Desiccation-induced loss of seed viability is associated with a 10-fold increase in CO2 evolution in seeds of the rare tropical rainforest tree Idiospermum australiense

• Here the relationship was investigated between metabolic activity, state of hydration and seed viability in the desiccation-intolerant (recalcitrant) seeds of Idiospermum australiense, a rare and primitive angiosperm tree restricted to wet tropical forest. • Seed CO2 evolution rate, R, was monitored in fully hydrated (control) seeds and seeds that were allowed to desiccate under ambient conditions over a period of c. 90 d. • During desiccation R increased dramatically toward a peak at a seed relative water content of 39 ± 3% (relative to maximum water content, which corresponded to 0.45 ± 0.03 g water g-1 d. wt) followed by a decline toward zero with total desiccation. This peak constituted a 10-fold increase in mean R, relative to the control. Exposing seeds to O2-free air at this peak induced a further large, but transient, increase in CO2 evolution, indicating that the peak developed in the presence of oxidative phosphorylation, rather than due to the absence of it. • The magnitude and mode of the observed increase in CO2 evolution in response to desiccation is unlike any reported so far and thus adds new information about metabolic changes that may occur as the water content of desiccation-intolerant seeds declines

Differential responses of Mimusops elengi and Manilkara zapota seeds and embryos to cryopreservation

Several dehydration protocols were evaluated for their ability to cryopreserve intact seeds and excised embryonic axes of Mimusops elengi and Manilkara zapota (Sapotaceae). Both interspecific and intraspecific variations in cryotolerance were found. M. zapota embryonic axes were more tolerant of cryopreservation than those of M. elengi, and showed higher desiccation tolerance, higher post-thawing survival and development, and a much wider range of moisture contents for cryopreservation. Maximum development rates were 94% and 27% for M. zapota and M. elengi, respectively. Intact seeds of both species tolerated desiccation to low moisture levels, but were sensitive to liquid nitrogen exposure, and cryopreserved seeds failed to germinate. Assessment of developing embryos excised from cryopreserved seeds associated nonviability of cotyledons and plumules with germination failure. Other structures survived at variable rates; most hypocotyls and radicles (up to 76% and 98% for M. elengi and M. zapota, respectively) were viable. The different cryotolerance between hypocotyls and cotyledons is a critical cause for failure in cryopreservation, contributing to the difficulty in developing protocols for such intermediate oily seeds