Low temperature stress in plants can be caused by chilling or freezing temperatures. Chilling stress occurs between 0-15°C and can cause physiological disorders and chilling injury symptoms like reduced growth, leaf lesions, and increased susceptibility to decay. Freezing stress below 0°C damages cell structures through ice crystal formation and causes symptoms like foliage desiccation and weakened roots. Both stresses impact plant metabolism and photosynthesis. Plants have mechanisms like certain genes and calcium signaling pathways that enhance their tolerance to low temperatures.

2. Concept of Low Temperature Stress
What is Stress?
Stress in physical terms is defined as mechanical force per unit
area applied to an object.
OR
The most practical definition of a biological stress is an adverse
force or a condition , which inhibits the normal functioning and
well being of a biological system such as plants.

3. Low temperature stress
Chilling stress
when plants are
exposed to a low temperature
above 0 ºC, but below the optimum
temperature.
Freezing stress
when plants
are exposed to a low
temperature below 0 ºC

4. CHILLING STRESS
Plants may develop physiological disorders when exposed to low but non-
freezing temperatures.
Chilling Injury
Plant chilling injury refers to an injury that is caused
by a temperature drop to 10 to 15°C but
above the freezing point.

5. Symptoms of Chilling injury
Common Symptoms
– Reduced plant growth and death
– Surface lesions on leaves and fruits
– Abnormal curling, lobbing and crinkling of leaves
– Water soaking of tissues
– Cracking, splitting and dieback of stems
– Internal discolouration (vascular browning)
– Increased susceptibility to decay
– Failure to ripen normally
– Loss of vigour (potato lose the ability to sprout if
chilled)

6. Freezing stress
When plants are exposed to a low temperature below 0 ºC
. Freezing damage occurs primarily due to the formation of ice crystals, which
damage cell structure when the temperature falls below 00C.
Symptoms of Freezing injury
1. Desiccation or burning of foliage.
2. weakened root system or split bark.
3.Wilting and/or desiccation, as caused by direct drought
stress.

7. Effects on Plants
Chilling effects
Chilling injury causes several metabolic
or physiological dysfunctions to the plant
including,
• disruption of the conversion of starch to
sugars
• decreased carbon dioxide exchange
• reduction in net photosynthesis.
• the destruction/degradation of
chlorophyll.
Freezing effects
The major effect of freezing is that it
induces severe membrane change.
Plants exposed to freezing at
reproductive stage for long time show
reduced metabolic rates leading to low
yield.

8. Low temperature tolerance;
Gene, eskimo has a major effect on freezing tolerance, plants having this
gene are more freeze tolerant.
Calcium is an important messenger in a low temperature signal transduction
pathway , change in cytosolic ca levels is a necessary first step in
temperature sensing mechanism which enables the plant to with stand future
cold stress in a better way.

9. Questions
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