1 Lecture #5 – Plant Transport

2 Key Concepts: The importance of water Water potential: Ψ = P - s How water moves – gradients, mechanisms and pathways Transpiration – water movement from soil to plant to atmosphere The pressure flow model of phloem transport

3 Diagram – movement of water through a tree WHY WATER??? Required for metabolism and cytoplasm Nutrients are taken up and transported in water-based solution Metabolic products are transported in water- based solution Water movement through the plant affects gas exchange and leaf T

4 Water Potential (Ψ): Controls the movement of water A measure of potential energy Water always moves from an area of HIGH water potential to an area of LOW water potential Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity Ψ = P - s

5 Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides

6

7 minus 4 is MORE NEGATIVE than minus 1

8 High Low

9 Diagram – water potential is universal, including with waterfalls

10 Water Potential (Ψ): Controls the movement of water A measure of potential energy Water always moves from an area of HIGH water potential to an area of LOW water potential Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity Ψ = P - s

11 P – Pressure Potential By convention, set to zero in an open container of water (atmospheric pressure only) In the plant cell, P can be positive, negative or zero  A cell with positive pressure is turgid  A cell with negative pressure is plasmolyzed  A cell with zero pressure is flaccid

12 Turgid P > 0 Plasmolyzed P < 0 Flaccid P = 0

13 Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides What are the little green things???

14 Turgid Plasmolyzed

15 Critical Thinking How can you tell this tissue was artificially plasmolyzed?

16 Critical Thinking How can you tell this tissue was artificially plasmolyzed?

17 Image – turgid plant on left, plasmolyzed on right Crispy means plasmolyzed beyond the permanent wilting point 

18 s – Solute Potential s = zero for pure water  Pure H 2 O = nothing else, not a solution Adding solutes ALWAYS decreases the potential energy of water  Some water molecules now carry a load – there is less free water s s s

19 Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane Remember, Ψ = P – s

20 Ψ = P – s Pressure can be +, -, or 0 Solutes always have a negative effect Simplest way to calculate Ψ is by this equation

21 Flaccid cell in pure water – what happens??? …..what do you know??? ….what do you need to know???

22 Flaccid cell in pure water – what happens???

23 Flaccid cell in pure water – what happens???

24 Flaccid cell in pure water – what happens???

25 Flaccid cell in pure water – what happens???

26 Flaccid cell in pure water – what happens??? …..what do you know??? ….what do you need to know???

27 Flaccid cell in pure water – what happens???

28 Flaccid cell in pure water – what happens???

29 Flaccid cell in pure water – what happens???

30 Flaccid cell in pure water – what happens???

31 Flaccid cell in pure water – what happens???

32 Then what happens???

33 Then what happens???

34 Then what happens???

35 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P

36 Diagram – osmosis across a semi-permeable membrane; next slide also Osmosis Critical Thinking: Where does water move by osmosis in plants???

37 Osmosis Critical Thinking: Where does water move by osmosis in plants???

38 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P

39 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P – no membrane, no solute gradient!

40 Critical Thinking Where does water move by bulk flow in plants???

41 Critical Thinking Where does water move by bulk flow in plants???

42 Diagram – apoplast, symplast and transmembrane pathways; same on next slide Cell Wall Cell Membrane Cytoplasm Routes of water transport soil  root  stem  leaf  atmosphere

43 Cell Wall Cell Membrane Cytoplasm Routes of water transport soil  root  stem  leaf  atmosphere

44 Diagram – Casparian strip; same on next 2 slides

45 The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells Water CANNOT PASS THROUGH the Casparian Strip Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

46 The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells Water CANNOT PASS THROUGH the Casparian Strip Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

47 Critical Thinking Apoplast water is forced into the symplast at the Casparian Strip What does this mean for the water??? What is the function of the Casparian Strip???

48 Critical Thinking Apoplast water is forced into the symplast at the Casparian Strip What does this mean for the water??? What is the function of the Casparian Strip???

49 Critical Thinking Apoplast water is forced into the symplast at the Casparian Strip What does this mean for the water??? What is the function of the Casparian Strip???

50 Diagram – review of membrane transport proteins Membrane Transport (review in text if necessary)

51 Water is on the move

52 Diagram – transpiration Transpiration Movement of water from soil  plant  atmosphere Controlled by HUGE water potential gradient Gradient controlled by P  Very little s contribution Ψ = P - s

53 Micrograph – stomata Stomates are the Valves: as long as the stomata are open, water will move through the plant

54 Diagram – transpiration Transpiration Movement of water from soil  plant  atmosphere Controlled by HUGE water potential gradient Gradient controlled by P  Very little s contribution Ψ = P - s

55 Solar Heating Drives the Process Air is dry because of solar heating  The air molecules bounce around more which causes air masses to expand  Warm air has tremendous capacity to hold water vapor Warm, dry air dramatically reduces the Ψ of the atmosphere Daytime gradient is commonly 30+ MPa

56 Critical Thinking Why do we have life on this planet and not the others in our solar system???

57 Critical Thinking Why do we have life on this planet and not the others in our solar system???

58 Critical Thinking Why do we have life on this planet and not the others in our solar system???

59 Model – our solar system Life is Random

60 Solar Heating Drives the Process Air is dry because of solar heating  The air molecules bounce around more which causes air masses to expand  Warm air has tremendous capacity to hold water vapor Warm, dry air dramatically reduces the Ψ of the atmosphere Daytime gradient is commonly 30+ MPa

61 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

62 Critical Thinking Under what conditions does atmospheric water potential approach zero??? Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

63 Critical Thinking Under what conditions does atmospheric water potential approach zero??? Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

64 Gradient is HUGE Pressure plumbing ~ 0.25 MPa Fully inflated car tire ~ 0.2 MPa Only in the pouring rain does atmospheric Ψ approach zero Soil Ψ is ~ zero under most conditions Remember – gradient is NEGATIVE Water is pulled into plant under TENSION

65 Gradient is HUGE Pressure plumbing ~ 0.25 MPa Fully inflated car tire ~ 0.2 MPa Only in the pouring rain does atmospheric Ψ approach zero Soil Ψ is ~ zero under most conditions Remember – gradient is NEGATIVE Water is pulled into plant under TENSION

66 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

67 Gradient is HUGE Pressure plumbing ~ 0.25 MPa Fully inflated car tire ~ 0.2 MPa Only in the pouring rain does atmospheric Ψ approach zero Soil Ψ is ~ zero under most conditions Remember – gradient is NEGATIVE Water is pulled into plant under TENSION

68 Diagram – transpiration gradient from soil to atmosphere The tension gradient is extreme, especially during the day Sunday, 1 October am – RH = 86% Noon – RH = 53% 4 pm – RH = 36% 8 pm – RH = 62% 5am, 23 September – 94% in light rain

69 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

70 Critical Thinking Tension is a strong force! Why doesn’t the water stream break??? Adhesion and cohesion Why doesn’t the xylem collapse??? Lignin!

71 Critical Thinking Tension is a strong force! Why doesn’t the water stream break??? Why doesn’t the xylem collapse???

72 Critical Thinking Tension is a strong force! Why doesn’t the water stream break??? Why doesn’t the xylem collapse???

73 Diagram – transpiration gradient plus pathways

74 Table – water use by various crops One hectare (2 football fields) of corn transpires about 6 million liters of water per growing season – the equivalent of 2’ of water over the entire hectare…

75 Transpiration is a powerful force! A single broadleaf tree can move 4000 liters of water per day!!! (about 1000 gallons) If humans had to drink that much water we would drink about 10 gallons per day! Transpiration accounts for 90% of evapotranspiration over most terrestrial surfaces Plants are the most important component of the hydrological cycle over land!!!

76 Image – deforestation snaps water cycle and also results in erosion Tropical deforestation is leading to ecological and social disaster Poverty, famine and forced migration 250 million victims of ecological destruction – that’s about how many people live in the US!  ….and just a tiny fraction of the world’s impoverished people Panama You can help change this!!! Guatemala

77 Tropical deforestation is leading to ecological and social disaster Poverty, famine and forced migration 250 million victims of ecological destruction – that’s about how many people live in the US!  ….and just a tiny fraction of the world’s impoverished people Panama You MUST help change this!!! Guatemala

78 Social Justice I’m not angry with you ……

79 Social Justice But I do expect you to DO something !!!

80 Transpiration is a Natural Process It is a physical process that occurs as long as the gradient exists and the pathway is open Under adequate soil moisture conditions the enormous water loss is not a problem for the plant

81 Critical Thinking What happens when soil moisture becomes limited???

82 Critical Thinking What happens when soil moisture becomes limited??? What then???

83 Critical Thinking What happens when soil moisture becomes limited??? What then???

84 What happens when soil moisture becomes limited??? Water stress causes stomata to close Closed stomata halt gas exchange  P/T conflict  P/T compromise Stomata are generally open during the day, closed at night  Abscissic acid promotes stomata closure daily, and under water stress conditions  Other structural adaptations limit water loss when stomata are open  Other metabolic pathways (C 4, CAM) limit water loss

85 Micrograph – turgid guard cells; same next 4 slides Normally, stomata open during the day and close at night in response to changes in K + concentration in stomata guard cells High [K + ] does what to Ψ??? K + accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

86 K + accumulation is triggered by increased light, low carbon dioxide, circadian rhythms Normally, stomata open during the day and close at night in response to changes in K + concentration in stomata guard cells

87 K + accumulation is triggered by increased light, low carbon dioxide, circadian rhythms Normally, stomata open during the day and close at night in response to changes in K + concentration in stomata guard cells

88 K + accumulation is triggered by increased light, low carbon dioxide, circadian rhythms Normally, stomata open during the day and close at night in response to changes in K + concentration in stomata guard cells

89 K + accumulation is triggered by increased light, low carbon dioxide, circadian rhythms Normally, stomata open during the day and close at night in response to changes in K + concentration in stomata guard cells

90 Diagram – open and closed stomata

91 Diagram – hormone mediated stomatal opening and closing Abscissic acid is the hormone that mediates this response

92 Diagram – spoke-like orientation of cellulose microfibrils Cellulose orientation determines shape of turgid cells

93 What happens when soil moisture becomes limited??? Water stress causes stomata to close Closed stomata halt gas exchange  P/T conflict  P/T compromise Stomata are generally open during the day, closed at night  Abscissic acid promotes stomata closure daily, and under water stress conditions  Other structural adaptations limit water loss when stomata are open  Other metabolic pathways (C 4, CAM) limit water loss

94 Micrograph – location of stomatal gradient This is the gradient that counts

95 Images – structural adaptations to dry environments

96 Images and diagrams – metabolic adaptations to dry environments Spatial separation helps C 4 plants be more efficient in hot climates Temporal separation does the same for CAM plants Both use an enzyme that can’t fix O 2 to first capture CO 2 Both adaptations allow photosynthesis to proceed with stomata largely closed during the day

97 Phloem Transport Most of phloem sap is water (70% +) Solutes in phloem sap are mostly carbohydrates, mostly sucrose for most plant species Other solutes (ATP, mineral nutrients, amino acids, hormones, secondary metabolites, etc) can also be translocated in the phloem Phloem transport driven by water potential gradients, but the gradients develop due to active transport – both P and s are important

98 Diagram – pressure flow model of phloem flow; this diagram is repeated throughout this section The Pressure Flow Model For Phloem Transport Xylem transport is uni-directional, driven by solar heating Phloem flow is multi-directional, driven by active transport – source to sink

99 The Pressure Flow Model For Phloem Transport Sources can be leaves, stems or roots Sinks can be leaves, stems, roots or reproductive parts (especially seeds and fruits)

100 The Pressure Flow Model For Phloem Transport Sources and sinks vary depending on metabolic activity, which varies daily and seasonally Most sources supply the nearest sinks, but some take priority

101 Diagram – the transport proteins that actively transport sucrose into the phloem cells from the leaf cells Active transport (uses ATP) builds high sugar concentration in sieve cells adjacent to source

102 The Pressure Flow Model For Phloem Transport High [solute] at source end does what to Ψ???

103 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s increases???

104 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s increases???

105 The Pressure Flow Model For Phloem Transport

106 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ decreases???

107 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ decreases???

108 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ decreases???

109 The Pressure Flow Model For Phloem Transport High [solute] at source end decreases Ψ

110 Critical Thinking What will happen to water pressure in any plant cell as water moves in???

111 Critical Thinking What will happen to water pressure in any plant cell as water moves in??? Why???

112 Critical Thinking What will happen to water pressure in any plant cell as water moves in??? Why???

113 The Pressure Flow Model For Phloem Transport High [solute] at source end decreases Ψ Water moves into the source end of the phloem  This increases the pressure

114 The Pressure Flow Model For Phloem Transport Increased pressure at source end causes phloem sap to move to any area of lower Ψ = sinks

115 The Pressure Flow Model For Phloem Transport At sink end, the sugars are removed by metabolism, by conversion to starch, or by active transport

116 The Pressure Flow Model For Phloem Transport What then happens to the Ψ at the sink end of the phloem???

117 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s decreases???

118 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s decreases???

119 The Pressure Flow Model For Phloem Transport

120 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ increases???

121 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ increases???

122 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ increases???

123 The Pressure Flow Model For Phloem Transport

124 The Pressure Flow Model For Phloem Transport Thus the phloem sap moves – from source to sink  Some xylem water is cycled into and out of the phloem in the process

125 The Pressure Flow Model For Phloem Transport Active transport is always involved at the source end, but only sometimes at the sink end

126 Micrograph – sieve cells; same next slide Critical Thinking What about the structure of the sieve cells facilitates the movement of phloem sap???

127 Critical Thinking What about the structure of the sieve cells facilitates the movement of phloem sap???

128 The Pressure Flow Model For Phloem Transport Questions???

129 Key Concepts: Questions??? The importance of water Water potential: Ψ = P - s How water moves – gradients, mechanisms and pathways Transpiration – water movement from soil to plant to atmosphere The pressure flow model of phloem transport