15 to 25
40 to 45
Lichens of cold regions
-25 to -10
5 to 15
20 to 30
Source: Adapted from Larcher, 1980.
Source: Adapted from Larcher, 1980.
Where the temperature exceeds 33°C, leaves appear to suffer from water stress.
• Leaves exposed to sun: Thick leaves that are not transpiring actively in still air are several degrees warmer than the air. Under intense radiation and high humidity, some leaves may be at a temperature 150C higher than the air. Likewise, under very hot and low humidity conditions, leaf temperature can be as much as 10oC higher than the air. Where plants do not suffer for want of moisture, the difference between leaf and air temperature is very small.
• Leaves under shade: Leaves shaded from direct sunlight are usually somewhat warmer than the surrounding air.
• Leaves exposed to a clear night: At night when the sky is clear, leaf temperature is usually lower than the air temperature. During a cold and clear night, a leaf may be around 2°C cooler than the surrounding air.
• Leaves exposed to a cloudy night: With cloud cover, the difference in air and leaf temperature is small. In certain cases the leaf temperature may be slightly higher than air temperature.
Thermal death point of active cells ranges from 50 to 60oC for most plant species, but it varies with the species, the age of tissue, and the length of time of exposure to high temperature. It has been reported (Chang, 1968) that most plant cells are killed at a temperature of 45 to 55°C, but some tissues withstand a temperature of up to 105°C.
For aquatic and shade plants the lethal limit is 40°C, and for most xerophytes it is 50°C, when the plants are exposed to a saturated atmosphere for about half an hour. High temperature results in the desiccation of the plant and disturbs the balance between photosynthesis and respiration. Once the temperature exceeds the maximum up to which growth takes place, plants enter a state of quiescence. When the temperature becomes extremely high, a lethal level is reached. At temperatures higher than the optimum cardinal, the physiological activity declines as a consequence of inac-tivation of enzymes and other proteins. Leaf functions are disturbed at about 42°C, and lethal effects on active shoot tissues generally occur in the range of 50 to 60°C.
Many rice varieties subjected to high temperature just before and just after flowering result in more than 20 percent sterility. High temperature just before or during flowering decreases pollen size, causes a shortage of starch in pollen, and increases the proportion of anthers that did not dehisce. High temperature during ripening decreases grain weight. In wheat crops, a major effect of high temperature appears to be the acceleration of senescence, including cessation of vegetative and reproductive growth, deterioration of photosynthetic activities, and degradation of proteinaceous constituents (Xu et al., 1995).
Serious damage to fruit and vegetable crops resulting from excessively high temperature has also been recorded (Muthuvel et al., 1999; Atta-Aly and Brecht, 1995; Chen, Lin, and Chang, 1994; Oda et al., 1994; Inaba and Crandal, 1988). Apart from desiccation and disturbed photosynthesis and respiration balance, plants are injured in several ways, such as excessive respiration from seeds, sun scald, and stem girdle.
The higher the temperature, the greater is the rate of respiration, which results in the rapid exhaustion of food reserves of seeds. Temperatures on the sunny side of the bark on stems during hot afternoon and late night undergo great fluctuations. The injury inflicted because of this short period fluctuation in temperature is known as sun scald. Stem girdle is another injury associated with high temperature. Exceptionally high temperature at the soil surface and the adjoining laminar sublayer of the air frequently scorches the short stems. The scorching of the stem is known as stem girdle. This type of injury is most common in young seedlings of cotton in sandy soils where the temperature of the soil surface during summer afternoons may be as high as 60 to 65°C (Chaurasia, Mahi, and Mavi, 1985). Stem girdle injury is first noted through a discolored band a few millimeters wide. This is followed by shrinkage of the discolored tissues. It appears that stem girdle causes the death of plants by destroying the conductive tissues or by an injury that helps the establishment of pathogens.
Exposure to extremely low temperatures and heavy snowfall damages the plant in several ways including suffocation, desiccation, heaving, chilling, and freezing.
Small plants may suffer from deficient oxygen when covered with densely packed snow. When suffocated, certain toxic substances accumulate in contact with roots and crowns and tend to inhibit the diffusion of carbon dioxide.
Spring drought sometimes occurs in coniferous trees in cool temperate climates. This results from excessive transpiration and a time lag in absorption of moisture from the soil, caused by a warm period when the soil is still frozen. The result is an internal moisture deficit sufficient to cause death of the twigs. The decreased water absorption by plants at low temperatures is the combined effect of the decreased permeability of the root membrane and increased viscosity of water. This results in increased resistance to water movement across the living cells of the roots.
Injury to a plant is caused by the soil layer lifting upward from the normal position and causing the root to stretch or break at a time when the plant is growing. Sometimes the roots are pushed completely above the soil surface. After thawing, it is difficult for the roots to become firmly established, and the plants may die because of this mechanical damage and desiccation.
Plants of tropical origin are damaged by exposure to mild chilling for two to three days. Plants of temperate origin withstand chilling for long periods without suffering any injury. Rice, cotton, and cowpea are killed when exposed to temperatures near 0oC for about two to three days. Sudan grass and peanuts are injured by short exposure to chilling temperatures but recover if favorable temperature conditions return shortly afterward. Short duration mild chilling does not seriously injure corn, sorghum, and pumpkin plants. Plants of cool climate origin such as wheat and soybean are injured when exposed to prolonged chilling but recover with the return of favorable conditions.
In temperate climates, two types of injuries occur because of low temperature. These are delayed growth and sterility. For example, rice yield decreases due to insufficient grain maturation caused by low temperatures during the ripening period. When flowering is delayed by low temperatures at a certain stage before heading, insufficient time is available to the grains to ripen fully before frost occurs in autumn.
In the sterility type of injury, rice yields decrease due to sterile spikelets caused by low temperatures at the booting stage or at anthesis. The observed injury in developmental order is a stoppage of anther development; pollen unripeness; partial or no dehiscence; pollen grains remaining in anther loculi; little or no shedding of pollen grains on the stigma; and failure of germination on the stigma.
Chilling injury of fruits is of particular interest because they are often stored and shipped at low temperatures. Symptoms of chilling injury to fruits include surface pitting, lesions, discoloration, susceptibility to decay organisms, and shortening of storage life. Fruits subjected to chilling injury do not ripen normally. The critical temperature at which chilling injury occurs is 8 to 12°C for tropical fruits such as banana, avocado, and mango, and 0 to 4°C for temperate zone fruits such as apple (Kozlowski, 1983).
Plant parts or an entire plant may be killed or damaged beyond repair as a result of actual freezing of tissues. Freezing damage is caused by the formation of ice crystals, first in the intercellular spaces and then within the cells. Ice within the cells causes more injury by mechanical damage disrupting the structure of the protoplasm and plasma membrane. Freezing of water in intercellular spaces results in withdrawal of water from the cell sap, and increasing dehydration causes the cell to die.
In a study on freezing injury to fruits in Hungary, Szabo and colleagues (1995) found that apricot is the least cold hardy of stone fruit species grown in Hungary. In strawberry, flowers become more susceptible to freezing as development progresses (Ki and Warmund, 1992). In Belarus (Kozlovskaya and Myalik, 1998) the degree of damage within apple and pear seedling populations due to low temperature is variable. The hybrid seedlings of ap ple are more severely damaged by low temperatures than the other progenies. In the case of pear, the damage was more severe to the seedlings with a tap root system than to those with a branching root system.
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