SEED GERMINATION 239 



plished is uncertain. Glucose produced by amylase and nialtase action 

 is converted to glucose-6-phospiiate by hexokinase. A more detailed 

 discussion of this and succeeding processes appears in Chapter 7. 



Peas, beans, and potatoes are rich in phosphorylase. Most seeds 

 probably contain this starch-splitting and starch-forming enzyme. 

 Phosphorylase converts starch to glucose-1 -phosphate. Glucose-1 -phos- 

 phate is changed to glucose-6-phosphate by phosphoglucomutase. Thus 

 starch hydrolysis, by means of amylases, maltase, and hexokinase or 

 by phosphorylase and phosphoglucomutase, results in the formation 

 of the same compound, glucose-6-phosphate. 



(ilucose-6-phosphate is transformed by a series of changes which is 

 discussed on pages 168 to 173. This process involves splitting a 

 hexosephosphate to two triosephosphates and oxidation of the latter 

 to pyruvic acid. Pyruvic acid may be oxidized to carbon dioxide and 

 water through the Krebs cycle. The intermediary compounds and 

 the Krebs cycle acids can be used for the synthesis of fats, proteins, or 

 new carbohydrates required for the growth of the embryo and seedling. 



Some germinating seeds, for example, barley, contain large amounts 

 of sucrose. It is now known that sucrose can be formed by the reactions 

 found on page 220. 



In germinating seeds, it is probable that fructose or a fructose phos- 

 phate is necessary for the formation of sucrose. Fructose phosphates 

 are produced in the breakdown of starch. Fructose is produced by 

 the hydrolysis of fructose-6-phosphate by a phosphatase. Sucrose in 

 seeds can be hydrolyzed to glucose and fructose by the action of sucrase. 

 This reaction was discussed in Chapter 6 on enzymes. 



Protein Metabolism 



Protein reserves of the seed are rapidly changed into simpler com- 

 pounds during germination, and these are translocated to the point 

 of utilization. Protein breakdown is initiated by proteinases of seeds. 

 These plant enzymes belong to the papainase group, which is inactive 

 when in the oxidized form but is activated by reducing agents. 

 I'apainases hydrolyze proteins to mixtures of polypeptides and small 

 amounts of amino acids. The polypeptides are then hydrolyzed to 

 amino acids through the action of peptidases. 



The amino acids may be utilized for the production of essential 

 plant compounds such as enzymes and nucleoproteins or for the 

 formation of other compounds such as porphyrins. Amino acids can 

 be deaminized by deaminases (aminases), amino acid oxidases, or 

 transaminases. An illustration of the deamination of an amino acid 

 by deaminases of seeds is the action of aspartase. 



