Seed detoriation

Mechanism of seed deterioration
contact: [email protected]

1. Seed Detoriation Presented by Debashish Hota PhD 1 st year

2. Introduction Seed deterioration can be defined as deteriorative changes occurring with time that increase the seed’s vulnerability to external challenges and decrease the ability of the seed to survive. Seed deterioration is an undesirable attribute of agriculture The physiology of seed deterioration is a separate event from seed development and/or germination Seed deterioration is cumulative process.

3. Harrington thumb rule on seed storage The following thumb rules by Harrington are useful measures for assessing the effect of moisture and temperature on seed storage. For every decrease of 1% seed moisture content, the life of the seed doubles. This rule is applicable when moisture content between 5 and 14%. For every decrease of 5°C (10°F) in storage temperature the life of the seed doubles. This rule applies between 0°C to 50°C. Orthodox  – the seeds able to tolerate moisture loss and less seed moisture favours the storage. i.e. decreased moisture increased storage period. Eg . Rice, sorghum , and most of the cultivated species. Recalcitrant  – just opposite to the orthodox. Seeds not able to tolerate moisture loss. Required high moisture for viability maintenance. Ex- Mango, Citrus

4. Facts about Seed Detoriation Seeds generally exhibit an initial period of deterioration under dry storage during which germination percentage is relatively constant, but germination rate decreases. Cellular damage accumulates in dry seeds, repair processes following imbibition can successfully restore a functional physiological state, up to a certain point, and allow germination to be completed. Seed damage in lag period is reversible.

5. Seeds should be stored at <50% RH. Accelerated ageing 75-100 % RH It is equally important to distinguish between damage that occurs in the dry state and that which occurs subsequently upon imbibition . Seeds vary in their resistance to aging.

6. Why it is difficult to critically evaluate seed deterioration ? The physiological processes governing seed deterioration vary. The rate of seed deterioration is influenced by confounding environmental and biological factors such as growth of storage fungi that create their own biological niche. Seed treatments influence seed deterioration, and, when applied, their impact on seed quality must be recognized. Most seed deterioration studies examine whole seeds. Most seed deterioration studies report effects on a seed lot, but seed deterioration is an individual event.

7. Changes during seed detoriation Enzyme activities : increases in amylase activity or changes in free radical scavenging enzymes such as superoxide dismutase, catalase , peroxidase , and others. Protein or amino acid content : The consensus is that overall protein content declines while amino acid content increases with seed aging. Nucleic acids : A trend of decreased DNA synthesis and increased DNA degradation has been reported. Membrane permeability : Increased membrane permeability associated with increasing seed deterioration

8. Hydration levels and deterioration and protection mechanisms in seeds

9. Some enzymes have activity near level II range. The rates of reaction are extremely low or are limited to the lipid phase. Below an equilibrium RH of about 50% (depending upon the temperature), glass formation can occur, further increasing viscosity and limiting mobility of molecules. Between 70 and 90% RH (hydration levels II and III), some enzyme activities can be detected (at very low rates) The minimum limit for respiration is about 90% RH Higher rates of respiration and protein and nucleic acid biosynthesis only become possible as water availability increases to hydration level IV Active physiological processes only occur in hydration level V, or above 99% RH

10. Deterioration Mechanisms in dry seed Oxidative and peroxidative processes play the primary roles in initiating the damage that occurs in dry seeds. Free radicals can be generated spontaneously, and can trigger oxidation of various seed constituents. At the lowest seed moisture contents, the extreme viscosity of the glassy state restricts the molecular motion and diffusion of substrates But the availability of oxygen and its tendency to form reactive oxygen species allow free radical reactions to occur in a slower rate. Presence of solvent water tends to quench free radical mechanisms and enable antioxidant mechanisms to be effective.

11. Peroxidation of lipids, initiated by the abstraction of a hydrogen by a hydroxyl radical, can result in a chain reaction that causes breakdown of the lipids (particularly unsaturated lipids) and release of by-products such as reactive aldehydes that can cause further damage to proteins and nucleic acids. Changes in membrane lipids due to peroxidation may be involved in the increase in membrane permeability and cellular leakage that is associated with seed aging. Changes in membrane lipids due to peroxidation may be involved in the increase in membrane permeability and cellular leakage that is associated with seed aging. At the low water contents of dry seeds, enzymic mechanisms to regenerate antioxidants are inoperative

12. Deterioration Mechanisms in wet seed Hydrolytic reactions become possible as free water becomes available. Metabolic imbalance has been proposed as a possible cause of damage during dehydration Lipid phase transitions also occur in this range of hydration, as sufficient water is present to reverse the water replacement by sugars and amphiphiles that stabilize bilayer membrane structures in dry seeds. Holding seeds in hydration level III results in rapid deterioration, particularly at elevated temperatures

13. Methods for testing seed detoriation Germination test Tetrazolium test Vital colouring test Enzyme activity test Electrical conductivity test Free fatty acid test Hydrogen peroxide test Indoxyl acetate test Fast green test Ferric chloride test Sodium hypochlorite test Excised embryo test X-ray Test

14. Loss of viability

15. REPAIR OF SEED DAMAGE Increasing seed moisture content hastens the repair process. Oxygen also increases the repair of highmoisture (27-44 %) lettuce and high-moisture (24-31 %) wheat seeds, suggesting that respiratory activity is an essential component of repair. Repair of seeds deteriorated by lipid peroxidation occurs during hydration.