4. Salinity stress

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4. Salinity stress

• Two kinds of problems due to high soil salinity
• 1. Osmotic stress
• 2. Specific ion effects of high Na, Cl-,
  SO4-2
• inactivate enzymes, inhibit protein synth.
• Plant Strategies
• 1. Osmotic adjustment to allow water uptake
• use inorganic ions, organic solutes
• 2. Salt exclusion or compartmentation to
• deal with ion effects.
• prevent entry at roots
• prevent transport to shoot
• keep away from sensitive organelles
• vacuolar compartmentation
• extrude salt into glands

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Facultative CAM switch to CAM during salt stress
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5. O2 deficiency Typically only a problem in
flooded soils or heavily compacted soils. O2
diffusion to roots is inhibited. Flooding
sensitivity and tolerance vary greatly and are
related to anatomical and biochemical differences.

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Hypoxia and root function growth
inhibition active transport reduced Flooding
sensitive plants Flooding tolerant plants Soil
conditions associated with anoxia toxic forms of
some ions, e.g. Fe2 H2S from SO4-2
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Corn roots develop large gas spaces when oxygen
deficient.
Fig. 25.18
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6. Air pollution What are the major air
pollutants? What are their effects on
plants? What determines variation in
sensitivity?
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Ozone enters leaves through stomata during normal
gas exchange. As a strong oxidant, ozone (or
secondary products resulting from oxidation by
ozone such as reactive oxygen species) causes
several types of symptoms including chlorosis and
necrosis. It is almost impossible to tell whether
foliar chlorosis or necrosis in the field is
caused by ozone or normal senescence. Several
additional symptom types are commonly associated
with ozone exposure, however. These include
flecks (tiny light-tan irregular spots less than
1 mm diameter), stipples (small darkly pigmented
areas approximately 2-4 mm diameter), bronzing,
and reddening. Ozone symptoms usually occur
between the veins on the upper leaf surface of
older and middle-aged leaves, but may also
involve both leaf surfaces (bifacial) for some
species. The type and severity of injury is
dependent on several factors including duration
and concentration of ozone exposure, weather
conditions, and plant genetics. One or all of
these symptoms can occur on some species under
some conditions, and specific symptoms on one
species can differ from symptoms on another. With
continuing daily ozone exposure, classical
symptoms (stippling, flecking, bronzing, and
reddening) are gradually obscured by chlorosis
and necrosis.
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EPA estimates an agricultural crop loss of 2
billion to 3 billion dollars per year
attributable to ozone exposure the extent of
forest damage is currently being studied.
Heagle, A.S. 1989. Ozone and crop yield. Annual
Review of Phytopathology 27397-423.
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1. YW YS YP
2. YW of xylem and phloem lt 0
3. YS lt 0 if there are any solutes!
4. YP of phloem gt 0! pressure flow
5. Water moves passively from higher to lower Yw

Xylem Phloem
?w ?S ?p ?w ?S ?p
??w ?w
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1. YW YS YP
2. YW of xylem and phloem lt 0
3. YS lt 0 if there are any solutes!
4. YP of phloem gt 0! pressure flow
5. Water moves passively from higher to lower Yw

Xylem Phloem
?w -1.8
??w -1.6
?S -2.8
?p 1.0
?w -1.0
?S -1.8
?w - 1.2
?p 0.8