Survival of tropical weed species propagules after immersion in fresh, brackish and salt water

Abstract

Neem (Azadiractha indica A.Juss.), Navua sedge (Cyperus aromaticus (Ridl.) Mattf. & Kük.), Hymenachne (Hymenachne amplexicaulis (Rudge) Nees), Bellyache bush (Jatropha gossypifolia L.) and Leuceana (Leucaena leucocephala ssp. glabrata (Rose) Zarate) are serious weeds in tropical north Queensland. Water movement is a significant dispersal vector for seeds of each of these species, particularly along water courses, but little is known about seed survival following immersion and movement in water. To improve knowledge of the effect of water immersion on seed viability seeds from each species were placed in fish tanks containing fresh, brackish and salt water for set periods of time before testing their viability.

Seeds of four of the species tested remained viable for the maximum test period of 98 days (14 weeks). Only Neem lost all viability after seven days in salt water and 14 days in the fresh and brackish water treatments. After 98 days in salt water, Leucaena seed retained almost half its original viability, whereas Bellyache bush and Navua sedge seed viability was reduced to approximately a third. Notably, Hymenachne seed viability remained relatively unaffected by immersion in all water types.

Although seeds can be transported in water, a suitable habitat is still required for germination and plant establishment and survival. By determining the length of time seed can remain viable after immersion in water, and whether immersion promotes, delays or inhibits viability gives an insight into the role water plays in the dispersal of these introduced weed species.