Witchweed (Striga asiaticaLour. ♯4STRLU) seed remained viable for 6 yr under open-shelf laboratory conditions and for 14 yr when deep buried under field conditions. Seed kept in normal atmosphere of the laboratory for < 1 through 6 yr had the following germination percentages: 2, 95, 95, 94, 83, 38, and 7. Newly harvested seed will not germinate because of postmaturity ripening requirements. There were no viable seed after 7 yr of shelf storage in the laboratory. Seed buried in the field gave 60 to 90% germination the first season after production. Annual recovery and testing showed a gradual reduction in viability to 10% at year 14 for the deepest buried seed; shallower buried seed expired sooner. No germination occurred thereafter.

Chinee apple (Ziziphus mauritiana Lam.) is a thorny tree that is invading tropical woodlands of northern Australia. The present study reports three experiments related to the seed dynamics of chinee apple. Experiment 1 and 2 investigated persistence of seed lots under different soil types (clay and river loam), levels of pasture cover (present or absent) and burial depths (0, 2.5, 10 and 20 cm). Experiment 3 determined the germination response of chinee apple seeds to a range of alternating day/night temperatures (11/6°C up to 52/40°C). In the longevity experiments (Expts 1 and 2), burial depth, soil type and burial duration significantly affected viability. Burial depth had the greatest influence, with surface located seeds generally persisting for longer than those buried below ground. Even so, no viable seeds remained after 18 and 24 months in the first and second experiment, respectively. In Expt 3 seeds of chinee apple germinated under a wide range of alternating day/night temperatures ranging from 16/12°C to 47 /36°C. Optimal germination (77%) occurred at 33/27°C and no seeds germinated at either of the lowest (11/6°C) or highest (52/40°C) temperature regimes tested. These findings indicated that chinee apple has the potential to expand its current distribution to cooler areas of Australia. Control practices need to be undertaken for at least two years to exhaust the seed bank.

If treatments imposed to control exotic plants also have a deleterious impact on their residual seed bank, the duration and extent of follow-up control may be reduced. Fire is one such technique that has this ability, particularly if seeds are located on or close to the soil surface. Three studies were undertaken in a riparian habitat in the dry tropics of northern Queensland to quantify the effects of spring burning on the seed bank of the exotic weed bellyache bush (Jatropha gossypiifolia). The first determined the distribution of seeds within the vertical profile of the trial site. The other 2 studies were experiments that quantified the effects of fire on germination and viability of both dispersed bellyache bush seeds and seeds held in mature capsules. Dispersed seeds of 2 types (intact and ant-discarded) were placed at the following 6 positions in the vertical profile of bellyache bush infestations: 0.5, 1, 2 and 4 cm depth below ground, on bare ground, and below fuel. Seeds held in capsules were located at random on the crown of bellyache bush plants. For both experiments, comparisons of burnt plots were made with unburnt controls. Fire was imposed in spring (September); the season in which burning for weed control in northern Queensland generally occurs. While maximum fire temperatures averaged 590 ± 46°C, the temperatures that seeds or capsules were exposed to depended on their location within the vertical profile, with temperatures decreasing in the following order: below fuel > crown of bellyache bush > 1 cm > 0.5 cm > bare ground > 2 cm > 4 cm below ground. There were negative correlations between seed germination and peak fire temperature and between viability and peak fire temperature. Seed viability was nil for seed under fuel but >80% for seeds placed on bare ground or ≥2 cm below ground. Fire reduced germination and viability of seeds held in capsules by 31 and 35%, respectively, when compared with unburnt seeds. While ant-discarded seeds generally had a higher germinability than intact seeds, they were more susceptible to fire. This may be attributed to loss of the external protective barrier of the seed coat (exotegmen) caused by the feeding of ants. Bellyache bush seeds were recorded across all soil depths, reaching a peak of 3.8 million seeds per hectare at 1–5 cm soil depth. These results suggest that while bellyache bush seeds are susceptible to fire, many are buried beyond the reach of lethal temperatures. Therefore, viable seeds will be available for post-fire recruitment and other measures, such as chemical control, may need to be employed in conjunction with burning.

Understanding the reproductive biology of Calotropis procera (Aiton) W.T. Aiton, an invasive weed of northern Australia, is critical for development of effective management strategies. Two experiments are reported on. In Experiment 1 seed longevity of C. procera seeds, exposed to different soil type (clay and river loam), pasture cover (present and absent) and burial depth (0, 2.5, 10 and 20 cm) treatments were examined. In Experiment 2 time to reach reproductive maturity was studied. The latter experiment included its sister species, C. gigantea (L.) W.T. Aiton, for comparison and two separate seed lots were tested in 2009 and 2012 to determine if exposure to different environmental conditions would influence persistence. Both seed lots demonstrated a rapid decline in viability over the first 3 months and declined to zero between 15 and 24 months after burial. In Experiment 1, longevity appeared to be most influenced by rainfall patterns and associated soil moisture, burial depth and soil type, but not the level of pasture cover. Experiment 2 showed that both C. procera and C. gigantea plants could flower once they had reached an average height of 85 cm. However, they differed significantly in terms of basal diameter at first flowering with C. gigantea significantly smaller (31 mm) than C. procera (45 mm). On average, C. gigantea flowered earlier (125 days vs 190 days) and set seed earlier (359 days vs 412 days) than C. procera. These results suggest that, under similar conditions to those that prevailed in the present studies, land managers could potentially achieve effective control of patches of C. procera in 2 years if they are able to kill all original plants and treat seedling regrowth frequently enough to prevent it reaching reproductive maturity. This suggested control strategy is based on the proviso that replenishment of the seed bank is not occurring from external sources (e.g. wind and water dispersal).

Understanding the dispersal of weed species is important for the development of effective control strategies. In this study, a series of experiments was conducted to clarify the role that meat ants (Iridomyrmex spadius) play in dispersing bellyache bush (Jatropha gossypiifolia), an exotic shrub currently invading the rangelands of northern Australia. The nutrient composition of food [lipids (fatty acids), fat and soluble carbohydrates] provided by bellyache bush seed components [caruncle, exotegmen and seed (without caruncle and exotegmen)] was identified. Seed components were rich in lipids, particularly palmitic, oleic, stearic, linoleic and eicosenic acids. Oleic and palmitic were most abundant in the caruncle (30% each), linoleic in the seed (61%) and palmitic in the exotegmen (36%). Over all seed components, fat concentration was relatively high (6.3%) compared with soluble carbohydrates (2.3%). The impact of feeding was then determined by comparing germination and viability of intact, non-carunculate (caruncle manually removed) and ant-discarded bellyache bush seeds. Feeding by meat ants significantly increased seed germinability, whilst having no adverse affects on viability. The quantity of seeds dispersed and the seasonal pattern of dispersal was recorded by collecting seed from the middens of randomly selected meat ant nests on a monthly basis. On average, 12 330 ± 603 seeds were retrieved from the middens of individual meat ant nests over 12 months, with highest numbers recorded between February and June (>1200 seeds/ant nest). The effect of this dispersal was determined through comparisons of plant densities within core infestations of bellyache bush, meat ant nest middens and pastures located directly adjacent to core infestations and that were being invaded primarily through localised ballistic dispersal. The density of bellyache bush plants growing from the seed reserves within middens averaged 79 plants/m2, just 18% less than that within core infestations. Seedling survival (1 year) and growth within core infestation and meat ant sites was also quantified. The middens of meat ant nests provided an environment conducive to higher seedling survival and faster growth rates than occurred within core infestations. Mutualistic interaction between bellyache bush and meat ants is likely to build local ‘infestation pressure’ that may be conducive to range extension in years of exceptionally wet seasons. Management of seed dispersal by meat ants may reduce that risk.

No abstract

Ethylene gas stimulated seed germination of the pearl millet [Pennisetum americanum(L.) K. Schum] strain ofStriga hermonthica(Del.) Benth. # STRHE) in two soil types in the North Kordofan Region of Sudan. Soils were the noncracking clay pediplain or ‘Gardud’, in Arabic, and the stabilized sand locally called ‘Qoz’. Ethylene at 1.5 kg ai/ha induced greater seed germination (suicidal seed germination) in the Gardud soil than in the Qoz soil. However, ethylene at 3.0 kg ai/ha was better on Qoz than Gardud soil. The effect of injection depth of ethylene was dependent on soil type. Ethylene was more effective when injected at a 15-cm depth in the Gardud soil and at a 30-cm depth in the Qoz soil. The viable population ofS. hermonthicaseed was reduced by suicidal seed germination at soil depth to 60 cm and to a horizontal distance of 25 cm from the point of ethylene injection. Ethylene-induced germination reduced seed population by 67 and 34% in Gardud and Qoz soils, respectively. A single injection of ethylene diffused up to 300 cm horizontally and up to 60 cm vertically and caused 12.6 and 8.5% reduction in viable 5.hermonthicaseed in Gardud and Qoz soils, respectively. The potential of ethylene gas as an effective tool in striga control operations in a pediplain and stabilized sand soil such as is found in N. Kordofan is dependent upon the availability of the gas and the injection apparatus and the economics of its application.

The parasitic weed striga had significantly greater effects on sorghum yield components than non-parasitic weeds. Grain reduction caused by striga amounted to 65% compared with 32% reduction caused by non-parasitic weeds. Comparable reductions were also manifest in straw yield, grains/head and plant height of sorghum. Early removal of striga (a week after its emergence), and thereafter at regular weekly intervals, may help to improve crop yields.In Sudan, Sorghum bicolor cultivars commonly grow in an agricultural background of mixed non-parasitic weeds. However, the incidence of Striga hermonthica Benth., a parasitic weed of sorghum, has recently become of frequent occurrence under arable conditions. Striga seeds normally germinate when stimulated by root exudates from sorghum or from other (non-host) plants (Andrews, 1947). As a result of a striga attack, yields of grain sorghum can be reduced to 30% or even to zero, especially during seasons of low rainfall (Andrews, 1945;Basinki, 1955;Last 1960a;Kambal, 1979). Thus it is of considerable importance to know the time at which parasitic weeds begin to affect the growth and yields of sorghum. An experiment was therefore conducted to find out, first, how late in the growth of a sorghum/striga mixture the control of striga could be delayed before the parasitic weed produces an irreversible depression of host yield? Second, what degree of crop recovery is possible if the striga population is removed after it has started to reduce the growth of sorghum? And, third, how sorghum yield reduction, caused by non-parasitic weeds, relates to that caused by the parasitic weed striga? MATERIALS AND METHODSSeeds of two sorghum cultivars, Feterita and Safra, chosen because of their susceptibility to striga attack and collected from Gedaref area, were sown in a split-plot with completely randomized block design. The cultivar treatments were allocated at random to the main plots and the weed treatments to subplots, whose dimensions were 4.8 x 5 m. Six sub-plot treatments, replicated four times, included (A) weedy check -affected by both parasitic and nonparasitic weeds, (B) cultivated check -free of both parasitic and non-parasitic weeds, (C) non-parasitic weeds only, (D) striga only, (E) striga removed 4 weeks