To hunger or to thirst: plant water-use and photosynthesis Flashcards
Stomata are
the main site of H2O loss and CO2 uptake by plants
Give the equation for water potential in the vapour phase
psiWV = (RT/Vw) x (lnRH(%) / 100)
Describe RT/Vw
135MPa at 20 degrees
What is the relative humidity inside lead air spaces
- 100%
- 0.00MPa psiWV
vpd
vapour pressure deficit
- depends very strongly on ambient temperature
Describe the relationship between CWV(sat.) in molm-3 against air temperatures in degrees C
- positive
- non-linear
- exhibits vpd
CWV(sat.)
saturation water vapour concentration
Describe the importance of stomata in regulating transpirational water loss
- boundary layer adjacent to leaf surface determines transpirational flux
- transpiration largely controlled by stomatal aperture when boundary layer effects are small
- not when boundary layer is large
when are boundary layer effects small?
in moving, turbulent air
when is the boundary layer large
in still air
Describe the relationship between transpirational flux (grams water vapour per cm2 leaf surface per second) against stomatal aperture (micrometers)
much more positive in moving air
Rate of CO2 influx and photosynthesis depend on
- physical resistances
- biochemical ‘resistance’
Describe physical resistance in the leaf
boundary layer, stomata, diffusion in liquid phase to chloroplast
Describe biochemical resistance in the leaf
activity of the Calvin cycle
Describe TR
= H2O lost/CO2 fixed; approx. 500 to 700
TR
- transpiration radio
- defined on either molar or mass basis
Describe WUE
= CO2 fixed/H2O lost; 0.0020 to 0.0014
WUE
- water-use efficiency
- defined as either molar or mass basis
List some stomatal feedforward loops
- direct humidity effect
- direct light effect
List some stomatal feedback loops
- hydro passive
- hydro active
- CO2
How to optimise photosynthesis and transpiration rates
dE/dA = lambda; constant
Describe the relationship between transpiration, E (mmolm-2s-1) against photosynthesis, A (micromolm-2s-1)
- at the bottom end, too little photosynthesis
- at the top end, too much transpiration
- optimum found in the middle
Optimal stomatal behaviour in terms of changes in conductance appears to be that which
maintains the marginal cost (H2O lost) equal to the marginal benefit (CO2 gained)
Constancy of λ leads to:
- maximal amount of CO2 fixation for a given amount of water transpired
- minimal amount of water transpired for a given amount of CO2 fixation
- maximal WUE
Describe the midday depression of photosynthesis
- an example of the optimisation theory in practice
- in seasonally arid, Mediterranean-type ecosystems
- e.g. in Quercus suber (cork oak)
Describe stomatal responsiveness to stimuli
- high sensitivity to CO2 (eudicots)
- high ABA sensitivity + active stomatal control (seed plants)
ABA
- abscisic acid
- stress hormone
Where were stomata innovated?
before the mosses
Describe the evolution of the Tracheophytes
- Late Silurian/ Early Devonian (430Mya): Cooksonia, small
- Early Devonian (410Mya): Rhynia, 15-20cm
- Late Devonian (380Mya): Archaeopteris, Cordaites, Lepidodendron; trees to 35m
Describe the evolution of planate leaves
Describe microphylls
- Early Devonian (400Mya)
- CO2 c.3000 p.p.m. low stomatal density
Describe megaphylls
- Late Devonian (360Mya) / Carboniferous
- CO2 c.300 p.p.m. high stomatal density
What are megaphylls
large planate leaves
Describe the evolution of megaphylls
First Embryophytes in the early Devonian (430–400Mya) had axial form and microphylls
- laminate, planate leaves did not evolve for another 40Ma or so
Did low atmospheric CO2 concentrations and transpirational cooling permit the evolution of megaphylls?
- dramatic decline in [CO2]atm started during the Devonian
- well-established inverse relationship between [CO2]atm and stomatal density
- higher transpiration rates would have become possible and evaporative cooling of the leaves more effective
- permitted the evolution of megaphylls without tissue temperatures reaching lethal values
Plant survival in water-limited environments is mediated by
Functional traits and drought tolerance mechanisms
List some drought tolerance mechanisms and functional traits that allow plant survival in water-limited environments
- life-history strategies
- morphological adaptations
- biochemical mechanisms
Describe plant life history strategies and drought tolerance
- leaf deciduousness
- short life cycle
- dormancy (drought escape)
Describe plant morphological adaptations for drought tolerance
- small leaves
- surface characteristics (leaf hairs; cuticle)
- stomata (number; topography; regulation)
- extensive or deep root systems
Describe some plant biochemical mechanisms for drought tolerance
- turgor maintenance (solute accumulation)
- protective (compatible) solutes in cytoplasm
List some drought-tolerant lineages
- cactus shrubs
- mangrove vegetation on littoral fringes
- thorn woodland or semi-deciduous thicket
List habitat water reliance in lowlands of Trinidad and NE Venezuela
- cactus shrub
- thorn woodland
- deciduous seasonal forest
- semi-evergreen seasonal forest
- evergreen seasonal forest
Describe coastal cactus shrub
terrestrial and epiphytic succulents (bromeliads, orchids, cacti)
Describe thorn woodland or semi-deciduous thicket
many drought-tolerant woody shrubs and small trees
‘Tropical rain forest’ aka
- lowland rain forest
- evergreen broadleaf forest
Describe the understory of a tropical rainforest
sparse vegetation; low light levels
Describe the lower montane rainforest
- more open canopy
- abundant epiphytes on tree limbs
Describe the upper montane forest
- lower canopy
- steeper slopes
- open aspect
- more profuse undergrowth
Describe the dwarf montane forest
- aka elfin forest
- low canopy (~2m) of stunted trees
- due to exposure, high wind speeds, and thin nutrient-poor soils
PAR
- Photosynthetically Active Radiation (400–700nm)
- only a part of the whole solar radiation spectrum
Describe leaf radiation balance
- the energy budget equation
- energy into leaf – energy out of leaf = energy stored by leaf
List the energies that enter a leaf
- absorbed solar irradiation
- absorbed infrared irradiation from surroundings
List the energies that leave a leaf
- emitted infrared (long-wavelength) radiation
- heat loss by conduction and convection
- heat loss by water evaporation
(i.e. latent heat loss by transpiration)
Heat loss by conduction and convection
sensible heat loss
Describe the energy stored by a leaf
- photosynthesis and other metabolism
- leaf temperature changes
Describe leaf and stem absorptance
- inverse of reflectance
- α
- reduced by surface covering of hairs, cuticular waxes and spines (e.g. desert shrubs - Encelia farinosa [Asteraceae], Sonoran Desert, California, USA)
- = 0.8 in spring, 0.3 in late summer
- reduces energy input to the leaf by irradiation
- phenotypic plasticity
In hot, water-limited environments, small, finely dissected leaves have…
- low boundary layer resistance
- efficient sensible heat exchange by conduction and convection (because of high SA:V ratio)
- leaves do not heat up too much above ambient temperature
- e.g. Acacia sp. (Fabaceae), Madagascar
Transpiration (latent heat loss) can cool leaves of some desert plants to … ambient air temperature.
15°C below
Without transpirational cooling, some leaves could be up to … ambient, exceeding lethal temperature for photosynthesis.
20°C above
Describe Death Valley, California, USA
- light: ~ 2000 μmol m−2 s−1
- air temperature: > 50 °C (max.)
- soil temperature: >70°C
- humidity: < 5% RH (min.)
- water potential: < −10MPa
- soil salinity: >3x seawater
Give an example of an halophytic plant
Atriplex hymenelytra (Amaranthaceae, Chenopodiaceae)
Hairs
- enhance reflectivity at visible and near IR wavelengths (0.4–1.6 μm)
- at longer, mid-IR wavelengths (> 8 μm), act as an antireflection layer, enhance emissivity, helping to dissipate heat
The paradox of high-altitude giant rosette plants…
- close to the snow line on tropical mountains (3500–5000 m)
- can be explained by leaf radiation balance
- large mass and nyctinastic leaf movements serve to maintain central meristematic tissues above ambient temperature during the night, so preventing freezing of cellular water
Describe the ‘cushion’ or ‘rosette’ habit in arctic- and alpine-type environments
- maximize energy input by irradiation during the day
- create high boundary-layer resistance to energy loss during the night
large mass has large
heat capacity
