Stoma as an ‘interphase’ with environment

SIH1003

Aim..

• Regulation of transpiration• Stomata: structure and function• Sensing the environment

Soil-Plant-Atmosphere Continuum

Normal plant water potential ≈ -1 MPa (water content> 75%)

Atmospheric water potential ≈ - 100 MPa

(T = 25oC and RH = 60%)

Water potential gradient is extremely high (99 MPa) !!!

Due to the different water saturation levels between atmosphere and substomatal pores, water has high tendency to transpire into the atmosphere. This effectuates the substomata to close, except when it’s necessary to open.

Water saturated

This “interphase” gradient is STOMA.

• “interphase”: the exchange of H2O and CO2 is due to different water concentration (plant ─ atmosphere).

-- highly temperature dependent -- Evaporation will lead to leaf cooling

• Stomatal “interphase” is complex: the rate of efficiency is very low, 1/30 to 1/1000, due substomatal pores. (higher leaf temperature, the higher the rate)

“Optimisation Theory”

Transpiration – Photosynthesis Compromise 500 kg soil water to produce 1 kg dry matter - transpiration is a waste?????

• Modus operandii - to regulate the rate of water loss (i.e. transpiration)

• To increase the efficiency of H2O and CO2

exchanges through stomatal pores.

• Boundary layer and stomatal resistances control water loss from leaf• RESISTANCE THEORY – plant as one physical system that exchanges water &

CO2 (refer diagram 1).

• Stomatal conductance; leaf stomatal opening • Important factors- diffusion conduction at leaf surface• Stomata function: control the rate of water loss and recruitment allow influx

of CO2 gaseous during photosynthesis process.

• Instrument : Porometer• Unit : cm s-1; gs = 1/rleaf = 1/ra + 1/rb

• Conductances: calculated simply as (cm s-1) equivalent to distance one molecule diffuses in one second, are finite and easier to quantitate in practise

Stomatal Conductance

rs rs

ra

ra ra

Mesophyll

Epidermis

Boundry layer

Heat H2O CO2

rc rc

rm

ra= Air boundary layer resistance

rs = Stomatal resistance

rc = Cuticular resistance

rm = Mesophyll resistance

ALL RESISTANCES IN

PHOTOSYNTHETIC PROCESS

Diagram 1

• Ecologically, we can make some generalisations about maximal leaf conductance

• Largely, this will tie in with the need to restrict cavitation and capacity for the plant to recharge water status overnight

Stomata: structure and function•Antagonism between guard cell and epidermal turgor•Ultrastructural modifications – alterations in the shape & vol of guard cells by internal changes in the hydrostatic turgor pressure

Figure 1: The radial alignment of the cellulose microfibrils in guard cells and epidermal cells of (A) a kidney-shaped stoma and (B) a grasslike stoma (source: Taiz L., Zeiger E., 2010)

•Guard cells are found in leaves of all vascular plants.

•In grasses, guard cells have a characteristic dumpbell shape, with bulbous ends

(Figure 1).

•These guard cells are always flanked by a pair of differentiated epidermal cells

called subsidiary cells, which help the guard cells control the stomatal pores.

•In dicots and nongrass monocots, guard cells have an elliptical contour (often

called “kidney-shaped”) with the pore at their center.

•Subsidiary cells are often absent, the guard cells are surrounded by ordinary

epidermal cells.

•A distinctive feature of guard cells is the specialized structure of their walls.

•The alignment of cellulose microfibrils, which reinforce all plant cell walls and are

an important determinant of cell shape, play an essential role in the opening and

closing of the stomatal pore.

Stomata: structure and function

• An increase in guard cell turgor pressure opens the stomata• Guard cells function as multisensory hydraulic valves. • Environmental factors such as light intensity and quality, temperature,

leaf water status, and intracellular CO2 concentrations are sensed by guard cells, and these signals are integrated into well-defined stomatal responses.

• The early aspects of this process are ion uptake and other metabolic changes in the guard cells.

• The decrease of osmotic potential (Ψs) resulting from ion uptake and from biosynthesis of organic molecules in the guard cells. Water relations in guard cells follow the same rules as in other cells. As Ψs decreases, the water potential decreases, and water consequently moves into the guard cells. As water enters the cell, turgor pressure increases. Because of the elastic properties of their walls, guard cells can reversible increase their volume by 40 to 100%, depending on the species. Such changes in cell volume lead to opening or closing of the stomatal pore.

• Subsidiary cells appear to play an important role in allowing stomata to open quickly and to achieve large apertures.

Response of stomata to environmental conditions (top), and typical daily patternsof stomatal opening. (From Salisbury and Ross, 1992)

Daily patterns of stomatal activity

Stomatal Regulation

• How do plants ‘decide’ on the most appropriate stomatal aperture for any given situation?

• Stomata seem to sense a variety of environmental parameters and respond accordingly

• The precise response depends on the crop species and the history of the plants

• Environmental factors discussed here: light, CO2, leaf water status & temperature

Sensing the environment• Feedback from internal CO2 and leaf water content (sensed partly by

carbohydrate supply; hydropassive feedback due to direct effects on water supply); Abscisic acid a key signal from roots and mesophyll.

• Feedforward : guard cells have chloroplasts (sense light) and water is evaporated directly around the guard cell complex to alter GC turgor

LightBlue light • Has a direct effect (perceived by guard cells)- similar to the new theory of stomatal

opening • *photoreceptor (flavin, caretenoid or phytochrome, Hinckley & Braatre, 1994)) ─

change the permeability of plasma membrane – stomata open• Has indirectly effect – PAR increasing photosynthesis and decreasing leaf internal CO2

(Ci). This will cause K+ ions to be actively pumped into guard cells, in turn decreasing solute potential in guard cell which causes the water to enter the guard cells, thus increasing turgor and causing the stomatal pore to increase in size

• A decrease in leaf water potential causes the opposite to occur – induce stomatal closure

Red light• red light photoreceptor (chlorophyll, Zeiger, 1990) modification of chloroplast

metabolism (sugar >) stomata open• similar to the old theory of stomatal opening, which is not totally wrong

CO2•CO2 and stomata opening are inversely related, regardless intensity of light (Mansfield et al., 1980; Morrison, 1987)•Stomata is more sensitive to Ci, not Ca (Mott, 1988; Mansfield et al., 1990)• photosynthesis decreases, (i) leaf internal CO2 increases Ci ) (ii) stomatal close (iii) low rate of water loss Overall effect : [CO2] WUE

Drastically increasing [CO2] in the air around a leaf will usually cause at least transient stomatal closure.

Changes in photosynthesis (AN), stomatal conductance (g) andleaf internal CO2 concentration (ci) of a corn leaf in response tofluctuations in incident PPFD. (Data of H.J. Earl)

Leaf water status• As leaf water potential drops, stomata tend to close.

Again, this effect may be direct or indirect.• If water potential is very low, the guard cells themselves,

and the surrounding subsidiary cells, may lose turgor, causing stomatal closure directly.• Much more often, the response to leaf water potential is

indirect. As leaf water potential drops, the content of ABA in the leaves increases. This ABA appears to sensitize the stomata to other signals that would normally cause closing, and so the average stomatal aperture is decreased.

• Evidence that guard cells respond to vapour pressure independent of leaf water status – other evidence?

• Shoot water potential is constant, but stomatal conductance declines in drying air

Relative humidity (RH)• Observation showed when leaf is exposed to low RH, transpiration

rate increased. But this mechanism is still unclear.

• Postulation: • Influence of RH through L (leaf water potential) RH L gs E (Schulze et al., 1987) • Direct effect of RH on i.e. guard cell which is sensitive towards

surrounding RH (ea) and respond accordingly (Turner, 1987). • But the problem with this postulation is that cutical layer is RH

(water) proof.

ei

Substomatal space

RH (ea)

Cutical layer (guard cell is protected from external RH)

ei cannot influence stomata ~ “saturated”

•Influence of RH through metabolism processes which provide energy required for stomatal opening process. e.g.* K+ into guard cell requires energy, * changes in permeability towards H+ also requires energy (Famous postulation : Tallman, 1993)•Stomata has a capability to trace and regulate the rate of water loss “metering volumetric fluxes of H2O” (Meinzer & Grantz,

1991).

Leaf temperature• Leaf temperatures may indirectly affect stomatal opening in several ways:

Changes in temperature affect the photosynthetic rate, and therefore alter leaf internal CO2 stomata

High temperatures cause leaf internal vapor pressure and therefore transpiration to increase.

lead to a reduction in leaf water potential, causing stomata to close.

Temperature may also have a more direct effect on stomata.

In most species, deleteriously high leaf temperatures may induce stomatal opening, even when leaf internal CO2 is not limiting to photosynthesis. (The effect occurs even in darkness.)

This appears to be a strategy designed to decrease leaf temperatures through evaporative cooling.