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Lecture 20

Organismal Lecture 20: Water and Ion Balance in Plants

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Biology 2601A/B
Brent Sinclair

Organismal Lecture 20 Water and Ion Balance in Plants Plants Need Water • For biochemical reactions • To maintain structure (turgor pressure) • For photosynthesis Boundary Layer • The boundary layer is the layer of unstirred air on the outside of the leaf; it provides resistance to the water vapour molecules as they pass through it • The boundary layer has a higher vapour content, meaning that it isn’t as dry as the environment ○ Due to this, less water is lost from the leaf tissues when the stomata is open The Boundary Layer is Disturbed by Wind • The higher the wind speed is, the smaller the boundary layer will be, and subsequently the length of the diffusion path decreases • Therefore, more wind = more transpiration • Wind speed also tends to cool the leaves and may cause sufficient dryness to close the stomata; therefore this occurs at extremely high wind speeds • Boundary layers are also thicker over larger leaves (as seen in diagram) 1 How Does Water Get Into the Plant? • Through the roots • Roots aren’t just a single structure; roots provide a lot of surface area • Roots will interact with water particles in-between the soil particles • Roots will absorb and transport upward to the stem virtually all water and minerals taken up by plants Roots • The root cap is not very permeable; therefore water will not be exchanged at the top • The root hair zone is where water and nutrients will be transported into the plant Root Hairs • Root hairs enhance water uptake by their ability to penetrate water-containing capillary spaces between soil particles • Increase several fold the volume of water extracted from the soil • Extend between the film grains into films of water 2 1. Apoplastic: around the outside of the cells 2. Transmembrane: across cell’s membranes into different cells 3. Symplastic: water moves through the cell wall without any cell membranes Water Can’t be Moved Actively • There is nothing in the cells that will act as “active water transporters” • Therefore, water absorption will be driven by the active ion uptake which will create a concentration gradient that water can follow ○ Aquaporins allow H₂O to move passively along a concentration gradient • The problem with saline/salty environments is that H₂O will evaporate and leave salt behind ○ When the soil has more salt than the plants, water will want to move out into the soil (not into the plant) ○ Due to this, many plants can’t survive in salty environments Active Absorption by the Roots • The roots will bring in ions from elsewhere (soil, environment etc) in order to give them to the tissues • These ions are micronutrients such as nitrogen (as NH₄ or NO₃), Mg, Ca.....etc. • This active absorption will cause root pressure in the xylem of vascular plants ○ Roots pressure is a form of osmotic pressure within the cells of a root system that cause sap to rise up through the plant stem to the leaves ○ Root pressure will provide a force to push water up the stem of the plant Casparian Band • The Casparian band will ensure that
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