Growth and Development of Horticultural Crops (1+1)

Physiological and biochemical changes during seed germination

Physiological changes
1. Water uptake

• Seed germination starts with the imbibition of water by dry seed coat. Due to imbibition of water the seed coat becomes more permeable to O2 and water and less resistant to outward growth of embryo. After imbibition, the inner contents of the seed increase in volume, thereby exerting pressure on the seed coat leading to rupture of the seed coat. The plumule and radical emerge thereafter.
2. Respiration

• After initiation of germination process, enormous energy is required for various biochemical changes which are met through rapid increase in respiration rate. Sucrose is probably the respiratory substrate at this stage which is provided by endosperm. In oilseeds and pulses, the lipids and proteins respectively are converted into sucrose by suitable biochemical reactions.
3. Mobilization of reserve materials

• As germination progresses, there is mobilization of reserve materials to provide.
i) Building blocks for the development of embryo
ii) Energy for the biosynthetic process
iii)Nucleic acids for protein synthesis and embryonic development

Biochemical Changes
1. Nucleic acids

• In monocots, during imbibition, there is a rapid decrease of DNA and RNA contents in the endosperm with a simultaneous increase in the embryonic axis. High concentration of RNA in the embryonic axis precedes cell division. Due to more cell division DNA content is increased.
2. Carbohydrates

• Insoluble carbohydrates like starch are the important reserve food of cereals in the endosperm. During germination, starch is hydrolysed first into maltose in the presence of alpha-amylase and ß - amylase and then maltose is converted into glucose by maltase. The glucose is further converted into soluble sucrose and transported to growing embryonic axis. During germination, the embryonic axis secretes gibberellic acid, into the aleurone layer which causes synthesis of alpha-amylase.
3. Lipids

• Many plants like castor bean, peanut, etc, store large amount of lipids or fats as reserve food in their seeds. During germination, the fats are hydrolyzed into fatty acids and glycerol by lipase enzyme. Fatty acids are further converted into acetyl – COA by the process of ß - oxidation. The acetyl COA is further converted into sucrose via glyoxylate cycle and is transported to the growing embryonic axis.
4. Proteins

• Some plants store proteins as reserve food in their seeds. Proteins are hydrolysed into amino acids by peptidase enzyme. The amino acids may either provide energy by oxidation after deamination (removal of amino group) or may be utilized in the synthesis of new proteins.
5.Inorganic nutrients

• A number of inorganic nutrients such as phosphate, calcium, magnesium and potassium are also stored in seeds in the form of phytin. These stored nutrients are liberated during germination due to the activity of various phosphatases including phytase.