Planta (Ber].) 71, 81--86 (1966)
E F F E C T OF "OSMOTIC" SYSTEMS ON GERMINATION OF PEAS (PISUM SATIVUM, L.)* MAN SI~o~ MANO~A~ Received July 4, 1966
Summary. 1. I t has been shown that the percentage germination of seed in contact with an aqueous solution depends (a) on the extent of differential permeability of the seed coat to the solute and (b) on whether the solute is toxic. 2. In investigations with "Meteor" pea seeds, polyethylene glycols of nominal molecular weights of 4000 or higher have proved to be suitable osmotic agents. 3. Solutions of sodium chloride, glycerol and mannitol may enter the seed with little restriction through micropyle and hence their influence on germination should not be ascribed to osmotic potential. 4. High concentrations of sodium chloride seem to be toxic to germinating peas. The glycerol solution on the other hand had no such toxic effect. In the earlier work published by MANOHAR and HEYDECKER (1964), it was concluded that matrix and osmotic potentials produced b y tension plates and mannitol solutions respectively, may differ in their effects on seed germination, largely because differences in the spatial systems involved tend to increase the effect of matrix and reduce the effect of osmotic potentials. Further investigations were therefore carried out in search of a better osmotic agent. The terms water potential, osmotic potential and matrix potential are used in accordance with the recommendations of SLATYER and TAYLOl~ (1960). The potential of pure flee water is taken as zero Joules/kg and therefore the solutes will lower the water potential. Materials and Methods Sodium chloride, glycerol, mannitol and polyethylene glycols "4000", "6000" and "15,000--20,000" (nominal molecular weights) were used with "Meteor" pea seeds in a series of experiments which were carried out in the dark at 25 o C.
Preparation o/ Osmotic Potential Solutions I t is known that sodium chloride, glycerol and mannitol solutions behave in accordance with Van t' Hoff's equation but, as reported by LAGE~WERFF, OGATA and EAGL~ (1961) and JACKSON (1962), with the polyethylene glycols of higher molecular weights the relationship of potential to molecular weight deviates from the predicted value. A thermocouple psychrometer ( M _ ~ o ~ , 1966) was there* This Investigation formed a part of a P h . D . thesis in the University of Nottingham, U. K. 6 Planta (Berl.), Bd. 71
M.S. M_~o~AI~: Effect of "Osmotic" Systems on Germination
fore used to investigate this relationship for solutions of polyethylene glycols "4000", "6000" and "15,000--20,000" by comparing their vapour pressures with those of sodium chloride solutions of known potentials (ROBINSONand STOKES, 1955).
Permeability o/Pea Seed Coats The degree of permeability of the seed coat as a whole is determined by the permeability of its most permeable part, i. e. the tissues associated with the micropyle. The permeability of this region at potentials of --1520 Joules/kg produced by solutions of sodium chloride, glycerol, marmitel and polyethylene glycols "4000" and "6000" was investigated; polyethylene glycol "15,000--20,000" could not be used because of its high viscosity. Osmometers were prepared much as already described by M i N o ~ and HEYDECKER (1964) but by fitting to the end of each 3 mm capillary tube an intact excised seed coat, with its micropyle in the centre, and filling the tubes, in duplicate, with each of the individual solutions so as to obtain 30 cm columns of solution above the water level.
Germination Tests o/"Meteor" Pea Seeds In order to establish a possible relationship between germination and the degree of differential permeability exhibited by the seed coats, germination tests were conducted in which pea seeds ("Meteor") were placed in contact with solutions at potentials of 0, --506, --1013 and --1520 Joules/kg, prepared from sodium chloride, glycerol, mannitol and polyethylene glycols "4000", "6000" and "15,000-20,000", using 25 seeds per 9 cm glass petri dish each containing a pad of three Whatman No. 1 filter papers, moistened with 25 ml of solution. Some seeds which had not germinated after 14 days at the lower potentials were then transferred to distilled water, after wiping the surface of the individual seeds, in order to ascertain whether they were capable of germination.
Results The polyethylene glycols lowered the water potential more t h a n would be predicted b y Van t ' t I o f f ' s equation and the discrepancy increased b o t h with increasing concentrations and particularly with increasing molecular weight. The results plotted on Fig. 1 were used to obtain the appropriate concentrations of polyethylene glycols to produce the range of potentials required. The degree of differential permeability shown b y the p a r t of seed coat associated with the micropyle to isotonic solutions of different chemicals (Fig. 2) varied a great deal. The degree of restriction imposed b y the seed coat was roughly proportional to the size of the molecules of the chemicals of lower molecular weight but, in contrast, the seed coats were impermeable to solutions of the two polyethylene glycols. The rate and total germination of pea seeds in isotonic solutions of different chemicals varied a great deal. E x c e p t the effect of potentials produced b y sodium chloride solutions, the rate and total germination of pea seeds (Fig. 3a and 3b) in isotonic solutions produced b y different chemicals varied in accordance with their molecular weights. The residual viability results presented in Table showed t h a t polyethylene
~ -150( o
NOMINAL MOLALITy OF CARSOWAXES ( POLYETHYLENE GLYCOLS}
Fig. 1. :Relationship b e t w e e n concentration a n d w a t e r potential of polyethylene glycols of different molecular weights. F i g u r e s represent the nominal molecular w e i g h t s CW =4000'
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