Unravelling the paradox in physically dormant species: elucidating the onset of dormancy after dispersal and dormancy-cycling

Abstract

Background: For species that produce seeds with a water-impermeable coat, i.e. physical dormancy (PY), it has been widely recognized that (1) seeds shed at a permeable state cannot become impermeable after dispersal; and (2) dormancy-cycling, i.e. a permeable ↔ impermeable transition, does not occur. Given a tight relationship between moisture content and onset of seed-coat impermeability, seeds maturing at low relative humidity (RH) and occurring in a high-temperature environment are inferred to produce impermeable coats, and ex situ drying of permeable seeds can lead to the onset of impermeability.

Scope and conclusion: It is proposed here that permeable seeds dispersed at low RH and in high-temperature soils might become impermeable due to continuous drying. Similarly, seeds with shallow PY dormancy (with higher moisture content immediately after becoming impermeable) can cycle back to a permeable state or absolute PY (complete dry state) when RH increases or decreases, respectively. A conceptual model is developed to propose that seeds from several genera of 19 angiosperm families at the time of natural dispersal can be (1) impermeable (dormant), i.e. primary dormancy; (2) impermeable (dormant) and become permeable (non-dormant) and then enter a dormant state in the soil, often referred to as secondary dormancy; (3) permeable (non-dormant) and become impermeable (dormant) in the soil, i.e. enforced dormancy; or (4) dormant or non-dormant, but cycle between permeable and non-permeable states depending on the soil conditions, i.e. dormancy-cycling, which is different from sensitivity-cycling occurring during dormancy break. It is suggested that this phenomenon could influence the dormancy-breaking pattern, but detailed studies of this are lacking.

Keywords: Impermeable seed coat; primary dormancy; secondary dormancy; sensitivity-cycling; shallow and absolute PY.