Title: Some thoughts about the desiccation tolerance continuum and the basis for “intermediate” seed physiologies Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: November 20, 2011
Publication Date: N/A
Technical Abstract: Desiccation-sensitive seeds succumb when dried below a critical water content. Critical water contents range widely among species and with experimental conditions, implying a continuum in the amount of water that can be non-lethally removed from diverse cells. However, water potentials corresponding to critical water contents usually fall within discrete steps that are specific to cell type and cell constituents, and provide support for categorical classification of desiccation tolerance. Moreover, mechanisms of desiccation damage usually invoke specific, discrete first order structural changes to biomolecules, such as membrane lipid phase transitions. The expression of desiccation tolerance as either a continuous or discrete trait influences the explanation of seeds having storage physiologies intermediate between recalcitrant and orthodox. Perhaps desiccation damage is a discrete event, but the timing or rate of expression is a quantitative feature. This possibility is consistent with the idea that inevitable structural change is delayed by increased viscosity when glasses form. This perspective leads us to consider desiccation damage and longevity interchangeably, and to view “intermediate” seeds as those that survive desiccation but age relatively rapidly while dry. Both qualitative and quantitative aspects of desiccation damage and tolerance can also be rationalized by considering desiccation in the context of critical volume changes. About 40 years ago, Meryman proposed that desiccation damage resulted from the mechanical stress of cell shrinkage and observed that 50% volume reduction was the distinguishing feature between cells that did and did not survive. Loading cells with more than 50% non-compressible dry reserves is a straightforward way to avoid 50% volume loss, regardless of the degree of water stress imposed. Orthodox seeds are extremely efficient at accumulating dry matter reserves, while recalcitrant seeds do not appear to meet the 50% dry mass criterion. We hypothesize that cells of “intermediate” seeds contain near 50% dry mass and that slight variation in dry matter accumulation, final water content and temperature can have major effects on seed survival.