Imbibitional Damage and Desiccation Tolerance of Wild Rice (Zizania palustris) Seeds

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Abstract
Seeds (caryopses) of North American wild rice (Zizania palustris var. interior), a temperate aquatic grass, have been reported to lose viability when desiccated (i.e. to be ‘recalcitrant’ or ‘homoiohydrous’). They are also deeply dormant at maturity and require as much as six months of stratification to break dormancy. It has been suggested that wild rice seed may have been misclassified as recalcitrant in some experiments due to the presence of dormancy. We report here that wild rice seeds can survive desiccation to seed and cmbryonic axis moisture contents as low as 6–8% (fresh weight basis). However, maximum survival of desiccation to these low moisture contents is possible only if dehydration occurs at temperatures ≥25°C and a slow imbibition period (3 weeks minimum) is allowed at temperatures between 10 and 25°C prior to stratification or dormancy-breaking treatments. The reduction in survival of dehydration at temperatures <25°C appears to occur only when embryonic axis moisture contents are reduced below about 8%. We also show that various techniques to assess viability of dry dormant seeds (moisture contents <30%), such as tetrazolium tests or scarifying the pericarp above the embryo, can cause a reproducible loss of viability due to imbibitional damage that has previously been interpreted as intolerance of desiccation. Most reports of the desiccation intolerance of wild rice seeds can be explained on the basis of the temperatures of dehydration or rehydration, failure to break dormancy, or imbibitional injury during viability testing. The successful dehydration and rehydration of wild rice seeds has important practical implications for seed storage and germplasm preservation. In addition, the temperature dependence of desiccation tolerance in wild rice seeds represents a novel relationship between seed viability, temperature, and moisture content.

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