Cultivated northern wild rice (NWR; Zizania palustris L.) has been bred and studied since the 1950s. One challenge facing researchers is the lack of storage options, due to the seed's unorthodox behaviour. This study evaluated varying storage temperature and moisture conditions for the maintenance of seed viability and dormancy breaking in Minnesota-grown NWR. First, seeds were placed in non-submerged, freezing storage (NSFS) for 12–26 weeks, then in submerged, cold storage (SCS) for 2 weeks. The addition of SCS increased germination (%G) relative to NSFS alone (<0.1% NSFS, 15% NSFS and SCS), indicating that NSFS does not kill seeds but also does not break seed dormancy. Next, the required length of SCS was evaluated by placing seeds in NSFS for 12 weeks and then in SCS for 2–14 weeks. A longer SCS period increased %G from 3 to 79%, at 2 and 14 weeks of SCS, respectively. Lastly, seeds were placed in NSFS, followed by SCS, at varying intervals over a 29-week period. Across lines, germination increased from 20 to 76% between 4 and 7 weeks of SCS, respectively, then plateaued. The results of this study indicate that NSFS could be used to store NWR seeds, but at least 7 weeks in SCS is required to overcome dormancy. Additionally, while NSFS did not break seed dormancy, physiological changes related to stratification processes were occurring in non-submerged, freezing conditions. Results also suggest that the genotypic variation in NWR could be utilized for selection to improve germination and storage viability.

The sorption isotherm for excised embryonic axes of recalcitrant (i.e. desiccation-sensitive) Quercus robur L. acorns was determined to find the relation between moisture content and water potential. Subsequently, physiological studies on the effect of desiccating the axes to a range of water potentials were undertaken. The respiratory capacity declined steeply after short exposure to water potentials from −5 to −30 MPa. The leachate conductivity increased significantly after exposure to −5 MPa and rose steeply after exposure to between −12 and −40 MPa. Axes were equilibrated at different relative humidities and the proline content showed a 15-fold increase with a peak value at −10 MPa. It was concluded that the critical water potential for initiation of damage was −5 MPa, and that axes accumulated proline as a response to desiccation stress. The embryonic axes from Q. robur behave more like typical vegetative tissue of angiosperms than like orthodox seeds.

A characteristic of recalcitrant seeds is that, if they are maintained under storage conditions that prevent water loss, they will ultimately lose viability. A current view is that hydrated recalcitrant seeds are metabolically active and undergo germination-associated changes in storage. Some of these changes, such as extensive vacuolation and increase in cell size, imply a requirement for water additional to that present in the seed on shedding. It is therefore suggested that, in storage, recalcitrant seeds are exposed to an initially mild, but increasingly severe, water stress. Deleterious events associated with a water stress of considerable duration are suggested to lead ultimately to the death of the tissue.

The damage that occurs on prolonged storage is unlikely to be associated with an inability to form glasses or prevent membrane lipid phase changes, as absolute water contents are higher than those at which these mechanisms become important. It is considered that the most likely process leading to death of water-stressed (as opposed to dehydrated) tissue is a breakdown of co-ordination of metabolism, leading to uncontrolled free-radical-mediated oxidative damage.

It is generally difficult to maintain tissue in a mild water-stressed condition for extended periods. Stored, hydrated, recalcitrant seeds may provide an ideal model system for studying the metabolic effects of prolonged mild water stress.

Changes in the properties of water in excised embryos were measured during the late stages of grain development in two cultivars of Zizania palustris and a population of the endangered species Z. texana. The relationships between water content and water activity were determined from water sorption isotherms, measured at temperatures between 35 and 5°C and then derived for lower temperatures. The freezing and melting behaviour of water in embryos at different water contents was determined using differential scanning calorimetry. The moisture content of embryos at high water activities decreased with maturation, as did the moisture content at which freezing transitions were not observed. While the temperatures of freezing and melting transitions decreased as the moisture content of embryos decreased, there were no discernible differences among embryos at different developmental stages. The properties of water measured in maturing Zizania embryos approached those for orthodox seeds as determined from the strength of water sorption, the enthalpy of the melting transition and the moisture content at which water is unfreezable. From these data we conclude that the properties of water in recalcitrant Zizania embryos change with development to resemble those of embryos of desiccation-tolerant seeds, but that the seeds never achieve the orthodox condition. The effects of interactions between moisture content and temperature on desiccation damage, freezing damage and germination in Zizania are predicted, based on the physical properties of water reported here and the correspondence of these properties with physiological function reported for other species. The resulting ‘phase diagram’ defines possible combinations of moisture content and temperature for storage under equilibrium conditions.

Seeds of four African tree species from the Sahelian zone (Boscia senegalensis) and the Sudanian zone (Butyrospermum parkii, Cordyla pinnata and Saba senegalensis) lose viability after moisture contents drop below 22 and 30%, depending on the species. Seed longevity in wet and airtight storage does not exceed a few months. Temperatures close to zero elicit symptoms of chilling injury leading to rapid seed death. The optimum storage temperature is 15°C. These results allow the four species to be classified as recalcitrant seed species. Boscia senegalensis is therefore an exception in arid zones where most species have orthodox seeds.

The mature seeds of durian (Durio zibethinus L.) are recalcitrant, with the cotyledons storing both starch (amylose, 78% dry weight) and some protein (7%), but having few triglyceride reserves (<1%). Reserve accumulation occurs during the latter third of the 110-d seed developmental period prior to fruit shed. The storage proteins of durian seeds consist of two immunologically related unglycosylated albumins, light and heavy zibethinin (20.2, 21.4 kDa), that account for 37–46% of the total protein during development. Light zibethinin is a single polypeptide, most likely having internal sulphydryl linkages, whereas heavy zibethenin is composed of two sulphydryl-linked subunits. The zibethinins are deposited in swellings of the endoplasmic reticulum (ER) which then either expand or coalesce to form protein storage vacuoles. This mode of protein body formation in durian varies from both the ER–Golgi complex–vesicle–vacuole pathway characteristic of many dicotyledonous seeds and the direct ER to protein storage vacuole pathway involved in deposition of certain prolamins in cereal endosperm cells. Protein storage vacuoles in durian seed storage parenchyma never become fully filled, but contain only diffuse peripheral clumps of zibethinin protein. There is a 15% loss in starch, a precipitous loss of total protein, and a concomitant loss of the zibethinin storage proteins during the 5 d prior to fruit shed, suggesting that both reserve mobilization and germination has begun.