30 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 



THE PHYSIOLOGICAL CONDITION OF THE PLANT. 



The influence of the physiological condition of the plant on injury 

 has until recently received scant attention by writers on fumigation. 

 It is evident that plant injury from hydrocyanic acid is influenced by 

 the chemical condition of the cells at the time of treatment, for other- 

 wise how could the fact, well known to every fumigator, be explained 

 that tender growing citrus plants are less resistant to gas than those 

 in a dormant and hardened condition, as during the winter. This is 

 equally true with the young leaves as with the mature ones, which 

 indicates that in becoming resistant young growth passes some sort 

 of maturation process. In fact it appears that the condition of citrus 

 plants which renders them hardy or resistant to frost injury likewise 

 develops increased resistance to hydrocyanic acid. 



Harvey (8) has shown that in the case of cabbage the hardening 

 process results in an increase in the glucose and sucrose content over 

 that present in nonhardened plants and quotes Lidforss as authority 

 for the statement that this is a common transformation in plants 

 generally during the cold season. Chemical changes increasing or 

 reducing the percentage of other substances are also shown to be a 

 result of hardening tissues. It is further stated that the hardening 

 of plants which results in an increase in the cell-sap concentration is 

 an accommodation brought about by low temperature. Plants can 

 also be rendered resistant by growth in a dry soil. 



Stone (18), working with cucumber plants grown under different 

 light and soil-moisture conditions, showed that the development of 

 tissue is influenced by these factors as well as their susceptibility to' 

 burning with hydrocyanic acid. The weaker tissue produced by 

 inferior light or excessive moisture was decidedly more injured than 

 that grown under full light conditions or in dry soil. 



More recently Clayton (1) experimented with tomato plants and 

 similarly observed that resistance to hydrocyanic-acid gas was modi- 

 fied by the conditions under which the plants were grown. Slow- 

 growing plants with a high chlorophyll content per unit area were 

 found to be more resistant to hydrocyanic acid than plants grown 

 rapidly with low chlorophyll content per unit area, and his conclu- 

 sions that the water supply was the underlying cause of these differ- 

 ences is in full accord with the prior work of Stone. Chemical 

 examination of the two sets of plants gave results in agreement with 

 those of Harvey (8) and others for hardened and nonhardened or 

 actively growing plants, that the more resistant forms have greatly 

 increased carbohydrate content, especially of the reducing sugar 

 calculated as dextrose. Experiments conducted by this writer with 

 plants infiltrated with dextrose showed that resistance to hydrocyanic- 

 acid gas was developed by this procedure and the conclusion was 

 reached that glucose in a plant acts as a protective agent against 

 injury by cyanid. 



