4. Salinity stress

Two kinds of problems due to high soil salinity1. Osmotic stress

2. Specific ion effects of high [Na+], [Cl-], [SO4-2]

inactivate enzymes, inhibit protein synth.

Plant Strategies1. Osmotic adjustment to allow water uptake

use inorganic ions, organic solutes

2. Salt exclusion or compartmentation todeal with ion effects.

•prevent entry at roots •prevent transport to shoot•keep away from sensitive organelles

vacuolar compartmentation• extrude salt into glands

Facultative CAM switch to CAM during salt stress

5. O2 deficiency

Typically only a problem in flooded soilsor heavily compacted soils.

O2 diffusion to roots is inhibited.

Flooding sensitivity and tolerance vary greatlyand are related to anatomical and biochemicaldifferences.

Hypoxia and root functiongrowth inhibitionactive transport reduced

Flooding sensitive plants

Flooding tolerant plants

Soil conditions associated with anoxiatoxic forms of some ions, e.g. Fe+2

H2S from SO4-2

Corn roots develop large gas spaces when oxygen deficient. Fig. 25.18

6. Air pollution

What are the major air pollutants?

What are their effects on plants?

What determines variation in sensitivity?

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.

Heagle, A.S. 1989. Ozone and crop yield. Annual Review of Phytopathology 27:397-423.

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.

w = S= p =

w = S= p =

w =

w =

Xylem Phloem

1. W = S + P

2. W of xylem and phloem < 03. S < 0 if there are any solutes!4. P of phloem > 0! “pressure flow”5. Water moves passively from higher to lower w

Xylem Phloem

1. W = S + P

2. W of xylem and phloem < 03. S < 0 if there are any solutes!4. P of phloem > 0! “pressure flow”5. Water moves passively from higher to lower w

p = 1.0

p = 0.8

S= -2.8

S= -1.8

w = -1.8

w = -1.0

w = - 1.2

w = -1.6