Water absorption by swelling seeds. II. Surface condensation boundary condition

Abstract

The previously developed model of water absorption by a swelling seed has been applied to imbibition and germination of wheat seed with the limiting supply condition of purely water vapour transport both with and without the presence of a porous medium. Experimental imbibition and germination data show that with vapour transport, the rate of imbibition and germination of wheat decreases with increasing distance from the supply source and that competition for water vapour occurs between seed. This competition effect was verified by values of the flow pathway shape factor for vapour transport to seed, measured in a three-dimensional electrical analogue. After accounting for heat production and losses with vapour absorption and using appropriate values of the flow pathway shape factors, the equilibrium seed absorption isotherms were used to derive surface condensation boundary conditions for several imbibition situations. These calculations suggest that in the absence of porous media a significant temperature rise of the seed retards the rate of imbibition. By comparison, with a close-packed porous medium around the seed, the large thermal conductivity of the medium should maintain nearly isothermal conditions. Using a finite difference approximation, the swelling seed model and condensation boundary conditions are analysed to give predicted imbibition data which can be compared with experimental data. These results show good agreement between predicted and experimental imbibition in the absence of a porous medium. For seed embedded in a porous medium, the predicted imbibition rate significantly exceeds the experimental rate, but no account could be taken for restriction of seed swelling by the porous medium. The results are discussed in terms of their relevance to the sowing of seed in 'dry' seed bed conditions.

Vapor Transport vs. Seed–Soil Contact in Wheat Germination

“…They did not specify the distance from the seed to the vapor‐supplying liquid. Collis‐George and Melville (1978) measured imbibition by wheat under purely vapor transport, and found that distance from the liquid surface was a factor in imbibition rates. If the distance between the liquid surface and the seed surface influences both imbibition rates and final moisture content of seed, then studies of vapor imbibition will give very different results, depending on the methods used.…”

Vapour is the principal source of water imbibed by seeds in unsaturated soils

“…Few experiments have been conducted in which seeds have been intentionally germinated in vapour alone, although several reports exist where this has occurred while pursuing other goals. As mentioned earlier, Owen (1952) determined that wheat imbibed with vapour alone germinated in 48 h at 20°C, and Collis-George and Melville (1978) observed germination in vapour in 60–80 h. In addition, Etherington and Evans (1986) used sealed Petri dishes packed with soil to germinate various native seeds in contact with soil of different water contents. They found that a piece of fibreglass filter paper placed between the soil and a seed made it easier to observe the seed, and caused no noticeable delay in germination.…”

“…Bruckler (1983) used a 1- to 6-mm air gap, and concluded that full imbibition of maize took three times longer in vapour than in liquid. Collis-George and Melville (1978) compared distances of 1 and 3 cm and measured 60 and 80 h for wheat germination to be completed. And, finally, Schneider and Renault (1997) placed maize grains 2 cm above water and concluded that it took ten times longer for them to imbibe in the presence of vapour than in liquid water.…”

“…Theoretical relationships between relative humidity and matric or osmotic potential are temperature dependent, and temperature gradients are likely. Collis-George and Melville (1978) measured a 2–3°C surface temperature rise for wheat grains, attributed to the heat of wetting and latent heat of condensation. (This is an alternative explanation for a lack of condensation on the Petri dish lids near a seed.)…”

Estudos sobre a absorção de água e o desempenho de sementes de soja

“…WEST, 1985), soluções com várias quantidades de produtos quimicos(POWELL & MATTHEWS, 1978; WOODSTOCK & TAO, 1981) e sistema de vapor expondo as sementes às diferentes condições de umidade relativa do ar(COLLIS-GEORGE & MELVILLE, 1978). Em todos os casos, a embebição pode ser realizada de maneira gradual e, para isso, basta controlar a quantidade de água disponível à semente(VERTUCCI,1989).Quando a embebição for realizada em substrato úmido, a água permanece retida no substrato pelo potencial mátrico e não há possibilidade de confundir os efeitos dos tratamentos com os efeitos devido a absorção de produtos químicos.…”