Plant biology Topic 9

Transport in the xylem of plants Topic 9.1

U: Transpiration is the inevitable consequence of gas exchange in the leaf. Leaf = primary organ of photosyn. Gas exchange in leaf = CO2 in, O2 out Occurs through stoma (stomata, pl) = pores in epidermis With gas exchange comes H2O loss Transpiration = loss of H2O vapor from leaves & stems Guard cells = minimize H2O loss by controlling aperture of stoma Liverworts = group of plants without stomata

How Stomata work

App: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots & capillary tubing In capillary tubes, water fills tube because of adhesion of H2O to glass, the cohesion of H2O causes water to be drawn up. Same thing happens in porous clay pots & filter paper. Adhesion & cohesion are not properties of mercury.

Water transport demo

Skill: Measurement of transpiration rates using potometers (Practical 7) Rate of transpiration difficult to measure directly Rate of water uptake can be measured instead, using a potometer Potometer = measures water uptake in plant

 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant Light Temperature Increasing light intensity increases rate of transpiration Light stimulates the opening of stomata (gas exchange required for photosynthesis to occur)  Some of the light energy absorbed by leaves is converted into heat, which increases the rate of water evaporation Increasing temp increases rate of transpiration Higher temps cause increase in water vaporisation in spongy mesophyll and increase in evaporation from leaf surface This leads to an increase in diffusion of water vapour out of leaf which increases rate of transpiration

wind Humidity Greater air flow around leaf surface increases the rate of transpiration  Wind removes water vapour (lower concentration of vapour on leaf surface), increasing rate of diffusion from within spongy mesophyll Increasing humidity decreases rate of transpiration Humidity is water vapour in air, thus high humidity means there is high conc of water vapour in air This reduces rate of diffusion of water vapour from inside leaf (concentration gradient is smaller resulting in less net flow)

Skill: design of an experiment to test hypotheses about the effect of temp or humidity on transpiration rates Questions to consider… How will you measure the rate of transpiration in your investigation? What biotic or abiotic factor will you investigate? How will you vary the level of this factor? How many results do you need, at each level of the factor that you are varying? How will you keep other factors constant, so that they do not affect the rate of transpiration?

U: The cohesive property of water & the structure of the xylem vessels allow transport under tension Structure of xylem vessels allows them to transport water inside plants efficiently: Long continuous tubes that transport water Walls thickened Reinforced & strengthened with spiral bands of lignin Lignin strengthens walls so they can withstand low pressures w/out collapsing Spiral bands allow xylem vessels to elongate & grow lengthwise Xylem cells are nonliving, so water flow is passive

Three ways by which terrestrial plants may support themselves are: Thickened cellulose:  Thickening of the cell wall provides extra structural support Cell turgor:  Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells turgid Lignified xylem:  Xylem vessels run the length of the stem and branches, lignification of these vessels provides extra support

Xylem vessels are formed from files of cells arranged end-to-end Xylem cells are dead! so water flow is passive Xylem cells are hollow & consist only of cell wall Cohesion = attraction of H2O molecules to each other due to polarity Adhesion = attraction of H2O molecules to hydrophilic parts of xylem cell walls Transpiration stream = H2O is pulled up from the xylem in a continuous stream

U: The adhesive property of water & evaporation generate tension forces in leaf cell walls First step of transpiration = evaporation Evaporation of one molecule of water moves another molecule with it due to adhesion & cohesion Transpiration pull = pulling force that is transmitted through H2O in xylem down stem to roots (by low pressure) Transpiration pull allows water to move up, against gravity Passive process Energy provided by thermal energy (heat) that causes transpiration Cavitation = breaking of column of a liquid Rarely happens with H2O

U: Active uptake of mineral ions in the roots causes absorption of water by osmosis Explain the process of mineral ion absorption from the soil into roots by active transport Minerals that need to be taken up from the soil include K+, Na+, Ca2+, NH4+, PO43- and NO3- Fertile soil contains (-) charged clay particles to which (+) charged minerals attach Root cells contain proton pumps that actively pump H+ ions into soil, which displaces (+) charged minerals allowing for their absorption (-) charged minerals may bind to the H+ ions and be reabsorbed with H+

Continued…U: Active uptake of mineral ions in the roots causes absorption of water by osmosis H2O is absorbed into root cells by osmosis Solute conc. inside the root cells is greater than that in the water in the soil Most solutes are mineral ions Conc gradients established by active transport (protein pumps) Separate pumps for each type of ion

U: Plants transport water from roots to leaves to replace losses from transpiration Movement of water from roots to leaves: Water leaves stomata by transpiration Replaced by water from xylem Water in xylem climbs stem through transpiration pull, cohesion, adhesion Water moves from soil to roots by osmosis due to active transport of minerals into roots Water in root travels to xylem through cell walls (apoplast pathway) and through cytoplasm (symplast pathway)

App: Adaptation of plants in deserts & in saline soils for water conservation Xerophytes = plants adapted to growing in dry habitat Increase rate of water uptake from soil Reduce transpiration Ephemeral = short life cycle during rainy season, then go dormant Perennial = more than 2 years, rely on storage of water in specialized leaves, stems or roots

Adaptations of xerophytes: Cactus Reduced leaves (spines) Stems point straight up Water storage in swollen stems Thick waxy cuticle CAM physiology (Crassulacean acid metabolism) Stomata in stems that open at night CO2 absorbed at night and stored as 4-C malic acid (C4 physiology) CO2 released during day, allowing photosyn during day when stomata closed

Adaptations of xerophytes: Merram Grass Rolled leaf to hold in water vapor Stomata sit in small pits within curls, making them less likely to open and lose water Hairs on inside of fold, to slow or stop air movement

Adaptations of halophytes: Reduced leaves (small scales or spines) Leaves are shed when low water, stem becomes green and takes over photosyn Water storage structures develop in leaves Thick cuticle and multiple layered epidermis Sunken stomata Long roots which search for water Structures that remove salt build-up

Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images

Draw stem & leaf!