Water Relations

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Chapter 5

• Water Relations

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Water Concepts

• Relative Humidity (water vapor density /
  saturation water vapor density) 100
• Water Vapor Pressure the pressure exerted by
  water in air
• Saturation Vapor Pressure pressure exerted by
  water in air that is saturated
• Vapor Pressure Deficit (saturation pressure
  water vapor pressure)

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Water in Air
Water vapor density and water vapor pressure are
two ways to measure the amount of water in air.
Water vapor pressure is easier to use when
considering organisms and the ecosystem.
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Vapor Pressure Deficit
5
Aquatic Solutions
When the organism has the same concentration of
solutes as water, the condition is isoosmotic.
(water and salts move in and out) When the
organism has a greater concentration of solutes
than water, the condition is hyperosmotic. (water
moves in, salts move out) When the organism has
a lower concentration of solutes than water, the
condition is hypoosmotic. (water moves out, salts
move in)
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Water Gradient
Water Potential relates to the concentration of
water in a location Water moves from areas of
high water potential to areas of low water
potential
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Water Movement Through Plants
Water moves down a water concentration gradient
from high water potential to low water potential
(from soil to roots, through the stem, to the
leaf, and into the atmosphere).
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Water Gains and Losses
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Beetle Smarts
Desert Beetles can take advantage of condensation
to acquire water from the air.
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Beetle vs. Kangaroo Rat
Different animals use different strategies to
gain water and suffer water loss in different
ways.
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Rooting Depth
The availability of water determines the depth to
which roots grow. On dry sites, plants grow a
large system of deep roots. On wet sites, plants
grow a modest system of shallow roots.
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Water as a Commodity
Organisms expend energy as appropriate to gather
and save resources in relation to their
availability. Energy Return On Investment
13
Leaf Drop
Some plants will drop their leaves during dry
periods to avoid water loss, only keeping leaves
when water is available for use in photosynthesis
and other metabolic processes.
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Wilting
A more common plant strategy for conserving water
is to wilt. This process shrinks the stomates
to reduce water loss as well as curving the leaf
to increase water potential near the leaf surface
to reduce evaporation.
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Similar Strategies
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Dissimilar Strategies
The cicada is active during the heat of the day,
in small cool pockets on trees and using
evaporative cooling, to avoid predation after
feeding on xylem fluids in the morning.. The
scorpion hides in the cool of the tree base
during the day and is active at night to reduce
water loss.
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Osmoregulation in the Sea
Hypoosmotic salts must be excreted
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Osmo-regulation in Freshwater
Hyperosmotic salt must be acquired
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Summary

• The availability of water to organisms is
  controlled by the movement of water down
  concentration gradients
• Terrestrial organisms regulate internal water by
  balancing water loss with acquisition
• Aquatic organisms use complementary mechanisms
  for water and salt regulation