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Plant response to the environment
How do plants sense, respond and adapt to environmental change? Special emphasis on response to stress.
Objectives:
• understanding of processes, mechanisms• be able to explain sequence of events
change in environment final response of plant
Years before present (billions)
4 3 2 1 0
900 myamarine algae
450 myanon-vascular plants colonize land
425 myavascular plants (microfossils of xylem tracheids)
Plants among the earliest organisms to appear in fossil record.
What problems were faced & solved during transition from watery env’t dry land?
need roots, vascular system to obtain H2O, carry to some height
need cuticle, epidermis, stomata to conserve H2O
need embryos capable of withstanding dry conditions
Plant-Water Relations
• How does H2O get into and out of plant cells?
• How .. enter roots move through the plant?
• How do plants regulate this to avoid dehydration?
read Taiz & Zeiger chapter 3
Plants and water
1. Water is essential for life
structural integrity of biological molecules (hydration sphere)
all biochemical-enzymatic rxns occur in an aqueous environment
vital role as a solvent• mineral nutrients• products of photosynthesis
Plants and water
2. Liquid continuity:
soil water
Liquid-gas interface at evaporating surfaces in leaf
unbroken continuity(SPAC)
Plants and water
3. H20 constitutes 80-95% of the mass of growing tissues (>50% for woody tissues)
e.g., corn at tasseling~ 800 g~ 700 g water
40,000-50,00 g water have passed through
Plants and water
4. Virtually every aspect of plant physiology is affected by water content. Many processes impaired by water deficit.
• growth• photosynthesis• cell division• protein synthesis• cell wall synthesis• hormone levels
water deficitdirect, physical
changes in gene expression
Plants and water
5. Productive agriculture is absolutely dependent upon supplies of freshwater.
water under increasing demand from farming, industrial, human uses
even wealthy industrialized societies are not immune from such pressure
slight (-) charge
slight (+) charge
polar molecule, net charge = 0
Water – physical properties:
Water – physical properties:
1. solvent:
water will dissolve more substances than any other common liquid
especially effective for electrolytes
H2O molecules form “cage” around ions, shielding their electrical charge increase solubility
H+ H+
- O
H+
H+
- O
H+H+
- O
K+
H+H+
- O
H+ H+
- O
Cl-
H2O as solvent:
Water – physical properties:
2. H-bonding: H+H+
- O
H+H+
- O
H+
H+
- O
(+) side of one molecule attracted to ( - ) side of another
thermal, cohesive, adhesive properties
H+
H+
- O
H+H+
- O
H-bonding among H2O molecules
high specific heat
= energy required to raise the temp. of a substance by a specific amount. For water:
1 cal to raise 1 g H2O 1 °C
Water – thermal properties
high specific heat: H2O molecules vibrate faster at high temperature but great deal of energy is required to break H-bonds.
i.e., H2O molecules absorb large quantities of energy without much temperature increase
What consequences?
Consequences of high specific heat of H2O:
buffers plant tissue (which is mainly H2O) from temperature fluctuations
provides temperature stability (even when gaining or losing heat energy)
Water – physical properties
cohesion: mutual attraction between H2O molecules (due to H-bonding)
adhesion: attraction of H2O to the solid phase (e.g., cell walls, glass surface, etc.)
Water molecules are more strongly attracted to their neighbors in the liquid than to those in the vapor. (H-bonded)H2O (liquid)
H2O (vapor).. . .
...
..
.. What consequence?
see Figure 4.8 Taiz and Zeiger (2010) p. 94
A meniscus forms, and the air-water interface assumes minimum surface area.
This creates a surface tension
Surface tensions and development of negative pressure:
surface tension of water an important contributor to pressure inside xylem elements
origin: sites of evaporation in the stomatal cavity
water adheres to cell walls – and coheres to each other – and that force (tension) is transmitted through rest of the fluid
Consider a single cylindrical pore:
- 2 Ts cos r
P =radius ( -P (bar)
5 -0.3
0.5 -3.0
0.05 -30
0.01 -150
0.005 -300
P = hydrostatic pressureTs = surface tension of H2Or = radius=contact angle
pore in a cell wall
Figure 4.10 Taiz and Zeiger (2010) p. 97
Water transport in plants:
1. diffusion: within a cell or tightly localized
2. bulk flow (mass flow): long distance; no membranes crossed
3. osmosis: cell to cell, crossing membranes
1. Diffusion
Fick (1855) discovered that the rate of solute transport is directly proportional to the concentration gradient and inversely proportional to distance traveled.
Fick’s Law describes passive movement of molecules down a concentration gradient. Substances move from high [ ] to low [ ].
Diffusion:
Cs
X- Ds Js =
difference in concentration
distance
diffusion coefficient
flux of a solute in solution
=
(mass/surface area/time)