthesis during the day.
• C4 Plants: Plants (maize, sugarcane) that separate gas exchange in space. They fix CO2​ into a 4-carbon molecule and deliver it to specialized bundle sheath cells deep inside the leaf, preventing O2​ from interfering with the CO2​-fixing enzyme Rubisco.

Free Air Carbon Enrichment (FACE) experiments are conducted to model the impact of climate change.

• These experiments expose ecosystems to high levels of carbon dioxide to simulate future atmospheric conditions, allowing scientists to study the effects of elevated CO2​ on a wide variety of plants.
• FACE experiments test the hypothesis that C3 plants are expected to benefit more from rising atmospheric CO_2​ (which can increase their photosynthetic efficiency) compared to C4 plants, which already concentrate CO₂​ internally.
• Researchers observe changes in plant growth, water use, behavior, and nutrient cycling.
• Results indicate that while increased CO₂​ can enhance overall photosynthesis and growth, the specific effects are highly variable and depend significantly on the plant species, the availability of soil nutrients, and water supply.