Plant water regime

• Stomatal conductance– Effects of environmental factors– Diurnal changes– Interactions among factors– Acclimation– Adaxial and abaxial epidermis– Ontogeny– Mathematical modelling– Opening and closing rate– Important characteristics of guard cells

Stomata and environmental conditions

• The primary role of stomata is optimalization of CO2 influx and H2O efflux under various environmental conditions

• Environmental conditions:

• immediate effects

• acclimation

• adaptation

• Responses of stomata are dependent on plant species, age, leaf insertion level, may be different on adaxial and abaxial epidermis or in different parts of a leaf

Sengbush 2006 podle Raschke 1975

Stomatal opening

• Stomata: a pair of guard cells + 1-3 pairs of subsidiary cells

• Opening of stomata is dependent on:

• Pressure potential of guard cells and elasticity of their walls

• Pressure potential of subsidiary cells and of other epidermal cells

• Effects of environmental factors are direct and/or indirect

• Feedbacks

• Isohydric a anisohydric plants

Simultaneous effects of different factors

Diurnal changes in stomatal and cuticular transpiration as affected by water stress development

Response of stomatal conductance to air humidity in different plant species

Effect of irradiance and internal CO2 concentration on stomatal conductance

Effect of leaf water potential and irradiance on stomatal conductance of adaxial and abaxial epidermes

Effect of water potential on adaxial and abaxial stomatal conductance in leaves of different age of plants grown under two different irradiances

Effect of irradiance on adaxial and abaxial stomatal conductance in leaves of different age

Effect of elevated CO2 concentration during plant growth on response of stomatal conductance to different factors

Effect of water potential on stomatal conductance and hydraulic conductance in Ceratonia siliqua leaves (Lo Gullo et al. 2003)

The dependence of stomatal conductance on water potential and irradiance

The dependence of stomatal conductance on water potential and irradiance

Velocity of stomatal movements

• Important for diurnal regulation of gas exchange and during sudden changes of environmental conditions

• Closing is usually more rapid than opening

• Different in different plant species, age, growing conditions

• Advantage of high velocity is protection against water deficit

• Advantage of low velocity - CO2 influx is not decreased at variable irradiance due to clouds or sunflecks

Light-induced stomatal opening as affected by plant age and water stress

Stomatal closing induced by ABA

• Influence of pH on stomatal closing induced by ABA on isolated epidermis from Commelina communis (A) or Arabidopsis thaliana (B). Epidermal strips were incubated at pH 7 (open points) or pH 5 (closed points) under irradiance and low CO2 concentration. After complete opening they were treated by 10 M (squares) or 10 nM (triangles) ABA for 2 h.

• (Prokic et al. 2006)

Important characteristics of guard cells

• Guards cells

• as distinct from other epidermal cells– have not plasmodesmata,

– have chloroplasts (with exception of Paphiopedilum)

• as distinct from mesophyll cells have– lower chlorophyll content

– lower Rubisco activity

– higher PEPC a ATPase activity

– more mitochondria

– photosynthetic efficiency of PS 2 70 - 80 %

Different number and localization of chloroplasts in guard cells of different plant species (Lawson et al. 2003)

Guard cell structure

• low modulus of elasticity enables relatively high changes in volume at relatively low changes in pressure potential

• radial arrangement of cellulose microfibrils in cell wall matrix and different cell wall thickness are important for stomatal opening

• microtubules – arrangement may change during day or by ABA, they might be important for orientation of newly synthetized cellulose microfibrils

• actin filaments – under opening induced by irradiance or high air humidity actin filaments are radially arranged, in the dark or after ABA treatment irregular arrangement of short fragments can be seen

Arrangement of actin filaments and microtubules under irradiance or in the dark

Arrangement of microtubules