The adaptation to freezing tolerance of hydrated lettuce seeds: effects of regional climate and of seed characteristics.

BACKGROUND: With global warming, soil seed banks at high altitudes face dual challenges, excessive water absorption and thinner snow cover that increase underground temperature. A better understanding of freezing tolerance of hydrated seeds provides insights for conservation in natural soil seed banks. OBJECTIVE: To understand the adaptation mechanisms of seed freezing tolerance under various climates, in relation to cooling rate and seed size. MATERIALS AND METHODS: Twelve ecotypes of lettuce (Lactuca sativa) seeds were collected from different geographical locations around the world. Seeds were fully hydrated and tested for their freezing tolerance using programmed cooling methods. RESULTS: The size of seeds from different climate regions varied, and was correlated with the freezing tolerance of the hydrated seeds (P < 0.05). Larger seeds showed poorer freezing tolerance. The local climates of maternal plants were also well correlated to seed freezing tolerance (P < 0.05), especially under slow cooling conditions. The seeds collected in regions with high spring rainfall exhibited greater freezing tolerance. CONCLUSION: Freezing tolerance of hydrated seeds is affected by the climate of maternal plants and by seed size. Our data revealed the existence of an adaptation mechanism of freezing tolerance among various ecotypes of lettuce seeds. doi.org/10.54680/fr22410110412.

Cryopreservation of Borassus flabellifer L. (arecaceae) using excised embryos: a first report for the genus Borassus.

BACKGROUND: Embryos of many palm species that produce desiccation-sensitive, i.e. recalcitrant seeds have been successfully cryopreserved. However, storage protocols for some genera such as Borassus are still lacking. MATERIALS AND METHODS: Mature fruits of Borassus flabellifer L were collected in south India at the time of natural dispersal (June-July). After removal of the pericarp and mesocarp, the pyrenes were dried in silica gel and the desiccation tolerance level determined. Isolated embryos were also subjected to drying, exposure to liquid nitrogen temperature and recovery in vitro. RESULTS: Mature fruits weighed an average mass of 0.9 kg and germinated to 89%. The moisture contents (MCs) of the pyrenes and embryos were 48 and 78%, respectively. Pyrenes dried in silica gel to c. 20% MC lost viability, whereas embryos could be dried to 11% MC with 55% survival. These results indicate that the seeds are relatively desiccation-sensitive. Embryos dried to 21 and 11% MC had 23 and 36% survival following exposure to liquid nitrogen for 48 h, respectively. CONCLUSION: There is a hydration level window between c. 10 and 20% MC that is optimal for the cryopreservation of Borassus embryos. However, long-term storage possibilities remain to be explored.

Decoding the decisive role of seed moisture content in physical dormancy break: filling the missing links.

Species producing seeds with a water-impermeable seed coat, i.e., physical dormancy (PY), dominate the dry tropical forests. Despite increasing interest and understanding of the germination ecology of a PY species, less is known about how PY break occurs, particularly what changes lead to the opening of the 'water gap'. Based on the moisture conent (MC) attained, two ranges of PY may exist: shallow PY, a state with higher MC and seeds could reverse to a permeable state when the relative humidity increases; and absolute PY, a completely dry state. Here, we demonstrate that this MC variation between seeds affects preconditioning and the 'water-gap' opening stages. A conceptual model developed shows a strong relationship between temperature and duration, with high temperature breaking PY in seconds, but seasonal temperature fluctuations and constant temperatures require a longer time. The duration required at any conditions to break PY is purported to depend on the hydrophobic bonds of the lipids, which are likely weakened during the preconditioning, and the amount of water influences hydrolysis, leading to the 'water-gap' opening. We argue that the moisture content of the seeds and its interaction with biochemical compounds are a possible explanation for why only a proportion of PY seeds become permeable to water each year. Nonetheless, empirical investigations must validate these notions.

Role of the lens in controlling physical dormancy break and germination of Delonix regia (Fabaceae: Caesalpinioideae).

Physical dormancy occurs in all three subfamilies of Fabaceae, namely Mimosoideae, Papilionoideae and Caesalpinioideae, making it one of the largest plant families in terms of number of species with physical dormancy. However, little is known about the water gap structure and germination ecology of species in Caesalpinioideae. Freshly collected seeds of Delonix regia (Caesalpinioideae) did not imbibe water, thus they had physical dormancy. Both dry heat and wet heat were effective in breaking dormancy, however, longer duration was required at 80 °C and shorter duration at 90 °C. Seeds buried in the field for 2 years germinated to 21% and 42% after the first and second summer, respectively, compared with 3% germination in seeds at the time of maturity. Seeds incubated at 15/60 °C in the laboratory (mimicking summer conditions) for 3 months supported this conclusion, as dormancy was relieved in 18% and 24% of seeds stored dry and watered intermediately, respectively. All the dormancy breaking treatments resulted in lifting of palisade layers in the lens region to form a circular lid-like opening, i.e. water gap (Type II simple). Blocking experiments confirmed that water entered only through the lens and no secondary water entry point was observed. No apparent changes in morphology/anatomy of the hilum region were noted in dormant and non-dormant (water permeable) seeds. These results suggest that summer temperatures could open the lens in a proportion of seeds every year and that germination occurs during the subsequent wet season in the tropics.