Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2015
Publication Date: 8/1/2015
Citation: Walters, C.T. 2015. Orthodoxy, recalcitrance and in-between: describing variation in seed storage characteristics using threshold responses to water loss. Planta. 242(2):397-406.
Interpretive Summary: Some seeds (like wheat) survive nearly complete drying and remain viable for decades; these seeds are called orthodox. Other seeds (like oak acorns) don’t survive drying very well and die within a few months; these seeds are called recalcitrant. This paper presents the premise that the differences in physiology between orthodox, recalcitrant and so-called intermediate seeds lies in how they transition from fluid to solid, much like how a plastic changes from malleable to rigid. Cells from recalcitrant shrink too much during drying and so lose structure. In contrast, orthodox seeds have the remarkable capacity to load cells with food reserves, allowing them to keep cell shape even when severely dried. Once in a solid, rigid state, orthodox seeds survive for different periods. Differences in seed longevity are considered analogous to differences in the shelf-life of plastics or dried foods, and attributed to the types of molecular movements that are constrained and allowed in the material. Application of a materials approach to the study of seeds is providing a new perspective that helps us identify how water, temperature and biology during seed development interact to impart orthodox seeds’ unique ability to survive drying and to explain why die at different rates.
Technical Abstract: Tolerance of desiccation is typically described by a threshold or low-water-content-limit to survival. This convention provides fairly good distinction between orthodox and recalcitrant seeds, which show thresholds of less than about 0.07 and greater than about 0.2 g H2O g dw-1, respectively. Threshold water contents, however, are not direct measures of the intensity of water stress tolerated by seeds, nor are they measures of cell response to water stress. More direct criteria, that accommodate both spatial and temporal effects of water loss, are required to explain variation of desiccation tolerance and longevity in seeds from diverse genetic backgrounds and growth conditions. This essay presents the argument that changes in cellular volume directly quantify primary responses to desiccating stress in a context that also links damage, as cellular constituents compress, and protection, as compressed molecules form stabilizing structure. During desiccation, fluid cytoplasm solidifies, and the newly formed spatial relationships among molecules determine whether and how long viability is maintained. The diversity of seed behaviors suggests complexity and opportunity to discover molecules and mechanisms that regulate survival and perception of time in cells that lack metabolic function.