Lecture notes on stomatal conductance. Agron 516: Crop physiology. Dr. Mark Westgate.

Lecture notes on stomatal conductance. Agron 516: Crop physiology. Dr. Mark Westgate.

Diurnal variation of stomatal conductance has direct consequences for leaf and canopy gas exchange Measure diurnal pattern of transpiration and photosynthesis by a corn canopy Idealized diurnal pattern of stomatal conductance at three levels of water availability Adopted from Christy, A.L., et al. 1986

Representative values of leaf conductances, RH, and water vapor affecting movement of water out of a leaf through a stomatal pore. Ψw at 25 C -1.38 MPa -7.04 MPa Fig. 8.6 P.S. Nobel -95.2 MPa 1. Stomatal conductance typically controls transpiration rate 2. Transpiration rate = stomatal conductance * ΔΨ wv (air - leaf) Nobel, Fig 8.6 At 25 C, Ψ wv = (137.3 MPa) * ln (RH/100)

Formalizing these concepts Flux = conductance X force Transpiration rate = leaf conductance X ΔC wv (leaf - air) J wv = g total wv * (Cias wv - Cai wv ) r but g st wv << g ias wv and g bl wv therefore _ J wv for transpiration ~ _ g st wv

General components of diffusive resistance within the leaf to CO 2 exchange with the atmosphere 4. Mesophyll/Chloroplast (CO 2-liq ) 3. Intercellular air spaces 2. Stomata 1. Boundary layer Taiz & Zeiger, Fig. 19.7

Stomata also control the rate of CO 2 flux into/out of leaves Table 8.4 P.S. Nobel

Again, formalizing these concepts Flux = conductance X force Photosynthesis rate = leaf conductance X ΔC CO2 (air-leaf)* J CO2 = g total CO 2 * (C air CO 2 C chl CO 2 ) Noting that flux through the gaseous and liquid parts of the path are equal g gas CO 2 * (C air CO 2 C ias CO 2 ) = g liq CO 2 * (C ias CO 2 C chl CO 2 ) And that stomata have the lowest conductance along the path g st CO 2 << g bl CO 2 and g ias CO 2 Therefore, J CO2 ~ g st CO 2 *gradient opposite for water vapor

Calculation of internal [CO2] and water use efficiency For water vapor exchange: J wv = g total wv * (Cias wv - Cair wv ) = g total wv (eias wv eair wv ) P atm Where C wv = mole fraction (eg. ppm) e wv = partial pressure (eg. Pa) = C wv * P atm For CO 2 gas exchange: J CO2 = g gas CO 2 * (C ias CO 2 - C air CO 2 ) = g gas CO 2 (a air CO 2 a ias CO 2 ) P atm Where a CO2 = C CO2 * P atm

To calculate internal [CO2]: Water vapor and CO2 diffuse along the same path but CO2 is 60% heavier than water Therefore. g CO2 = g wv 1.6 Substituting into the J CO2 equation. J CO2 = g total wv * (aair CO 2 a ias CO 2 ) Then solve for a ias CO 2 1.6 * P atm a ias CO 2 = a air CO 2 1.6 * P atm * J CO2 g total wv

To calculate Water Use Efficiency (WUE): WUE = J CO2 /J wv ~ Biomass/Crop Water Use ~ Grain yield/seasonal Transpiration J CO2 J wv = g gas CO 2 * (aair CO 2 a ias CO 2 )/ P atm g total wv * (eias wv eair wv )/ P atm Recall that: g CO2 = g wv 1.6 Substituting: J CO2 = (a air CO a ias 2 CO ) 2 1.6 * (e ias wv eair wv ) J wv For C3 plants this ratio is about 1:1000, for C4 plants the ratio is about 1:400

What determines when/how stomatal open?? Open and closed stomata of Vicia faba. Taiz & Zeiger, Fig. 18.10 Guard Cell Turgor (Ca +, K +, sucrose, malate, etc.)

Schematic representation of events leading to opening and closing of stomatal pores Fig. 8.2 P.S. Nobel

Stomatal aperature is regulated by many internal and external factors: External: epidermal cell turgor apoplast ph N-nutrition xylem ABA cytokinins PAR Blue light Vapor pressure deficit CO 2 Internal: Ion channel activity ATPases secondary messengers Nitric Oxide zeaxanthin Diurnal change in stomatal aperature in broad bean in relation to K + and sucrose content. Taiz & Zeiger, Fig 18.17

Stomata close when leaf Ψw decreases Brodribb and Holbrook, 2003

Leaf hydraulic conductance can decrease rapidly at low leaf Ψw Ψp = 0 What causes the decrease in hydraulic conductance? Brodribb and Holbrook, 2003 Leaf hydraulic conductance (K leaf ) is determined by the rate of leaf re-hydration K leaf = C * ln(ψo/ψf) / t

How is the change in leaf hydraulic conductance related to stomatal closure? Tritcum aestivum 1. The water in the xylem is under tension (i.e. Ψpxyl < 0) 2. As leaf Ψw decreases, the tension on the water in the xylem vessels increases (i.e. Ψpxyl < < 0) 0.1 mm Eucalyptus crenulata 3. Large tensions cause embolisms to develop in the xylem vessels 4. Water flow to affected sections of the leaf decrease Then what happens?? 0.3 mm

Stomatal closure is closely associated with an increase in xylem ABA content Sunflower leaves xylem [ABA] increased in water stressed leaves natural soil drying ABA fed to cut leaf No H 2 O Zhang and Davies, 1989a ABA: abscisic acid

ABA appears to be involved in stomata closure even before there is a change in leaf water status No H 2 O Maize leaves Stomata start to close before leaf water status changes WW WS Zhang and Davies, 1989b

But the water status of roots in the upper part of the soil profile does change, and these roots produce ABA No H 2 O Stomata start to close on day 6 WW WS Root ABA content increases dramatically as the soil dries Zhang and Davies, 1989b

How can the leaves remain at high Ψw when the roots are drying? Roots in the upper soil profile dry first, and produce ABA Roots lower in the profile remain hydrated, and supply the shoot with water No H 2 O Maize roots Open symbols: WS Closed symbols: WW 20-100 cm 0-20 cm Zhang and Davies, 1989b

ABA produced in dehyrdating roots in upper soil layers and transported to the shoot may enable plants to anticipate continued soil drying by inducing stomatal closure (feed forward response). How does the ABA get to the leaves? How does the ABA get to the guard cells? What does the ABA do when it gets there?

Re-distribution of ABA from the xylem during water stress Increase in xylem ph (apoplast ph in general) converts some ABAH to ABA -, which decreases uptake by the mesophyll cells, so more ABA reaches the guard cells. Taiz & Zeiger, Fig. 23.3

Cytoplasmic Ca 2+ mediates ABA-induced turgor loss in guard cells J. Schroeder, et al. 2001 [high ] [low] (1, 9, 10) ABA increases Ca 2+ in cytoplasm by increasing uptake, and release from vacuole (2,3,6) Ca 2+ activates anion channels and K + out channels, and inhibits H+ ATPase pump (4,5) Ca 2+ promotes K + release from vacuole, increases cytoplasm ph to promote K + out channels (8,9,10,11) Ca 2+ as second messenger: PLC, InsP3, cadpr, CICR systems. Sustained stomatal closure requires sucrose removal and conversion of malate to starch. PLC: phospholipase C, InsP3: inositol tri-phosphate, cadpr: cyclic ADP ribose

Cytosolic Ca 2+ levels oscillate! Disrupting the oscillation pattern prevents stomatal closure. Hypothesis: Ca 2+ oscillations encode information required for processing closure signals ABA-induced Ca 2+ oscillations in Arabidopsis guard cells expressing fluorescent dye (yellow cameleon 2.1) J. Schroeder, et al. 2001

Proposed regulators mediating guard cell response to external ABA NB: integrated involvement of transporters, pumps, sugar metabolism J. Schroeder, et al. 2001 ROS: reactive oxygen species, PP2C: protein phosphatase, --PK: protein kinases FTase: farnesyl transferase, PLD: phospholipase D, ABC: ATP-binding cassette

References Nobel, P.S. 1991. Physicochemical and environmental plant physiology. Academic Press, Inc. San Diego. Schroeder, J.I., J.M. Kwak, and G.J. Allen. 2001. Guard cell abscicis acid signalling and engineering drought hardiness in plants. Nature 410: 327-330. Zhang, J., and W.J. Davies. 1989. Abscisic acid produced in dehydrating roots may enable the plant to measure the water status of the soil. Plant, Cell, Environ. 12: 73-81. Zhang, J., and W.J. Davies. 1989. Sequential response of whole plant water relations to prolonged soil drying and the involvement of zylem sap ABA I the regulation of stomatal behaviour of sunflower plants. New Phytol. 113: 167-174. Brodribb, T.J., and N.M. Holbrook. 2003. Stomatal closure during leaf dehydration, correlation with other leaf physiological traits. Plant Physiol. 132: 2166-2173. Christy, A.L., D.R. Williamson, and A.S. Wideman. 1986. p 11-20. In: J.C. Shannon et al (eds.) Regulation of carbon and nitrogen reduction and utilization in maize. Amer. Soc. Plant Physiol. Rockville, MD. Taiz, L. and E. Zeiger. 1998. Plant Physiology, second edition. Sinauer Associates, Inc. Sunderland, MA.