Movement of Water and Substances in Plants
Plants move tremendous amounts of water great distances
•	Tomato:  125 L per year; maize: 206 L per year	
–	Animals cycle water between systems
•	Heights of 110 m, over 360 feet

Water loss is a considerable problem
•	Lack of water is number one constraint on plant productivity	
•	Water loss from rivers and reservoirs by plants tapping water 
–	bank vegetation
–	floating plants
Water loss is a necessary evil
•	For photosynthesis to occur, CO2 must enter leaf and dissolve in water
–	membranes quite impermeable to gas
–	must expose damp surface of mesophyll		
•	Necessarily means that moisture is lost through stomates
Energy is the imperative for life, so photosynthesis is favored at the cost of water loss.

Opening and closing stomata regulate water loss, but may limit CO2 uptake as well.
Opening and closing stomata regulate water loss, but may limit CO2 uptake as well.

Some concepts:
•	Turgor pressure:  the pressure inside an enclosure caused by the volume of material inside
•	Water potential:  a term referring to the free energy status of water in various conditions
–	water moves from high to low water potential   (energy status)

•	Factors that reduce water potential:
–	Quantities of solutes inside a cell
–	Reduction in amount of water in a cell
–	Attraction of membranes and walls for water
•	Factors that increase water potential
–	High turgor pressure in a cell

Water enters plants because their water potentials are below those of the soil
•	Osmotic potential and matric potential (--) overwhelm turgor potential (+)
Water is lost from leaves because the potential energy of water in the air is less than inside the leaf.

 (lower concentration of water in air than in leaf)
Mechanics of stomatal opening
•	Guard cells remain open when they are turgid
–	opening is caused by pattern of cellulose microfibrils around cell
•	Loss of turgor causes guard cells to close
•	Loss of turgor is triggered by hormones most of the time, and is not a response to leaf water status except in extreme cases

Abscisic Acid (ABA) is a key stress hormone in stomatal response
Factors regulating stomatal opening
•	Low CO2 in mesophyll stimulates opening
•	Red and blue light stimulatory
•	Low humidity causes closure
–	air currents redistribute humidity and affect process
How does water move?
•	Push vs. pull
Root pressure (push) was plausible, but not correct
•	Solutes in tissues cause water to enter roots by osmosis, raising pressure and pushing water upward
–	Support: cut stumps “bleed”
–	Refutation: not enough energy to do so
–	Refutation: water still moves in stems without roots (remember celery stalk in colored water?)
Water is pulled up roots and stems through evaporation off leaves
•	Accepted explanation
•	A continuum of water potential:
–	air  (LOWEST)
–	leaf
–	twig
–	trunk
–	root
–	root hair
–	soil  (HIGHEST)

•	Depends on the chemistry of water:
–	High tensile strength (water molecules stick to each other)
–	No air bubbles, whose surface tension blocks flow of water
–	alternate pathways in xylem are critical
–	tracheid structure helps too
Confirmation
•	Sap flow should begin in twig, then move to lower trunk
Confirmation
•	Shrinkage of stems and trunks, starting in a.m. with stems
Confirmation
•	Water should be under tension (Scholander pressure bomb)
Movement of water is a passive process (no energy is spent by the plant to move water)
•	Implied by fact that conducting cells of xylem are dead
•	One can mimic system with nonliving components
Routes for water
Hydraulic Lift is a Consequence of the Mechanism of Water Uptake
Hydraulic Lift is a Consequence of the Mechanism of Water Uptake
Hydraulic lift benefits other organisms
Nutrient Uptake Requires Energy
•	Starved plants cannot absorb nutrients
•	Plants accumulate nutrients and reverse the concentration gradient that drives passive processes
Mycorrhizae facilitate nutrient uptake for most plants
•	K, N, S, Ca, Fe, Mg -- important required soil nutrients
•	Important role for P, perhaps Zn, Cu, Mn
Movement of Substances in Phloem Requires Energy
•	Role of phloem in movement of sugars confirmed by:
–	 girdling
–	isotope studies
Münch’s Pressure flow hypothesis remains predominant explanation of phloem transport