Effects of burial and age on viability of rubber vine (Cryptostegia grandiflora) seeds

Abstract

One of the main difficulties in controlling the woody weed rubber vine (Cryp�tostegia grandiflora Roxb. ex R.Br.) is its capacity to regenerate from seed, particularly in riparian habitats. It is not known how long seeds can remain viable under natural conditions that favour germination, such as during wet seasons, or under conditions that preclude germination, such as during drought, or dry storage. Two experiments were undertaken to determine the viability of rubber vine seeds exposed to a range of contrasting conditions. Experiment 1 compared changes in
the viability of seeds that were placed at six positions within the soil profile (0 cm on bare ground, 0 cm ground level with seeds covered with 10 cm thick, slightly pressed, dry vegetation mulch and 5, 10, 20 and 40 cm below ground) and exposed to either natural rainfall or had rainfall excluded. Retrieval of seed lots was undertaken annually for a maximum of four years. Experiment 2 compared the viability and vigour of rubber vine seeds that had been stored for different durations (0, 1, 9, 11 and 20 years) under conditions conducive to prolonged life (dry storage at 7 ± 1°C).
In the field, the most rapid decline in viability occurred under natural rainfall conditions – with no viable rubber vine seeds remaining in the soil seed bank after one year, irrespective of burial depth. In contrast, viability of seed lots under conditions where rainfall was excluded averaged 68, 29 and 0% after 1, 2 and 3 years, respectively. Under dry storage, viability of 1 year old seed was extremely high (99%) and not significantly different to that of freshly collected seed. In comparison, viability of 9 and 11 year old seed averaged 87%, and only 20% of 20 year old seeds remained viable. Almost all viable seed had sufficient vigour to develop into seedlings, irrespective of age.