Outline • Functions of water in plants • Water potential concept • Water uptake and transport • Water use efficiency • Hydrologic cycle • Precipitation effectiveness • Plant adaptations to water stress
Why do plants need water? • Major component of cytoplasm • Solvent, reactant or by-product in reactions • Transport e.g. • Nutrients from roots to growing parts • Photosynthate from leaves to other parts • Hormones e.g. cytokinins (growth regulators) from root to buds, abcissic acid from root to epidermis (affects stomata) • Structure and growth • Turgor pressure (rigidity) in mature cell • Growth in young cell
Water potential (ψ) • Measure of “free energy” of water relative to pure water • Water moves from high energy to low energy • Measured in megapascals (pressure unit related to energy/mass) • Pure water potential = 0 therefore all water in biosphere at – ψ.
Water potential (ψ) • Three components: ψtot = ψm + ψs + ψp M = matric potential (-ve) • S= osmotic potential (-ve) • P= hydrostatic or pressure potential (+ve) • Total is negative • Change in osmotic potential can be an acclimation to water deficit or an adaptation to xeric environments (lower osmotic potential – maintain turgor at lower water potentials)
Water movement (hydrodynamics) • Enters through roots (either cell to cell or between cell walls) • Moves into xylem (vascular tissue) • Moves into leaves, into mesophyll cells, then to substomatal cavities. • Vaporizes and transpires through stomatal pores
Water movement (hydrodynamics) • Driving force: transpiration (SPAC hypothesis) • Transpiration from leaves creates gradient of negative water potential ψsoil > ψstem > ψleaf > ψair • Requires continuous column of water • Regulated by stomates
Water movement (hydrodynamics) • Multiforce hypothesis: • Some evidence that pressure alone is not only driving force. • Some plants have “water capacitance” – can drive transpiration with cell water not just soil water • Osmotic potential may also be important (solutes in xylem) • Convection along bubble surfaces may speed movement
Water Use Efficiency • Trade-off between CO2 uptake and water loss • Transpiration ratio: moles water/moles CO2 • e.g. Corn 1 kg dry matter takes 600 kg water • Affected largely by leaf characteristics • Diffusive resistance • Boundary layer • Movement within leaf • Cuticular transpiration • Stomatal resistance • Leaf form (larger leaves – cool temps)
Soil water and wilting point • Water is held in soils by capillary action and matric forces. Field capacity is max amount of water held by soil (after gravitational flow). • Gravitational water flows through; significant only in saturated soils. • Permanent wilting point (PWP)= point at which plants can’t extract more soil water (held too strongly to particles).
Hydrologic cycle USGS
Precipitation effectiveness • “Precipitation” can include condensation, rain, snow. • Season, precipitation type, intensity, variability etc. can all affect water availability • e.g. central Australia less xeric than many areas, but huge variability in rainfall (from 58 to 1150 mm per year) leads to xeric vegetation
Site water balance • Attempts to assess “droughtiness” and potential vegetation of habitats. • Based on soil water storage, growing season precip., and potential evapotranspiration. • 13C ratios: diffuses more slowly than other 12C. Plants with high WUE amplify the difference in diffusion and have higher 13C ratio. • e.g. Stewart et al (1995): 13C ratios paralleled rainfall gradient in Queensland.
Interception • Plant structure affects amount of water entering soil: • Interception and stem flow • Throughfall • Causes uneven distribution of water; can affect vegetation composition (shrub VS grassland)
Adaptations to water stress • Drought escape: ephemerals. Finish life cycle while conditions good • Dehydration tolerance – rare in vascular plants • Dormancy e.g. bunchgrasses • Dehydration postponement – osmotic adjustment, water storage (succulents), reduced transpiration (e.g. deciduous leaves)
Adaptations to water stress • CAM photosynthesis: usually in succulents. Store water and acid in central vacuole; can store sufficient water to continue CO2 fixation at permanent wilting point. • Xeromorphic leaves: • Reduce transpiration rate, increase boundary layer • Small, reduced cell size, thick blades, sunken stomata, stomata on lower leaf surface, less intercellular space
Adaptations to water stress • Phreatophytes (“well plants”) roots remain in contact with permanent ground water (riparian zones, basins) • May have very high transpiration rates (e.g. mesquite, tamarisk) • May need to be salt tolerant (desert depressions accumulate water and salt) e.g. shadscale – salt excreting glands.
Vegetation types and water Purves et al.
Example • Model of hydrodynamics to predict forest vegetation and production: http://www.wsl.ch/projects/LAASim/laasim.html