Outline 1.Functions of water in plants 2.Water potential concept 3.Water uptake and transport 4.Water use efficiency 5.Hydrologic cycle 6.Precipitation effectiveness 7.Plant adaptations to water stress

Why do plants need water? 1.Major component of cytoplasm 2.Solvent, reactant or by-product in reactions 3.Transport e.g. 1.Nutrients from roots to growing parts 2.Photosynthate from leaves to other parts 3.Hormones e.g. cytokinins (growth regulators) from root to buds, abcissic acid from root to epidermis (affects stomata) 4.Structure and growth 1.Turgor pressure (rigidity) in mature cell 2.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: ψ tot = ψ m + ψ s + ψ p 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 ψ soil > ψ stem > ψ leaf > ψ air 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 CO 2 uptake and water loss Transpiration ratio: moles water/moles CO 2 –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. 13 C ratios: diffuses more slowly than other 12 C. Plants with high WUE amplify the difference in diffusion and have higher 13 C ratio. e.g. Stewart et al (1995): 13 C 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 CO 2 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: