Seed responses to change in ambient humidity and the consequences for storability

The study of seed moisture sorption isotherms helps us to understand the response of seeds to changes in moisture in their environment and how this might impact their longevity. A seed moisture isotherm shows the relationship between moisture content (MC) and water activity (a_w) at a constant temperature. Seed MC is higher when it dries from a hydrated state (desorption) than when it rehydrates from a very dry state (adsorption) to the same RH, an effect known as hysteresis. Seeds on the desorption isotherm will age faster than seeds on the adsorption isotherm at the same RH due to higher MC, resulting in a more rapid decline in seed viability and vigour. In this study, moisture isotherms of seeds from diverse species were determined and modeled using the Guggenheim-Anderson-de Boer (GAB) model; the hysteresis effect was then quantified. The maximum hysteresis magnitude varied between 0.3 and 2.7% MC (occurring between 49 and 57% RH) when seeds were dried over silica gel (below 11% RH) and rehydrated. Cycling seeds between 30 and 50%, 20 and 50%, and 15 and 50% RH, it was found that seeds of barley, lupin, buckwheat, wheat and pea switch to the adsorption isotherm upon rehydration after drying at intermediate moisture levels. Thus, adsorbing seeds of these species have greater longevity than desorbing seeds at the same RH. In contrast, seeds of oilseed rape and yellow mustard do not show a hysteresis effect unless they are dried to very low MC (over silica gel), so the adsorbing seed longevity remains the same as desorbing seeds at a constant RH. It is important to consider the moisture history of seeds depending on species for better seed storage management.