E.A. Oelke

Survival of Zizania embryos in relation to water content, temperature and maturity status

The interactions between water content and temperature on freezing and desiccation damage were examined for Zizania (wild rice) embryos at several stages of maturity. The water content of excised embryos was manipulated by flash drying at 35°C or room temperature to between 2.5 g H 2 O/g dw and 0.05 g/g. Embryos were then exposed to temperatures ranging from 5 to −50°C. Viability following the drying and cooling treatments was assayed by leakage of electrolytes and germination in culture. Viability of embryos decreased when embryos were dried below a critical water content. The critical water content was greatest for the least mature embryos. Critical water contents were also temperature dependent and increased with decreasing temperature. Even though the critical water content varied with developmental status and temperature, the water activity corresponding to the critical water content appeared to be constant at 0.90. The most mature embryos survived temperatures as low as −50°C while the least mature embryos survived only to −18°C. These trends were predicted by ‘phase diagrams’ based on the physical properties of water in embryos at different stages of maturity (Vertucci et al. , 1994a). Our results confirm the earlier prediction that long term preservation of Zizania grains is possible at −20°C and the degree of success will be related to the maturity status of the embryos.

Physical properties of water in Zizania embryos in relation to maturity status, water content and temperature.

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.

Formation of Panicles and Hermaphroditic Florets in Wild-Rice

Formation of panicles and flower primordia in wild-rice, Zizania palustris L., was investigated by scanning electron microscopy (SEM). Floral development involved the formation of both staminate and pistillate floral primordia in the wild-rice spikelet. The SEM study indicated that wild-rice shared a similar developmental process in panicle formation with that of cultivated white rice (Oryza sativa L.). Variations of hermaphroditic floret formation and sex expression were observed in the transition zone between the male and female portions of the panicles in several different wild-rice populations. The frequencies of plants with hermaphroditic floret formation ranged from 27% in the Pistillate population (Z. palustris L.) to 70% in the Peterson Pond (Zizania aquatica L.) population. There was a high degree of phenotypic variation in sex expression in the transition zone of panicles from different wild-rice populations.