The effect of nutrients on pyrrolizidine alkaloids in Senecio plants and their interactions with herbivores and pathogens

The aim of this review is to combine the knowledge of studies on effects of nutrients on pyrrolizidine alkaloids (PAs) in Senecio with those studies of effects of PAs on herbivores and pathogens in order to predict the effects that nutrients may have on herbivores and pathogens via changes in PAs. We discuss whether these predictions match with the outcome of studies where the effect of nutrients on herbivores and insects were measured. PA concentrations in S. jacobaea, S. vulgaris and S. aquaticus were mostly reduced by NPK fertilization, with genotype-specific effects occurring. Plant organs varied in their response to increased fertilization; PA concentrations in flowers remained constant, while shoot and roots were mostly negatively affected. Biomass change is probably largely responsible for the change in concentrations. Nutrients affect both the variety and the levels of PAs in the plant. The reduced PA concentrations after NPK fertilization was expected to benefit herbivores, but no or negative responses from insect herbivores were observed. Apparently other changes in the plant after fertilization are overriding the effect of PAs. Pathogens do seem to benefit from the lower PA concentrations after fertilization; they were more detrimental to fertilized plants than to unfertilized control plants. Future studies should include the effect of each element of nutrients separately and in combinations in order to gain more insight in the effect of specific nutrients on PA content in Senecio plants

Managing saffron thistle in pastures with strategic grazing

Saffron thistle ('Carthamus lanatus' L.) is a widespread and troublesome weed in grazing land throughout much of eastern Australia. Conventional control measures are not feasible in much of this area, so the prospect for controlling saffron thistle through strategic grazing with sheep was investigated.By sowing seeds, and counting seedlings that emerged, we found that as little as 2 cm of pasture cover reduced the number of seedlings that emerged by 96% compared with bare ground. Grazing experiments showed that the survival of saffron thistle rosettes was reduced by 12% in plots that were strategically rested for one month in autumn, followed by grazing at high stocking rates. Grazing in late spring, when plants produced flowering stems, killed 20% of thistles before they set seed. A comparison of thistle density and pasture composition on farms that practice strategic grazing with neighbouring farms that used continuous grazing supported these results. Pastures had few thistles when perennial grasses accounted for over 35% of groundcover. We conclude that grazing management can help control saffron thistle

The Effect of Row Spacing and Weed Density on Yield Loss of Chickpea

The adoption of no-till farming and the desire to maintain stubble cover when sowing legumes in northern New South Wales and southern Queensland have resulted in an increase in commercial row spacing for chickpea ('Cicer arietinum' L.). This paper examines the effects of increasing crop row widths on weed competition in chickpea crops. Weed densities of 0, 2, 4, 8, 16, and 32 plants/m² of wild oats ('Avena sterilis' ssp. 'ludoviciana') and turnip weed ('Rapistrum rugosum') were established with chickpea crops planted with either 32 or 64 cm row configurations in northern New South Wales during 1996 and 1997. A rectangular hyperbolic model adequately represented the loss in chickpea yield with increasing density of either weed. Even low densities o

Can hybridization cause local extinction: a case for demographic swamping of the Australian native Senecio pinnatifolius by the invasive Senecio madagascariensis?

The outcome of interspecific hybridization between native and invasive species depends on the relative frequencies of parental taxa and viability of hybrid progeny. We investigated individual and population level consequences of hybridization between the Australian native, Senecio pinnatifolius, and the exotic S. madagascariensis, with AFLP markers and used this information to simulate the expected outcome of hybridization.A high frequency (range 8.3-75.6 %) of hybrids was detected in open pollinated seeds of both species, but mature hybrids were absent from sympatric populations indicating that sympatric populations represent tension zones. A hybridization advantage was observed for S. madagascariensis,where significantly more progeny than expected were sired based on proportional representation of the two species in sympatric populations. Simulations indicated S. pinnatifolius could be replaced in sympatric populations if hybridization was density dependent.For this native-exotic pair, prezygotic isolating barriers are weak, but low hybrid viability maintains a strong postzygotic barrier to introgression. Due to asymmetric hybridization, S. pinnatifolius appears under threat from demographic swamping, and local extinction is possible where it occurs in sympatry with S. madagascariensis

Assessing the efficacy and impact of management of an invasive species in a protected area: Poa annua on sub-Antarctic Macquarie Island

Plant eradication is difficult, particularly in remote, protected areas. The Southern Ocean Islands are very isolated and highly protected, but the flora contains many alien plants. Small restricted populations have been eradicated, but eradication of established species has proven difficult. A better understanding of the efficacy of control methods at sub-Antarctic temperatures and their off-target impacts may increase eradication success. With interest in controlling non-native Poa annua in the region, we aimed to determine if physical and chemical methods can control P.\ua0annua (the sub-Antarctic biotype) in sub-Antarctic conditions and examined their impact on native plants. We quantified the effectiveness of physical control methods on P.\ua0annua in\ua0situ on sub-Antarctic Macquarie Island through field-based experiments and assessed their selectivity on P.\ua0annua compared with native grasses. We also quantified the effectiveness of several herbicides on P.\ua0annua at sub-Antarctic temperatures and assessed their selectivity on native grasses. Of the four physical disturbance methods tested, none effectively reduced P.\ua0annua cover as one-off treatments. Of the herbicide treatments, glyphosate and trifloxysulfuron sodium were effective and were less damaging to native grass species, indicating potential selectivity. Physical control was of limited effectiveness, but did not affect native species richness. An integrated weed management programme utilising the strategic use of selective herbicides with follow-up chemical and physical intervention may balance control and biodiversity outcomes. This research highlights the importance of site-specific testing of control methods and understanding off-target impacts of control when managing alien plant species in protected areas