Resource Acquisition and Transport in Plants

Bulk flow is the movement of fluid driven by pressure.
Water and solutes move together through:
- Tracheids and vessel elements of xylem (xylem sap).
- Sieve-tube elements of phloem (phloem sap).
Structural adaptations enhance bulk flow in xylem & phloem:
- Mature tracheids and vessel elements lack cytoplasm.
- Sieve-tube elements have few organelles.
- Perforation plates connect vessel elements.
- Porous sieve plates connect sieve-tube elements.
Resources are transported throughout the plant by diffusion, active transport, and bulk flow.

Water and mineral absorption occurs near root tips via root hairs and a permeable epidermis.
Active transport concentrates essential minerals in roots, exceeding the concentration in surrounding soil.
The endodermis is the last checkpoint for selective mineral passage from the cortex into the vasculature.
- The Casparian strip (waxy) in the endodermis blocks apoplastic transfer from the cortex to the vascular cylinder.
- Water and minerals in the apoplast must cross the plasma membrane of endodermal cells to enter the vascular cylinder.
Water crosses the cortex via the symplast or apoplast.

Plants lose most water through evapotranspiration (water evaporation from the plant surface).
Water is replaced by bulk flow of xylem sap (water and minerals) from roots to stems and leaves.
The question is posed: Is the sap mainly pushed up from the roots, or pulled up by the leaves?

Early theories included:
- Mechanical pumps (never found).
- Root pressure (can only move water up a few feet).
- Capillarity (only reaches a few inches).

There is negative pressure in xylem and leaves, but positive pressure in roots.
- Osmotic pressure builds from epidermis to cortex to endodermis.
- Water pressure in roots is higher than in the rest of the plant.
Water or sap oozes from wounds in the stem due to root pressure.
Guttation (water drops from pores in leaf tips) occurs when:
- Transpiration is negligible (often at night).
- Soil moisture is high.

Evapotranspiration is the evaporation of water vapor from the plant, primarily through stomata in leaves.
Water moves from an area of higher water potential to lower water potential.
Water molecules are polar and form hydrogen bonds.
- Cohesion: Water sticks together.
- Adhesion: Water sticks to vessel walls.
Water evaporates from the mesophyll, creating tension on the water column.

Water from the xylem is pulled into cells and air spaces.
Increased surface tension pulls water from cells and air spaces.
The air-water interface retreats as water vapor is replaced from the water film.
Water vapor diffuses out via stomata.

Water potential: $\Psi$ (MPa = megapascals)
Outside air: $\Psi = -100.0 \text{ MPa}$
Leaf (air spaces): $\Psi = -7.0 \text{ MPa}$
Leaf (cell walls): $\Psi = -1.0 \text{ MPa}$
Trunk xylem: $\Psi = -0.8 \text{ MPa}$
Root hair: $\Psi = -0.6 \text{ MPa}$
Soil: $\Psi = -0.3 \text{ MPa}$
Water flows from soil, through the plant, into the atmosphere
Water uptake from soil is driven by this water potential gradient, aided by cohesion and adhesion in the xylem.

Cavitation is the formation of air bubbles in the xylem.
It can result from freezing or drought stress.
- As water freezes, air molecules leave solution as gas.
- With heat, strong transpirational pull may break hydrogen bonds.
Air bubbles break the chain of water molecules, preventing water from being drawn up to the leaves.
Vessels cavitate unevenly.
- Wide vessel elements (angiosperms) form large bubbles that often don’t dissolve.
- Narrow tracheids form small bubbles that are more likely to redissolve.
Tracheids may be less efficient but are "safer."

Large surface area, high surface-to-volume ratio in leaves increase photosynthesis but also increase water loss.
Guard cells open/close stomata to regulate water loss.

Stomata open via active transport:
- A H+/K+ pump concentrates K+ inside guard cells.
- Water follows the lower water potential via osmosis.
- Guard cells expand, opening the pore.

Stomata close via diffusion:
- Active transport stops bringing in K+.
- K+ diffuses out (down the concentration gradient).
- Water follows the lower water potential via osmosis.
- Guard cells contract, closing the pore.

Stomatal opening is triggered by:
- Blue-light receptors in guard cell plasma membrane.
- Low internal CO2 (if water is sufficient).
- Circadian rhythms maintaining a daily cycle.

Stomatal closing is triggered by Abscisic Acid (ABA) hormone:
- Released in high water stress.
- Causes membranes to leak (channels open).
- K+ leaves guard cells.
- Stomata close.

Ocotillo (Fouquieria splendens).
Oleander (Nerium oleander): thick cuticle, trichomes ("hairs") in crypts, sunken stomata.
Old man cactus (Cephalocereus senilis).

Sugars are translocated from sources to sinks via phloem.
At the source, sugar is loaded into the phloem by active transport.
Water follows solutes, entering the phloem by osmosis, creating high turgor pressure (and phloem sap).

At the sink, sugar moves from the phloem to tissues via active transport.
Water leaves via osmosis, creating low turgor pressure.
Phloem sap flows from higher pressure at the source to lower pressure at the sink.

Xylem water flows up (roots to leaves).
Phloem sap flows in every direction (sources to sinks).
Phloem provides systemic transport for:
- Sugars.
- Hormones.
- Viruses.
- Electrical signals (via ions), e.g., rapidly coordinates leaf movements in the sensitive plant (Mimosa pudica).