Chapter 3 



WATER-SOLUBLE 

 ORGANIC ACIDS 



The occurrence of a variety of free acids in plants has been well-known for many 

 years. Some plant organs accumulate rather large quantities of specific acids which are 

 primarily concentrated in the vacuolar sap. As a result of such accumulation the pH of 

 this sap may fall to values as low as 2 or 3. Many of the common plant acids are those 

 which participate in the familiar citric acid cycle of metabolism; and since this cycle is 

 believed to be of fundamental importance in the biochemistry of almost all organisms, 

 acids participating in it must occur to some extent in all plants. However, the mere oper- 

 ation of this cycle does not entail any accumulation of acids. Moreover, the reactions of 

 the citric acid cycle take place in the mitochondria, whereas acids which accumulate do 

 so in the vacuole. Therefore while this cycle may provide the reaction pathways for syn- 

 thesis of several common plant acids, it does not explain how a particular plant organ 

 often accumulates just one acid from the cycle in the vacuoles of its cells. That there is 

 such a selectivity is evident from a consideration of the major acids in some common 

 fruits — e.g. citric acid in lemons (Citrus limonica), isocitric in blackberries (Rubus 

 spp. ) and malic in apples (Mains spp. ). 



Besides the plant acids involved in the citric acid cycle several other water-soluble 

 acids are of very common occurrence in plants. Some of these may be grouped as lower 

 members of the fatty acid series (cf. Chapter 5); some are intermediates in the pathway 

 leading from carbohydrates to aromatic compounds (cf. Chapter 4); some are, in fact, 

 carbohydrates ( cf. Chapter 2); some are isoprenoid derivatives (Chapter 8); and, finally, 

 some are formed by peculiar or unknown metabolic pathways. Table 1 lists some of the 

 non-citric cycle acids with examples of plants where they are found in relatively high con- 

 centrations. 



The table on page 37 is by no means complete. The bibliography of Buch listed 

 under general references has about 30 water-soluble, aliphatic plant acids not including 

 sugar acids or lower fatty acids. While most of these acids occur free or as salts, a few 

 (chiefly the lower fatty acids) are often found as esters in essential oils (cf. Chapter 8). 



The common plant acids are colorless substances which are usually soluble not only 

 in water but also in organic solvents such as ethanol and ether. They are insoluble in the 

 very non-polar solvents like benzene or petroleum ether. Many of the plant acids are 

 optically active, and normally only one of the enantiomorphs is naturally occurring. Com- 

 pared With the mineral acids they are only weakly acidic. Their sodium and potassium 

 salts are water soluble, but calcium and barium salts are usually insoluble or only slightly 

 soluble in water. Crystals of calcium oxalate appear as raphides in the cytoplasm of 

 many plant cells. 



The function of plant acids in respiratory cycles is well-known; but aside from this 

 role in energy metabolism several other functions have been suggested for them, particu- 

 larly for those which accumulate or are excreted and apparently are not involved in ac- 

 tive metabolism. They are often considered as waste products which are not further 

 utilized. However, they may accumulate at one stage and disappear at a later stage-- 



- 36 - 



