WATER Plants most important chemical most often limits

WATER • Plants' most important chemical • most often limits productivity

Climate change will alter rainfall Overall prediction is that crops will suffer in many parts of world

WATER Constantly lose water due to PS (1000 H 2 O/CO 2 fixed) • Water transport is crucial! • SPAC= Soil Plant Air Continuum • moves from soil->plant->air

Water potential Driving force = water's free energy = water potential Yw • Important for many aspects of plant physiology

Water potential Driving force = water's free energy = water potential Yw Water moves to lower its potential

Water potential Driving force = water's free energy = water potential Yw Water moves to lower its potential

Water potential Water moves to lower its potential Depends on: 1. [H 2 O]: Ys (osmotic potential) 2. Pressure Yp 3. Gravity Yg 4. Yw = Ys +Yp + Yg

Water potential Measuring water potential Ys (osmotic potential) is “easy” • Measure concentration of solution in equilibrium with cells Yg (gravity potential) is easy: height above ground YP (pressure potential) is hard! • Pressure bomb = most common technique Others include pressure transducers, xylem probes

Measuring water potential YP (pressure potential) is hard! • Pressure bomb = most common technique Others include pressure transducers, xylem probes Therefore disagree about H 2 O transport in xylem

Measuring water potential Therefore disagree about H 2 O transport in xylem • Driving force = evaporation in leaves (evapotranspiration) • Continuous H 2 O column from leaf to root draws up replacement H 2 O from soil (SPAC)

Measuring water potential Driving force = evaporation in leaves (evapotranspiration) • Continuous H 2 O column from leaf to root draws up replacement H 2 O • Exact mech controversial

Measuring water potential Driving force = evaporation in leaves (evapotranspiration) • Continuous H 2 O column from leaf to root draws up replacement H 2 O • Exact mech controversial Path starts at root hairs

Measuring water potential Path starts at root hairs • Must take water from soil

Measuring water potential Path starts at root hairs • Must take water from soil • Ease depends on availability & how tightly it is bound

Measuring water potential Path starts at root hairs • Must take water from soil • Ease depends on availability & how tightly it is bound • Binding depends on particle size & chem

Measuring water potential Must take water from soil • Ease depends on availability & how tightly it is bound • Binding depends on particle size & chem • Availability depends on amount in soil pores

Measuring water potential Availability depends on amount in soil pores • Saturation: completely full

Measuring water potential Availability depends on amount in soil pores • Saturation: completely full • Field capacity: amount left after gravity has drained excess

Measuring water potential Availability depends on amount in soil pores • Saturation: completely full • Field capacity: amount left after gravity has drained excess • Permanent wilting point: amount where soil water potential is too negative for plants to take it up

Water movement in plants Water enters via root hairs mainly through apoplast until hits Casparian strip : hydrophobic barrier in cell walls of endodermis

Water movement in plants Water enters via root hairs mainly through apoplast until hits Casparian strip : hydrophobic barrier in cell walls of endodermis Must enter endodermal cell

Water Transport Water enters via root hairs mainly through apoplast until hits Casparian strip : hydrophobic barrier in cell walls of endodermis Must enter endodermal cell Why flooded plants wilt!

Water Transport Water enters via root hairs mainly through apoplast until hits Casparian strip : hydrophobic barrier in cell walls of endodermis Must enter endodermal cell Why flooded plants wilt! Controls solutes

Water Transport Must enter endodermal cell Controls solutes Passes water & nutrients to xylem

Water Transport Passes water & nutrients to xylem Ys of xylem makes root pressure

Water Transport Passes water & nutrients to xylem Ys of xylem makes root pressure Causes guttation: pumping water into shoot

Water Transport Passes water & nutrients to xylem Ys of xylem makes root pressure Causes guttation: pumping water into shoot Most water enters near root tips

Water Transport Most water enters near root tips Xylem is dead! Pipes for moving water from root to shoot

Water Transport Most water enters near root tips Xylem is dead! Pipes for moving water from root to shoot Most movement is bulk flow

Water Transport Xylem is dead! Pipes for moving water from root to shoot Most movement is bulk flow • adhesion to cell wall helps

Water Transport Xylem is dead! Pipes for moving water from root to shoot Most movement is bulk flow • adhesion to cell wall helps • Especially if column is broken by cavitation (forms embolisms)

Water Transport Most movement is bulk flow • adhesion to cell wall helps • Especially if column broken by cavitation In leaf water passes to mesophyll

Water Transport Most movement is bulk flow • adhesion to cell wall helps • Especially if column broken by cavitation In leaf water passes to mesophyll, then to air via stomates

Water Transport In leaf water passes to mesophyll, then to air via stomates Driving force = vapor pressure deficit (VPD) • air dryness

Water Transport In leaf water passes to mesophyll, then to air via stomates Driving force = vapor pressure deficit (VPD) • air dryness • ∆ H 2 O vapor pressure [H 2 O(g)] & saturated H 2 O vapor pressure

Water Transport In leaf water passes to mesophyll, then to air via stomates Driving force = vapor pressure deficit (VPD) • air dryness • ∆ H 2 O vapor pressure [H 2 O(g)] & saturated H 2 O vapor pressure • saturated H 2 O vapor pressure varies with T, so RH depends on T

Water Transport In leaf water passes to mesophyll, then to air via stomates Driving force = vapor pressure deficit (VPD) • air dryness • ∆ H 2 O vapor pressure [H 2 O(g)] & saturated H 2 O vapor pressure • saturated H 2 O vapor pressure varies with T, so RH depends on T • VPD is independent of T: says how fast plants lose H 2 O at any T

Water Transport In leaf water passes to mesophyll, then to air via stomates Driving force = vapor pressure deficit (VPD) • air dryness Rate depends on pathway resistances

Water Transport Rate depends on pathway resistances • stomatal resistance

Water Transport Rate depends on pathway resistances • stomatal resistance • Controlled by opening/closing

Water Transport Rate depends on pathway resistances • stomatal resistance • boundary layer resistance • Influenced by leaf shape & wind

Processes affected by p. CO 2 1)Pathways that use CO 2 as substrate • Calvin cycle (Carbon reduction pathway)

Processes affected by p. CO 2 1)Pathways that use CO 2 as substrate • Calvin cycle (Carbon reduction pathway)

Processes affected by p. CO 2 Calvin cycle (Carbon reduction pathway) 1)Ru. BP binds CO 2 2) rapidly splits into two 3 -Phosphoglycerate • therefore called C 3 photosynthesis

Processes affected by p. CO 2 Calvin cycle (Carbon reduction pathway) 1)Ru. BP binds CO 2 2) rapidly splits into two 3 -Phosphoglycerate 3) catalyzed by Rubisco (ribulose 1, 5 bisphosphate carboxylase/oxygenase) the most important & abundant protein on earth • Lousy Km • Rotten Vmax!

Processes affected by p. CO 2 Calvin cycle (Carbon reduction pathway) Reversing glycolysis converts 3 -Phosphoglycerate to G 3 P consumes 12 ATP & 12 NADPH/glucose

Processes affected by p. CO 2 Calvin cycle (Carbon reduction pathway) Reversing glycolysis G 3 P has 2 possible fates 1) 1 in 6 becomes (CH 2 O)n 2) 5 in 6 regenerate Ru. BP

Processes affected by p. CO 2 5 in 6 G 3 P regenerate Ru. BP Basic problem: converting a 3 C to a 5 C compound feed in five 3 C sugars, recover three 5 C sugars

Regenerating Ru. BP Basic problem: converting a 3 C to a 5 C compound must assemble intermediates that can be broken into 5 C sugars after adding 3 C subunit

Processes affected by p. CO 2 1)Pathways that use CO 2 as substrate • Calvin cycle (Carbon reduction pathway) • Rubisco must be carbamylated & bind Mg 2+ to be active!

Processes affected by p. CO 2 1)Pathways that use CO 2 as substrate • Calvin cycle (Carbon reduction pathway) • Rubisco must be carbamylated & bind Mg 2+ to be active! • Ru. BP binds & inactivates uncarbamylated rubisco

Processes affected by p. CO 2 1)Pathways that use CO 2 as substrate • Calvin cycle (Carbon reduction pathway) • Rubisco must be carbamylated & bind Mg 2+ to be active! • Ru. BP binds & inactivates uncarbamylated rubisco • Rubisco activase removes this Ru. BP (also CA 1 P)