TAMIL NADU AGRICULTURAL UNIVERSTIY

[2008]

Physiological Basis for Midstorage

Correction

S.SATHISH, M.Sc (Agri), Ph.D, HDCA

PHYSIOLOGICAL BASIS FOR MIDSTORAGE CORRECTION

Introduction:

Seeds have to be stored since there is usually a period of time between harvest

and planting. During this period, the seed have to be kept somewhere. While the time

interval between harvest and planting is the basic reason for storing seed, there are other

considerations, especially in the case of extended storage of seed.

Seed suppliers are not always able to market all the seed they produce during the

following planting season. In many cases, the unsold seed are “carried over” in storage

for marketing during the second planting season after harvest. Problems arise in

connection with carryover storage of seed because some kinds, varieties and lots of seed

do not carryover very well.

Seeds are also deliberately stored for extended periods so as to eliminate the need

to produce the seed season. Foundation seed units and others have found this to be an

economical, efficient procedure for seed of varieties for which there is limited demand.

Mid storage treatments

Seeds in storage accumulate damage to cell membranes during senescence .Mid

storage seed treatments are capable of reducing the age induced damages and restoring

the seed vigour to a certain extent besides, the seed viability and productivity of stored

seeds are also improved.

Hydration – Dehydration (H-D)

It is the process of soaking the low and medium vigour seeds in water with or

without added chemicals usually for short durations to raise the seed moisture content to

25 – 30% and drying back the seeds to safe limits for dry storage.

Types of H-D treatments

The wet treatments include soaking-drying, dipping-drying, spraying-drying,

stepwise hydration-drying, moisture equilibration-drying, moisture equilibration soaking-

drying, moist and conditioning-drying, etc. The choice of the treatment depends upon the

characteristics of seed and initial vigour status of the seeds.

Soaking – Drying (S-D)

Stored seed is soaked in water or solution of chemicals sufficient to cover it and

kept at room temperature for 2-6 hour depending on the material with occasional stirring.

The soaked seed is taken out and after surface drying in the shade for some time, dried

back to the original moisture content Dilute solution of chemicals such as sodium or

potassium phosphate (di and mono basic), sodium chloride, p-hydroxy benzoic acid, p-

amino benzoic acid, oxalic acid, potassium lodide, etc can also be used at 10-4

to 10-3

M

concentrations. Fungicidal and insecticidal formulations can also be incorporated in the

soak water.

Dipping – Drying (D-D)

Seeds are dipped in water or solutions of the aforesaid chemicals for only 2-5 minutes

and the wet seed is taken out immediately and kept covered for 2 – 6 hours depending on

the material, for absorption of surface water followed by drying back in S-D. This

treatment is effective in most high and high-medium vigour seeds of rice, wheat, jute,

summer and winter vegetables

Spraying – Drying

Seeds are spread in a thin layer and then an amount of water (approximately 1/5

to ¼ of the seed weight) is sprayed on to it in two equal installments (turning over the

seed layer after the first spray) and then kept covered by a polythene sheet for 2-4 hours

before drying back. This treatment is similar to D-D in its efficacy and suitability.

Moisture equilibration – drying (ME – D)

Here, the seeds are placed in thin layers on trays kept on a raised platform in a

closed moisture saturated chamber lined internally with moist blotters giving nearly

100% RH at room temperature. After 24-48 hours, depending on the material and

ambient temperature, the seed is dried back in the usual way. For soaking injury prone

seeds this treatment, which gives a slow and progressive rise in moisture content, is very

effective. ME-D, however, difficult to practice on a large scale and is not advocated for

low vigour non leguminous seeds because of possible aging effect of the treatment

especially when given for prolonged periods.

Moist sand conditioning – drying (MSC-D)

This treatment is similar to the moisture equilibration treatment but easier to

practice. For slow and progressive moisture uptake, the seed is thoroughly mixed with

pre-moistened sand, using 3 times the amount of air dry sand than seed. Moisture content

of sand is adjusted to 5-10 by adding the requisite amount of water or solution of

chemicals to previously washed and dried fine grain building grade sand. The addition of

water should be so adjusted as to get the required hydration effect without initiating the

germination process. After mixing the dry seed with the premoistened sand, the mixture

is kept at room temperature for 16 – 36 hours depending on the material and sand

moisture content. The seed absorbs moisture from sand and after incubation the hydrated

seed is separated from sand by sieving and dried back to the original weight.

Mode of Action: The main purpose of hydration is to raise the seed moisture content to

25 –30% (wet weight basis) before drying back to safe limits for dry storage. The

hydration - dehydration treatment may improve the vigour by controlling free radical

reactions and consequent peroxidative damage to lipoprotein cell membranes.

PRECAUTIONS:

The mid term wet treatment should be given only to stored medium and low

vigour seeds and not to freshly harvested seed

The moisture content of the seed should be much less than that is necessary for

germination

The duration of hydration should be so adjusted to avoid unnecessary germination

advancement which is incompatible with drying back so far as further storability

is concerned

Drying back the original weight is essential as otherwise, re-storage of sufficiency

dried seed would do more harm than good

Direct soaking of injury prone leguminous seeds should be avoided

Lastly it needs to be clearly emphasized that the treatment would maintain vigour

and viability in storage but would not make a seed germinable, which has already

lost viability.

Physiology of mid storage correction:

Exact mode of action or physiological basis of midstorage correction is not clearly

known. However the following are believed to be the physiological basis for extension in

seed vigour and storage life.

Repair of biochemical lesions by the cellular enzymatic repair system.

Metabolic removal of toxic deterioration-accelerating substances.

Counteraction of free radical and lipid peroxidation reactions.

Repair system:

As the H-D treatment is effective only in seeds which have lost some vigour, an

invigoration treatment involving the endogenous repair system (Villers and Edgcumbe,

1975) cannot be ruled out. However such repair may not require new protein synthesis

because even in the presence of the potent protein synthesis inhibitor, cycloheximide, the

ME-D treatment was quite effective in lettuce seeds (Pan and Basu, 1985). Any repair of

biochemical lesions requiring protein synthesis will depend on the activity of pre-existent

enzyme proteins.

Removal of toxic substances:

Removal of toxic metabolites during the hydration phase is also important

because any activation of enzymes like the superoxide dismutase (SOD) during the

hydration phase may lead to removal of damaging free radicals in cells. However,

whether during the short hydration phase the same would occur or not needs verification.

Deterioration of seed by free radical and lipid peroxidation reaction:

The earliest symptoms of ageing are loss of membrane permeability which most

likely is caused by the breakdown of the lipoprotein membrane structure. The electrical

conductance of the seed leachate is a good indicator of such damage (Matthews and

Brandnock, 1968). The double bond in the lipid moiety of the membrane cannot be

excluded (especially to the secondary and tertiary structures), the unsaturated lipids in the

plasma membrane, the membranes of the inner lysosomic structures and the endoplasmic

reticulum can readily undergo peroxidative changes.

Lipid peroxidation:

Lipid peroxidation may be caused by

Autoxidation (non-enzymic) due to exposure to oxygen; oxidation proceeds by

free radicals mechanism in which a chain reaction is initiated by removal of a

hydrogen atom from an α-methylene group.

Chemical catalysis (nonenzymic) by haeme compounds such as the cytochromes

Oxidation catalyzed by lipoxidase (lipoxgenase).

The first 2 nonenzymatic processes are slow, the third one catalyzed by the

lipoxidases enzyme, for which the substrates are unsaturated free fatty acids released

from storage lipids by lipases or from membrane lipids by nonspecific lipolytical

hydrolases, is rapid. Although the products of lipid peroxidation (hydroperoxides) are

similar in all the 3 cases; lipoxygenase catalyzed reaction shows the following

distinguishing features:

Activatin energy is smaller and

The enzyme has a very specific substrate requirement in that the fatty acid must

contain at least 2 cis double bonds interrupted by a α-methylene group and the

double bond should be present 6 carbon atoms away from the methyl end of the

chain (distance from the carboxyl end less critical).

Linoleic acid (18 C atoms), present in storage lipids and lipoprotein

biomembranes, having 2 double bonds, one between carbon atoms 9 and 10 and another

between 12 and 13, provide an ideal substrate for peroxidative attack. The production of

linoleate hydroperoxides from linoleic acid and the subsequent breakdown of linoleate-

13-hydroperoxide to caproic aldehyde (hexanal, a volatile aldehyde) are shown below.

Suggested pathway of oxidation of linoleic acid to linoleate hydroperoxides (3 forms) by

free radical induced lipid peroxidation reactions.

Possible steps in the breakdown of linoleate-13-hydroperoxide to hexanal (caproic

aldehyde, a volatile aldehyde) and an unidentified fragment.

Free radicals

Depending on the kind of resonance hybrid free radicals formed initially, different

volatile aldehydes such as hexanal, pentanal, butanal etc. may be produced. When

linolenic acid is the substrate, acetaldehyde, crotonaldehyde and other unidentified

fragments would be obtained.

Highly significant negative correlation between soybean seed vigour and

hydroperoxide level has recently been reported by hailstones and Smith (1989). Thermal

breakdown of such peroxides produced volatiles which have been positively identified by

gas chromatography as hexanal, pentanal and butanal. In lettuce also, volatile levels were

closely related to hydroperoxide levels and seed viability (Smith and Adamson, 1989). As

heating of solvent-extracted lipids of lettuce seeds yielded the same volatiles, these

authors implicated lipid autoxidation in lettuce seed deterioration.

The bioassay system developed by Basu et al., 1990 has clearly demonstrated the

presence of growth inhibitory gases in the emanations of low vigour seeds. A part of the

gaseous emanation could be absorbed by the aldehyde trapping reagent MBTH (3-

methyl-2-benzo-thiazolinone hydrazone), suggesting the production of volatile aldehydes

which are obviously products of lipid peroxidation.

Counteraction of free radical and lipid peroxidation reactions:

In the viability maintaining seed invigoration treatments, the production of

volatile toxic substances including aldehydes, was less implying reduced lipid

peroxidation (Pal & Basu, 1989). These results are in agreement with the redued post

ageing lipid peroxide formation in wheat (Rudrapal and Basu, 1979). Sunflower (Dey and

Basu, 1982), mustard (Dey and Basu, 1985), onion (In the viability maintaining seed

invigoration treatments, the production of volatile toxic substances including aldehydes,

was less implying reduced lipid peroxidation (Choudhuri and Basu, 1982) and soybean

(Saha et al, 1990) following hydration-dehydration treatments.

If free radicals induced lipid peroxidation is accepted as one of the basic reasons

of deteriorative senescence of the dry stored seed (Wilson and McDonald, 1986), the

termination of the same should extend seed longevity. The possible termination reactions

are summarized below.

Free radical-induced lipid peroxidation reactions.

The possible termination of free radial reations and ontrol mechanisms of peroxidative

breakdown of lipids

Conclusion:

In conclusion, besides the suggested involvement of the cellular repair system

during the hydration phase of the wet seed treatments, the counteraction of free radical

induced lipid peroxidaton reactions by the hydration-dehydration treatment is a highly

probable reason of its beneficial effect on vigour and viability and storability. The dry

treatment offer protection against ageing damage by their antioxidative, antioxidation

synergistic or biomembrane stabilizing effects. Further studies are necessary to elucidate

the fundamental reasons of seed deterioration and to work out still simpler and more

effective practical low-cost methodologies for seed preservation under less conductive

storage environments.

REFERENCES:

Bhaskaran.M, Bharathi.A, Vanangamudi.K, Natesan.P, Jerlin.R, Prabakar.K,

2003. Principles of Seed Production and Quality Control. Kaiser graphics limited.

Basu. RN (1989). Seed Research. “Seed Invigoration for Extended Storability”.

ISST. Pg: 217 – 229.