The biology of Protasparagus Africanus (Lam.) Oberm. in eastern Australia

Stanley, T. D. (Trevor D.) (1994). The biology of Protasparagus Africanus (Lam.) Oberm. in eastern Australia PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Stanley, T. D. (Trevor D.)
Thesis Title The biology of Protasparagus Africanus (Lam.) Oberm. in eastern Australia
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 1994-01-01
Thesis type PhD Thesis
Supervisor A/Prof R. W. Rogers
E/Prof H. T. Clifford.
Total pages 140
Language eng
Subjects 0701 Agriculture, Land and Farm Management
300204 Plant Protection (Pests, Diseases and Weeds)
Formatted abstract

Studies were undertaken to establish the regeneration strategies of Protasparagus africanus, to help explain the success of the species as a weed and to propose possible control mechanisms.

A study of the phenology of flowering and fruiting showed that the plants flower in response to the first major rains after the August-September dry period and that mature plants each set about 21 000 seeds per year. The mean percentage of flowers which set fruit per plant was 12.9% ± 0.7%. Self fertilisation was shown to occur. The first germinable seeds were collected in March, 14 weeks after anthesis and these seeds were from green fruits. All viable seeds were germinable by week 18. The first fruit ripened in May, 22 weeks after anthesis. All fruits had ripened by the end of September. Although seeds germinated at 14 weeks after anthesis, the speed of germination of the seeds did not reach its maximum until 18 weeks after anthesis. Studies on predation and dispersal of fruits and seeds were undertaken. No seeds were shown to be destroyed or damaged by predators. Secondary dispersal of seeds or fruits did not occur. The first seeds dispersed were in green fruits which fell in April. All fruits had been dispersed by the end of October. The first removal of fruits by consumers occurred in May, six months after anthesis, but most removal of fruits by consumers occurred in August and September. Overall 40.1% ± 2.6% of fruits per plant were removed by consumers. The bird species Zosterops lateralis, silvereyes, was the only animal observed taking fruit.

No seeds were shown to possess either innate or induced dormancy; all viable seeds germinated when placed in conditions suitable for germination.

Seeds were stored under a variety of conditions both in the field and in the laboratory. Seeds stored in the laboratory at 38ºC remained viable for less than two months while some seed stored at 25ºC with a relative humidity of 33% or at 2ºC and ambient relative humidity remained viable for 14 months. Seeds remained viable for less than 12 months when stored in the field. The speed of germination was shown to decrease as the seeds aged. Speed of germination was shown to increase when the fresh seeds were subjected to a period of osmotic potential at or below -0.6 MPa (osmoconditioning) before germination. Speed of germination was also shown to increase when seeds were imbibed, dried before germination and then reimbibed and allowed to germinate. It was proposed that the speed of germination of seeds stored in the field was the result of increased speed of germination due to osmoconditioning and/or imbibing-drying tempered by a decrease in speed of germination due to aging of the seeds. The increased in the speed of germination due to osmoconditioning and/or imbibing-drying is a mechanism which ensures that seeds which are shed late in the dry season do not germinate after the first light rains but germinate quickly after the first heavy rain and that seeds shed at other times of the year germinate quickly after periods of heavy rain which occur from time to time.

Thermogradient bars were used to study the germination of seeds. The seeds will germinate in a wide range of alternating or constant temperature conditions either in constant darkness or with a 12 hour photoperiod. Germination was most rapid in constant temperatures in the dark. Alternating temperatures reduced the speed of germination compared to constant temperatures but did not inhibit germination with the larger temperature alternations having the greatest effect. The addition of a photoperiod had no effect with constant temperatures but reduced the speed of germination but did not inhibit germination with alternating temperatures. There was also a light/temperature interaction with the reduction in germination speed in a 12 hour photoperiod being greatest at the lowest and highest temperatures and least at the optimum temperature. It was proposed that the observed reactions of germinating seeds to light and temperature was a depth· sensing mechanism which ensured seeds germinated only when moisture conditions were likely to be suitable for seedling establishment.

Studies were undertaken at two field sites to measure changes in the seed rain abundance, the seed bank abundance and in the abundance of seedlings. Prorasparagus africanus was shown to have a transient seed bank. However the seed bank was 'pseudo-persistent' in that the long period of seed rain, from April until late October together with some seeds having a longevity in the field of 8-1 o months, ensured that some germinable seeds were present in the seed bank at all times of the year. Few seeds in the soil seed bank were buried, the majority of seeds being found on the soil surface or in the litter on the soil surface. Seedlings were shown to emerge in the field from January to June. Large numbers of seedlings appeared in January following the first heavy rains after the dry period in August -September. Some seedlings were shown to persist for periods of more than 12 months and the species was shown to maintain a bank of persistent seedlings. The regeneration strategy of Protasparagus africanus was shown to be regeneration from a seedling bank, backed up by two other strategies, a seasonal germination strategy and a risk -spreading strategy which together ensured that favourable conditions for germination were taken advantage of throughout the year.

A list of biological characteristics which contribute to the success of the species as a weed was prepared. The lack of a true persistent seed bank was seen as a characteristic which could be taken advantage of in controlling the species and a suggested method and timing of control was proposed.

Keyword Weeds
Weeds -- Control
Climbing asparagus fern

Document type: Thesis
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