Lecture 25 - Long Distance Water Transport

Why Do Plants Need Water?

Water is needed for:

• Activation of enzymes in germination

• Photosynthetic reaction

• Nutrient uptake into roots

• Transpiration

  • Involves the uptake of CO2, cooling the plant, and moving water & nutrients throughout the plant

Long Distance Water Transport

Systems that have evolved moving water & other molecules around the plant over distances larger than transport between individual cells. This system includes both xylem & phloem

• Plant water relations is the study of water availability/potential in plant tissues & the environment around them

Short Distance Water Transport

Cell to cell transport of water through plasodesmata (in the case of symbplastic) or around cells (apoplastic)

Root & Water Distributions in Soil

The deeper we go into the soil, the less dense the roots will be. Most of the roots are densest/concentrated near the surface of the soil

As for water, generally, there’ll be very little water where it overlaps with roots (because plants are taking it up). Then, there is a peak right after the roots where it’ll plateau and be constant.

• The overall level varies from season to season (more dry during the summer, etc)

Relationship between Stomata Aperture & Transpiration

As stomata open more (increase in aperture), there'll be an increased rate in transpiration

When the air is moving, there is fresh dry air coming in to take water molecules away, so stomata will open more and have increased rates of transpiration

• However, when the air is still, transpiration will plateau

Plant Hydraulic System

This involves the phloem and the xylem, which carries water up the root, to the stem, and to leaves. 

In a leafy stem cross section, remember that we’d see vascular bundles composed of phloem & xylem.

In a woody stem cross section, the vasculature would’ve undergone secondary growth where they form rings. The bark would be the phloem while the inner secondary xylem sapwood would be responsible for transporting water

Complex Xylem

Complex xylem have more traceids and vessels that form pits and perforation plates that slow down water transport. 

However when soils are dry, air bubbles can form and prevent water movement (embolism). More complex xylem are able to resist embolism.

Details of Transpiration & Cycle

1. Water vapour will diffuse from the inner moist air spaces to the outer drier air via stomata

2. As they diffuse, water vapour is replaced by evaporation of water on the mesophyll cells

• Here, there is increased transpiration due to more surface tension to pull in more water from surrounding cells & spaces

3. Eventually, water from xylem is pulled into the surrounding cells & air spaces to replace the water that was lost

The overall cycle would be:

1. Water vapour in leaf inner air spaces diffuses out

2. Water from xylem of leaves enters air spaces

3. Bulk transport of water to leaves through xylem (adhesion & cohesion)

4. Water transferred into xylem for transportation from roots (apoplastic & symplastic)

5. Water is absorbed from soil by root cells

Different plant species have different rates of transpiration

Water Potential Gradient & As A Driver

There is a water potential gradient within plants and the outside environment. Water moves from high potential to low potential.

Water potential is highest within the soils, which will enter the plant roots, where it is the highest in the plant. As we go up the plant, water potential decreases, where it is at its lowest at the leaf/tips. Overall, the water potential is lowest in the atmosphere

• Water molecules will travel up through cohesion (to each other) and adhesion (to xylem walls)

• There are steep decreases in water potential as we go from soil to plant to the atmosphere (almost exponential). The steepest being between leaf to air and stem to leaf 

Under drought conditions, overall, the water potential of the plant will decrease over time

• This is also true for the water potential of soils

• In leaves & roots, water potential will drop a lot at first before rising back and then dropping more and rising and so on

  • The leaves experience a larger drop in water potential and will meet the wilting point first

Why Quantify Transpiration?

Quantifying transpiration allows us to better inform and understand:

• Irrigation

• Forest water budget

• Flood mitigation

• Climate change impacts

• Plant mortality

Sap Rise

• There are processes that drive water movement in the soil-plant-atmosphere continuum (SPAC)

• Long distance transport is driven by cohesion-tension, evaporation of water from leaves will pull water from roots

• The water potential gradient across SPAC moves continuous stream of water

• Bulk flow would be water transport over long distance through xylem (up to 45 m/h in trees)

• Some plants have root pressure where there is active transport of solutes in roots, enhancing flow of water into roots to essentially push water up the plant

Push or Pull of Water?

There is also a push of water from root pressure, where the roots will actively transport solutes into roots so that more water will flow in. This extra water will then physically push the water up.

Then, transpiration would be acting as the pulling force where as water leaves via transpiration, more water is pulled up (with adhesion & cohesion) to replace the lost water

Overall, the pulling force is the major driver of long distance water transport

Soil Compostion & Water Content

The water content tends to depend on the soil composition which makes the soil type. Soil grains need to be big enough to let water flow freely while also being small enough to actually retain it.

• Clay soils retain too much water and its hard for plants to access it

• Sandy soils can’t retain water

• Silty soils are the perfect balance

The smaller the particles, the more surface area for water cohesion & adhesion.

Soils can be in three separate states:

• Gravitational where water will drain from a soil saturated with water, leaving macropores filled with air

• Capillary where there are smaller micropores will with capillary water (best condition for plants)

• Hygroscopic where plants have withdrawn all the water they can, but some water is still leftover absorbed to soil particles

Hydraulic Lift

There is a difference in transpiration during the day and night.

• During the day, there is a pull of water from soils, through the plant, and then into the air

• During the night, stomata are closed, but water uptake continues where water is redistributed among roots to rebalance water potentials

Biotic & Abiotic Factors of Transpiration

Some biotic factors influencing transpiration are:

• Plant adaptations

• Plant cover (forest vs. grassland, which affects humidity, light exposure, temperature, etc)

• Leaf area (larger leaf areas would have more stomata to support more transpiration)

Some abiotic factors are:

• Soil moisture availability (includes soil type, organic matter presence, etc)

• Topography

• Climatic conditions (evaporative demand, solar radiation, wind speeds)