T4 Baena- To hunger or to thirst: plant water-use and photosynthesis

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Last updated 10:20 AM on 5/22/26
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7 Terms

1
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how is leaf conductance determined?

the conductance (efficiency of water transport) in leaves is determined by:

  • the xylem conductance- vessel composition, length, diameter + abundance and pit structure + density

  • out-of-xylem conductance- conductance of cells, path length to stomata (minimised by fine veins in angiosperms), stomatal density + aperture

    • the apoplastic pathway is initially restricted by the bundle sheath cells (lignin + suberin layer) around the xylem to control salts + pathogens- only path is through aquaporins

    • this means conductance is also dependent on aquaporin activity, osmotic adjustments + cell wall thickness

<p>the <strong>conductance</strong> (efficiency of water transport) in leaves is determined by:</p><ul><li><p>the <strong>xylem conductance</strong>- vessel composition, length, diameter + abundance and pit structure + density</p></li><li><p><strong>out-of-xylem conductance</strong>- conductance of cells, path length to stomata (minimised by fine veins in angiosperms), stomatal density + aperture</p><ul><li><p>the apoplastic pathway is initially restricted by the <strong>bundle sheath cells</strong> (lignin + suberin layer) around the xylem to control salts + pathogens- only path is through <strong>aquaporins</strong></p></li><li><p>this means conductance is also dependent on aquaporin activity, osmotic adjustments + cell wall thickness </p></li></ul></li></ul><p></p>
2
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how is water use efficiency determined and how does it vary?

  • water use efficiency = CO2 fixed in biomass/H2O used (mainly lost in transpiration)

  • this is important due to the optimisation theory of photosynthesis- plants have to strike a balance between maximising carbon fixation by photosynthesis and minimising water loss + risk of hydraulic failure (trade-off)

this varies between species (due to different photosynthesis types) and is regulated by changing stomatal aperture and density + resource allocation

<ul><li><p><strong>water use efficiency</strong> = <strong>CO<sub>2</sub></strong> <strong>fixed </strong>in biomass<strong>/H<sub>2</sub>O used </strong>(mainly <strong>lost </strong>in transpiration)</p></li><li><p>this is important due to the <strong>optimisation theory</strong> of photosynthesis- plants have to strike a balance between <strong>maximising carbon fixation </strong>by photosynthesis and <strong>minimising water loss </strong>+ risk of <strong>hydraulic failure </strong>(trade-off)</p></li></ul><p></p><p>this varies <strong>between species</strong> (due to different photosynthesis types) and is <strong>regulated </strong>by changing stomatal aperture and density + resource allocation</p><p></p>
3
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how is transpiration affected by temperature?

  • as temperature increases, the atmosphere has more capacity to hold water vapour (non-linear relationship)

  • at higher air temperatures, the difference between the maximum theoretical humidity and the real humidity is greater- greater vapour pressure deficit

  • this increases transpiration

<ul><li><p>as <strong>temperature increases</strong>, the atmosphere has <strong>more capacity</strong> to hold water vapour (non-linear relationship)</p></li><li><p>at <strong>higher </strong>air temperatures, the <strong>difference </strong>between the <strong>maximum </strong>theoretical humidity and the <strong>real </strong>humidity is greater- <strong>greater vapour pressure deficit</strong></p></li><li><p>this increases transpiration</p></li></ul><p></p>
4
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how efficiently is solar energy used by plants?

  • only about 5% of solar energy goes into biomass

  • 60% is not absorbed

  • some is lost as heat (reflection of long-wavelength radiation, ‘sensible’ conduction + convection of air, and ‘latent’ evaporative cooling from transpiration)

  • some is lost in metabolism, maintenance + repair

5
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why did plants evolve leaves?

  • in the late devonian period, there was a massive decrease in atmospheric CO2 (in photo)

  • this required the evolution of leaves and an increase in stomatal density

<ul><li><p>in the late devonian period, there was a massive decrease in atmospheric CO<sub>2</sub> <em>(in photo)</em></p></li><li><p>this required the evolution of leaves and an increase in stomatal density</p></li></ul><p></p>
6
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how have plants adapted to survival in water-limited environments?

life history stratgeties:

  • drought escape- short, ephemeral life cycles with dormant seeds that germinate after rainfall

  • drought-deciduous- long-lived, enters dormancy + sheds leaves (fertile soil to regrow leaves)’

morphological features:

  • lignified tracheid pores to resist embolism

  • dimorphic roots- tap roots (reach deep water + hydraulic lift), lateral roots (nutrient uptake) and/or ephemeral surface roots (after rain, secrete acids to solubilise + absorb phosphate)

  • small leaves- less transpiration + more sensible heat loss

  • leaf hairs in summer leaves (less photosynthesis but more water conservation)

biochemical mechanisms:

  • accumulation of compatible solutes to lower protoplast water potential in high salinity

  • CAM photosynthesis

<p>life history stratgeties:</p><ul><li><p>drought escape- short, ephemeral life cycles with dormant seeds that germinate after rainfall</p></li><li><p>drought-deciduous- long-lived, enters dormancy + sheds leaves (fertile soil to regrow leaves)’</p></li></ul><p></p><p>morphological features:</p><ul><li><p>lignified tracheid pores to resist embolism</p></li><li><p>dimorphic roots- tap roots (reach deep water + hydraulic lift), lateral roots (nutrient uptake) and/or ephemeral surface roots (after rain, secrete acids to solubilise + absorb phosphate)</p></li><li><p>small leaves- less transpiration + more sensible heat loss</p></li><li><p>leaf hairs in summer leaves (less photosynthesis but more water conservation)</p></li></ul><p></p><p>biochemical mechanisms:</p><ul><li><p>accumulation of compatible solutes to lower protoplast water potential in high salinity</p></li><li><p>CAM photosynthesis</p></li></ul><p></p>
7
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how do epiphytes adapt to water availability?

  • because epiphytes don’t live on the ground, their access to water is not reliable/consistent

  • they collect water and store it in water tanks

  • trichome hairs on leaves assist water capture

  • thick cuticle

  • CAM photosynthesis

  • pseudobulbs

  • aerial roots

  • specialised spongy epidermis around roots in epiphytic orchids

<ul><li><p>because epiphytes don’t live on the ground, their access to water is not reliable/consistent</p></li><li><p>they collect water and store it in water tanks</p></li><li><p>trichome hairs on leaves assist water capture</p></li><li><p>thick cuticle</p></li><li><p>CAM photosynthesis</p></li><li><p>pseudobulbs</p></li><li><p>aerial roots</p></li><li><p>specialised spongy epidermis around roots in epiphytic orchids</p></li></ul><p></p>