Submitted to: Seed Science Research
Publication Type: Review Article
Publication Acceptance Date: 10/5/2006
Publication Date: 3/13/2007
Citation: Wuest, S.B. 2007. Vapour is the principal source of water imbibed by seed in unsaturated soils. Seed Science Research. (2007) 17, 3-9.
Technical Abstract: The assumption that seeds imbibe most of the water they require for germination as liquid through seed-to-soil contact has been a dominant factor in germination research and seeding technology. Under most conditions seed is exposed to water vapour during imbibition, but the relative contributions of liquid and vapour are difficult to measure. In modelling efforts including a factor for imbibition of vapour, experimental procedures for estimating potential vapour imbibition have underappreciated the effect of distance on diffusion rate. At the same time, the amount of seed-to-soil contact and resultant liquid bridges from soil water films to the seed tend to be greatly overestimated considering soil water contents typically found in the field. Most researchers have measured very little decrease in germination over soil water contents ranging from field capacity to nearly permanent wilting point, and little response to bulk density, soil type, or seed-soil contact. While hydraulic conductivity decreases by several orders of magnitude as soil water content, bulk density, and seed-contact decrease, relative humidity remains near 100%. There are several experiments demonstrating that seeds of many species can germinate without any contact with liquid water at a rate very similar to those found when planted in soil. The combined evidence contraindicates the assumption that seed-soil contact is important for imbibition of water by seed. Water vapour should be considered the primary source in unsaturated soil.