Stomata and water fluxes through plants Bill Davies The Lancaster Environment Centre, UK
Summary Stomata and responses to the environment Conductance, a function of frequency and aperture Measuring/estimating conductance Mutant stomatal behaviour The transpiration stream Water uptake by roots The development of deficits
Mills, L. N. et al. J. Exp. Bot. 2004 55:199-204; doi:10.1093/jxb/erh027 Copyright restrictions may apply.
Wet cell surfaces - a problem for plant water retention!
Stomatal responses : Short term (minutes to hours) aperture regulation : light temperature increase aperture air humidity water deficit air pollutants (eg. O 3 ) decrease CO 2 partial pressure(p)
Stomatal responses to environmental variation Transpiration = conductance (g) x driving force ( vp)
Estimating leaf (stomatal) conductance The diffusion porometer Conductance changes with changes in stomatal aperture but also changes in size and frequency of stomata
Thermal sensing of stomatal conductance The leaf cools with increasing Evaporation rate
Increasing leaf temperature therefore indicates stomatal closure which itself is a good indicator of plant stress So we can use leaf temperature as an indicator of irrigation need 25 stomatal conductance Tleaf 20 15
Potential application to irrigation Monitoring for failures in irrigation Cotinus
Potential application to irrigation Monitoring for failures in irrigation Automated irrigation control using thermal sensing lavender
1.Take images Increasing the efficiency of water use applying less water effectively 2.Identify leaves sun/shade areas) (and 3.Read temperatures 4.Calculate stress index 5.Decide on irrigation (which pots and how much) 6.Apply irrigation
Change in stomatal density of 100 plant species with CO 2 enrichment (Woodward & Kelly 1995) % of all species 30 20 10 0-80 -70-60 -50-40 -30-20 -10 0 10 20 30 40 50 % change in stomatal density Impact of stomatal differentiation on leaf conductance/water loss
Mutant stomatal behaviour stomata that don t work Hot Mutants: leaf temperature higher than wt at ambient [CO 2 ], and no significant change in leaf temperature from ambient to elevated CO 2 conditions. N2 and HP5 Cool Mutants: no change in leaf temperature between ambient [CO 2 ] and elevated [CO 2 ] conditions. Some appear cooler then wt at ambient CO 2 concentrations. E26.2, EEA8, MQ1, F, and GV5
Mutant EEA8 No response to elevated CO2 assessed with thermal imaging camera Digital images Thermal images: Ambient vs. Elevated [CO 2 ]
MQ1(M3) MQ1 Col-0 Ambient vs. Elevated [CO 2 ] Note: leaves are more wilted then wt at low humidity, softer than wt. Plants grows as fast as wt.
F (M3) [CO 2 ] 360ppm 1500ppm Col-0 F Note: leaves are wider and more flat than wild type. Grows well.
The transpiration stream Xylem - mostly up (water, ions, hormones) Phloem - mostly down (carbohydrates, proteins water, amides, amino acids, organic acids, inorganic ions (no Ca, S and Fe), hormones) A range of compounds from the soil Carbon and a range of exudates
Vascular development from the tip of each shoot to the tip of each root
The Xylem Angiosperms broad leaves most transport through vessels Gymnosperms most transport through tracheids Vessels may be up to 0.3 mm in diameter and may be meters in length (Poiseuille s law -flux proportional to 4th power of the radius) Tracheids - a few tens of microns in diameter and a few mms in length
Relationship between vascular development and leaf area - can be problems
Measuring the hydraulic properties of plants
Hydraulic properties of plants - genotypic variation and functional significance
In the leaf, no cell is more than a few cells from a vein ending
The driving forces for the movement of water and solutes in the xylem
A) Osmotically driven water movement B) Can counteract this with pressure -osmotic pressure C) Excess pressure will drive water flow against an osmotic gradient D) Tension will also drive water flow
In the living cell, walls exert pressure, solutes and membranes generate osmotic forces. In the xylem - no membranes, tensions promote flow. A) Turgid cells will lose turgor as water is lost B) Solutes become more concentrated as water is lost C) Water potential = turgor + solute potential D) All water moves along water potential gradients - from high water potential to low water potential
Water movement into and out of the cell
Wet cell surfaces - a problem for plant water retention!
Water is sucked up the plant - the loss of water from the leaves generates a tension in the xylem A water potential gradient from the atmosphere to the leaf to the stem to the root to the soil
Drying soil will reduce the water potential in the shoots. In the xylem this is largely a function of increasing tension
Cavitation Water columns must be intact for water flux to occur Cohesive and adhesive forces are substantial Run-away cavitation will kill the plant - there is genotypic variation in this property
Water uptake from the soil Driven by a gradient in water potential Water potential of the soil declines as the soil dries For water to enter, the water potential of the root must be more negative - this means dehydration or solute regulation -unless it is pumped!
What is the pathway of radial water movement?
Cross section of a monocot root Casparian band
Aquaporins - water channels in the membranes Diurnal variation in Lp - is this a function of variation in aquaporin activity? From Clarkson et al. 2000
Everything is affected by water deficit!