Keeping Up With Conservation: Florida Ziziphus

Each year, the Rare Plant Conservation Program collects seeds from the ex situ population of Florida Ziziphus (Ziziphus celata) in the National Collection beds and sets up a germination trial to test seed viability. Along with freshly-collected seeds, seeds that were collected in previous years and stored at various temperatures are also included in the trials. This helps us determine how long seeds retain viability, learn which seed storage temperature is optimum for this species, and detect whether there is inherent variation in viability among seeds produced from different maternal genotypes.

Approximately 3,600 seeds were used in the 2021 germination trial that was set up in August. Germination began at Day 24. The elongated cotyledons emerge from a hard, woody endocarp, and true leaves begin to develop several days later. Germination rates are typically low for this species, with some years yielding much lower than the expected ~20-30% germination rate. Germination and seedling emergence can be inhibited by the physical restraints posed by the hard endocarp, and some seedlings fail to shed the endocarp even after successful emergence from the soil.

Our previous research has shown germination to be significantly enhanced by passage through the gut of gopher tortoises, and both this treatment and natural weathering and abrasion may historically have helped to promote seed germination in wild populations.

In greenhouse conditions, germination is influenced by factors such as depth of seed sowing (deeply sown seeds do not germinate, and shallow-sown seeds need to be just deep enough to maintain soil moisture and for the emerging seedling to move through enough sand to help mechanically remove the endocarp) and size of cells in the germination trays (cells need to be large enough to maintain steady soil moisture and provide enough room for some lateral movement of the endocarp during germination).

So far the 2021 germination trial appears to be successful, with several new seedlings arising each week. The peak of germination is expected to be in October through November, and because germination can continue for several months in this species, this trial will continue through spring 2022. This trial will greatly help us understand seed longevity among seed storage temperatures and inherent viability across maternal genotypes, as well as producing new plants for population introductions.