LOWER TEMPERATURE LIMIT AND CHILL INJURY 1219 



upon cooling gradually rather than suddenly, e. g., in some tropical plants, 

 after 1 hour exposure to 5°, and after 15 minutes exposure to 1° C. Mat- 

 thaei (1904) found that the photosynthesis of cherry laurel leaves declines 

 rapidly below 0° and ceases practically immediately at —6°. An exten- 

 sion of the temperature range of active photosynthesis, down to —16° for 

 certain alpine phanerogams and to -20° for alpine lichens, was again 

 claimed by Henrici (1921). In the interpretation of her results, the pos- 

 sibility of wide difference in temperature between the interior of the leaves 

 and the surrounding air should be taken into account. In the mountains, 

 where strong irradiation combines with low air temperature, the heating of 

 leaves by light absorption may be particularly strong. Ivanov and Orlova 

 (1931) found photosynthesis in pine trees down to -7° C, Printz (1933), 

 only down to —2° or —3°. Freeland (1944) noted that apparent photo- 

 synthesis still was positive, in three Pinus species, at an external tempera- 

 ture of —6°. 



It was mentioned above that the inhibition of photosynthesis by cold 

 requires time. Conversely, once photosynthesis has been stopped by cold, 

 plants may require a certain time for the recovery of their photosynthetic 

 ability. Ewart (1896) found that, when Elodea was chilled to 0° for 6 

 hours and then transferred to warm water, photosynthesis did not reappear 

 before 10 or 15 minutes; after chilling for 1 or 2 days, the recovery required 

 3 hours, and after 5 days, from 5 to 24 hours. 



The suspension of photosynthesis in chilled leaves and its recovery 

 upon thawing is one aspect of the broad problem of frost resistance of 

 plants, which is of vital importance in agriculture. Largely because of this 

 practical importance, extensive investigations have been carried out on the 

 way in which plant cells are injured by cold. Ice formation — inside as 

 well as outside the cell — certainly is a factor of importance, and frost re- 

 sistance has often been associated with the capacity of the protoplasm for 

 undercooling. Some plants can be cooled to —20° C. or lower without 

 visible formation of ice. This capacity for undercooling has been as- 

 sociated with the living state of the protoplasm; thus, Lewis and Tuttle 

 (1920) found that ice was first formed in living cells of Pyrola rotundifoUa 

 at —32° C, while in leaves of the same species, killed by immersion into 

 solid carbon dioxide, water was observed to freeze at —3.5° C. 



Ice formed on the outer walls drains water from inside the cells and 

 thus causes injuries similar to those induced by drought. Ice formed in- 

 side the cell can cause injury by mechanical pressure. 



Warburg (1919) found that Chlorella cells can resist immersion into 

 liquid air for several hours without loss of their capacity for photo- 

 synthesis. He attributed this striking property to the fact that the single 

 chloroplast contained in a Chlorella cell has the form of a bell, spread over. 



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