Macadamia ('Macadamia integrifolia' Maiden and Betche, 'M. tetraphylla' Johnson and hybrids) orchards in Australia are typically hedged around anthesis (September). Such hedging reduces yields, largely through competition for carbohydrates between early fruit set and the post-pruning vegetative flush, but also through a reduction in photosynthetic capacity caused by the loss of canopy. We examined whether hedging at other times might mitigate yield losses. Hedging time was found to affect yields across four cultivars: 'A4', 'A38', '344' and '816'. Yield losses were lower for trees hedged in November-December than for trees hedged in September. Yields for trees hedged in June were higher than for trees hedged in September in one experiment, but were similar in a second experiment. Yield losses for September and October hedging were similar. Hedging time changed the pattern of fluctuations in stem water-soluble carbohydrates (WSC). WSC declined shortly after hedging in September, October or November, and the declines preceded increases in fruit abscission relative to unpruned control trees. The increase in fruit abscission was less pronounced for the trees hedged in November, consistent with the idea that fruit become less sensitive to carbon limitation as they mature.

An experiment aimed to examine the effect of aqueous extracts of three weed species on soybean roottip- cell mitosis has been carried out at the Laboratory of Plant Physiology and Biotechnology. Department of Agronomy and Soil Science, University of New England, Australia. Aqueous extracts of fresh weed material ('Amaranthus Cyperus rotundus' and 'Paspalum dilatation') at the concentration of 25% ( w/v) were added to the germination medium of soybean seeds cultivar Banjalong, Melrose, and Valiant. The germination was conducted at a growth cabinet with 25°C and total darkness. All experimental units were arranged in a completely randomized design with 5 replication. Radicles (roots) from the germinating soybeans were harvested when they were about 1 – 1.5 cm long. The soybean root tips were used to observe the mitotic index following a series of hydrolysis and staining process. Results indicate that aqueous extracts of the weeds reduced cell division of the soybean regardless of the cultivars. This contributes to the understanding of the modes of action of the allelopathic effect of the weeds. 'Amaranthus powellii' had the most inhibitory effect to the soybean cell division.

Yields of macadamia ('Macadamia integrifolia', 'M. tetraphylla', and hybrids) orchards tend to increase with increasing tree size up to ≈94% light interception. Beyond this, there is some indication that crowding leads to yield decline, but the evidence is limited to one site. Increasing tree size and orchard crowding also present numerous management problems, including soil erosion, harvest delays, and increased pest and disease pressure. The aim of this study was to better characterize long-term yield trends in mature orchards and to assess the effects of manual and mechanical pruning strategies on yield, nut characteristics, tree size, and economics. We monitored yield at four sites in mature '344' and '246' orchards for up to seven years and confirmed a decline in yield with crowding for three of the sites. There was a small increase in yield over time at the fourth site, which may reflect the lower initial level of crowding and shorter monitoring period compared with the other sites, and highlights the need for long-term records to establish yield trends. Pruning to remove several large limbs from '246' trees to improve light penetration into the canopy increased yield relative to control trees but the effect was short-lived and not cost-effective. Removal of a codominant leader from '344' trees reduced yield by 21%. Annual side-hedging of '246' trees reduced yield by 12% and mechanical topping of '344' trees caused a substantial reduction in yield of up to 50%. Removal of limbs in the upper canopy to reduce the height of '344' trees had less effect on yield than topping but re-pruning was not practical because of the extensive regrowth around the pruning cuts. Tree size control is necessary for efficient orchard management, but in this study, pruning strategies that controlled tree size also reduced yield. Research into the physiological response to pruning in macadamia is required to improve outcomes.

Vietnam is one of the largest rice-exporting countries, and therefore a large amount of rice husk and rice straw is produced annually. To manage residues after harvesting, Vietnamese farmers commonly burn rice residues in the field which emits large quantities of gaseous and particulate pollution to the atmosphere and has a negative impact on the climate and the health of the population. In the last decade, using biomass to make biochar for application to cropland has received growing attention as a possible strategy to mitigate climate change by sequestering carbon from the atmosphere and suppressing soil greenhouse gas (GHG) emissions. Biochar can be produced from various biomass sources including crop residues, and at various scales from large industrial facilities, village scale and even at the household level using small-scale pyrolysis technologies.

This thesis investigated the climate change, human health (particulate matter and human toxicity) and economic impacts of biochar production from rice residues for addition to paddy soils compared with the conventional practice of open burning of residues. Life Cycle Assessment (LCA), a methodology that aims to assess impacts of products, processes, or services from “cradle to grave”, was employed to evaluate the environmental and health effects of alternative uses of biomass in rice growing systems in northern Vietnam. For this purpose, different studies related to crop residue management were defined and, for each study, the biochar system and a comparison reference system were modelled. The biochar produced in all studies was assumed to be returned to paddy rice fields from where the biomass was harvested. In study one, the carbon footprints (CF) of two rice production systems were calculated: one scenario in which rice residues are burned and another scenario where these are converted into biochar and incorporated into soil. The functional unit (FU) was the production of 1 kg of milled rice. It was assumed that households used pyrolytic cookstoves and drum ovens to produce biochar. Based on a literature review, I assumed that the agronomic effects of biochar compounds with increasing biochar application until reaching maximum benefits at 18 Mg ha^-1. This amount of biochar would take eight years to be produced in pyrolytic cook-stoves and drum ovens using the rice husk and straw available. Biochar addition reduced the CF of spring rice and summer rice by 49% and 38% respectively, compared with rice produced with conventional residue disposal, after eight years of biochar addition.

Study two assessed the CF of two different biochar production systems: one scenario in which rice straw-derived biochar in raw form was applied to the paddy fields, and a second scenario using enriched biochar (biochar made from rice straw enriched with lime, clay, ash and manure). In this study, the management of 1 Mg of dry rice straw was chosen as the FU. Applying enriched biochar showed an increase in GHG emissions abatement by 126% and 309% in spring and summer seasons, compared with using rice straw-derived biochar. This was mostly due to greater reduction of soil CH₄ emissions by enriched biochar, because of the larger area treated with enriched biochar at a lower application rate.

Study three involved a comparative analysis of the climate change and health impacts of various biochar-compost (COMBI) systems relative to the conventional practice of open burning of rice husks. Three COMBI systems, using different pyrolysis technologies (pyrolytic cook-stove, brick kiln and the BigChar 2200 unit) for the conversion of rice husk into biochar were considered. In this study, the FU was the management of 1 Mg of dry rice husk. All biochar systems substantially improved environmental and health impacts of rice husk management compared with the open burning of rice husks. The differences between the three COMBI systems in the climate change and particulate matter impacts were not significant, due partially to large uncertainties. The lowest human toxicity effect was offered by the BigChar 2200 system, where biochar is produced in a large-scale plant in which pyrolysis gases are used to generate heat energy. This result highlights the significance of pyrolysis gas recycling for sustainable biochar production.

At the household or village level, economic benefits are a key factor that drives producers to adopt a new agricultural technology. The scenarios modelled in study one were used to assess the costs and benefits, and non-renewable energy use of rice production. After eight years of biochar application, the net present value of rice was enhanced by 12% and the energy intensity decreased by 27%, compared with rice production with conventional residue management. The existence of a carbon market that recognises the reduction of soil GHG emissions and carbon sequestration due to the land application of biochar could considerably raise the profitability of rice production.

These results indicate that ceasing the open burning of residue in the field and using residues to produce biochar for addition to soils can provide important benefits in climate change mitigation, human health and economic returns in rice cropping systems in Vietnam. This conclusion relied on several uncertain assumptions, including the effect of biochar on CH₄ emissions from soil. The assumed suppression of soil CH₄ emissions is a major contributor to the reduced climate effects for the biochar systems, and sensitivity analysis showed that the CF of biochar systems was highly sensitive to any variation in this factor. Therefore the soil impacts of biochar need to be confirmed by further research to enable more accurate quantification of the climate effects of biochar use in rice production.

Sturt's desert pea ('Swainsona formosa') is an Australian native legume used as an ornamental pot plant and is also suitable for hanging baskets and as a cut flower. One of the impediments to the commercialisation of S. 'formosa' as a pot plant is its inability to produce flower under low light conditions. The work reported here investigated the effect of high light intensity (800 ± 50 μmol m⁻²s⁻¹) and low light intensity (150 ± 10 μmol m⁻²s⁻¹) on the flowering of 'S. formosa' with particular emphasis on the changes of glucose concentration in shoot apices from the vegetative to floral stages. Plants grown under high light initiated flowers within 45 days from seed germination, while plants grown under low light intensity remained vegetative and produced no flowers during the 60 days of experimental period. Using HPLC, trace amounts (0.52 mg g⁻​¹) of glucose were detected under high light intensity at the beginning of transition from vegetative to floral stage (40 days after germination) which increased to 2.70 mg g⁻​¹ with progressive floral development. No glucose was detected in shoot apices of plants grown under low light during the experimental period. Results indicate that adequate glucose production may be required for floral initiation and development in 'S. formosa'.

Okra and tomatoes are major vegetable crops commonly grown under irrigation, and understanding whether they respond to salinity by withstanding (tissue tolerance) or avoiding (salt exclusion) accumulation of salt in the shoots will assist with management for optimizing yield under declining soil and water resources. Both crops were grown in non-saline (0.0 dS/m) and saline (3.0 dS/m) loamy sand and drip irrigated with water of 0.0, 1.2 or 2.4 dS/m. Differences in the growth and yields of the two crops under saline conditions were associated with uptake and distribution of cations, especially Na. The tomato employed tissue tolerance mechanism in response to salinity and produced fruits even when shoot/root Na concentration was >3.0; concentrations of Na in tomato tissues was in the order shoots > roots ≈ fruits. Okra was sensitive to shoot Na such that a shoot/root Na concentration as low as 0.13 reduced yield by as much as 35%; this crop employed salt exclusion mechanism and minimized shoot accumulation of Na, which was distributed in the order fruits > roots > shoots. Root and shoot concentrations of Na, P and S were correlated with flower abortion and negatively correlated with yield and yield components in both crops. Fresh fruit produced on the saline soil were reduced by 19% in tomato compared with 59% in okra, relative to yields on non-saline soil. Water salinity reduced fresh fruit yields in the tomato by as much as 36% with every unit (dS/m) rise in water salinity compared with 27% in okra. Soil salinity significantly reduced water-use by 6% in tomatoes and 29% in okra, but had no impact on water use efficiency (WUE) that averaged 3.9 g of fresh fruits/L for tomatoes and 1.75 g/L for okra. Every 1.0 dS/m rise in water salinity reduced water-use by 0.33 L in okra and 3.31 L in tomatoes, and reduced WUE by 2.61 g/L in tomatoes and 0.53 g/L in okra. Soil salinity explained <5% of the variance in yields in tomatoes and 10-20% in okra, while water salinity explained 48-68% of the variance in tomatoes and about 40% in okra. We conclude that (1) water salinity was more injurious to yield in both crops than soil salinity, and (2) yield losses due to salinity can be minimized through frequent leaching of soil salt under okra and increased irrigation intervals in tomatoes.

In Australian macadamia ('Macadamia integrifolia' Maiden and Betche, 'M. tetraphylla Johnson' and hybrids) orchards, trees are generally side-hedged in early spring to maintain the inter-row for efficient orchard management. Hedging at this time increases fruit abscission and decreases yield due to competition for carbohydrates between fruit and post-pruning shoot growth, and to the loss of photosynthetic area. Girdling increases fruit set in many crops and the possibility that girdling might mitigate the effect of pruning on fruit abscission and yield was investigated in this study. Girdling was also investigated for its potential to control shoot growth and tree size. In the first experiment, in early spring around anthesis, trees were pruned (P), pruned and girdled (PG), or were left as unpruned and ungirdled control trees. Fruit per raceme over time, percent of racemes with fruit at 20 weeks post anthesis, fruit number per tree, nut weight, yield, and the shoot length and number of the post-pruning flush were measured. In a second experiment, trees were pruned (P) or pruned and girdled (PG) in autumn, and the shoot length and number of the post-pruning flush were measured. In a third experiment, trees were girdled at anthesis in 3 years over a 4-year period and shoot growth, tree height, yield, fruit number and nut weight were compared with ungirdled control trees. Girdling mitigated the effect of pruning on fruit abscission and yield but the mitigation was slight with PG trees producing 28% less yield than unpruned control trees. Girdling reduced shoot length and number but more so on unpruned branches (26% and 28%, respectively) than on pruned branches (10% and 14%, respectively). The tree height increment in girdled trees was around half that of ungirdled trees. In the 4-year study, girdling increased yield by around 10% in two out of four seasons, and decreased it in one season. Cumulative yields were similar for girdled and ungirdled trees. This study has shown that girdling was not useful in mitigating the effect of pruning on yield. However girdling was effective in controlling shoot growth and tree size, with no reduction in cumulative yield, and may be useful as a tree size control strategy. Further monitoring of long-term effects of repeat girdling is required to confirm this.

Crop residue management (RM) plays an important role in maintaining soil organic carbon (SOC) in horticulture, especially where annual crop rotations rely on frequent tillage. A trial investigating the short-term effects of sweet corn ('Zea mays' L. var. 'rugosa') residue incorporation on crop yields in a corn-cabbage ('Brassica oleracea' L.) rotation using organic (Org) and conventional (Conv) soil management systems (SMS) was established on 14 December 2009 in two contrasting soil types (Vertosol and Chromosol). The effect of mulched corn residue incorporation on weed biomass production was also studied. Corn was grown under the two SMS and residue was retained (+RES) or removed (-RES) after harvest on 23 April 2010. Cabbage was then grown from 4 May to 14 October 2010, under the same SMS in a three-way factorial design (SMS x RM x soil type). In both systems, equal quantities of macro-nutrients were supplied. Crop yields and weed biomass and apparent electrical conductivity (ECa) of soil were measured. There was no significant difference in total corn biomass for SMS or soil type. However, cabbage yield was significantly greater at the Chromosol site. The SMS x RM x soil type interaction was significant for weed biomass in cabbage, with Org having less weed biomass at the Vertosol site, especially in -RES. The +RES treatment had reduced weed biomass by 20 and 64% in conventional and organic SMS, respectively, in comparison to -RES in Chromosol. Soil ECa was significantly different for soil type only. The reduction of weed biomass in +RES treatment could be attributed to the mulching effect of the incorporated corn residue, the differences in weed seed bank and drainage between two sites. In conclusion, crop yields and soil ECa were not influenced by SMS or RM in short-term, but incorporation of residue in soil reduced weed biomass.

Swainsona formosa (G.Don) J. Thompson (Sturt's desert pea) is an Australian native legume which is used as an ornamental pot plant. It is also suitable for hanging baskets and cut flowers. One of the Impediments to the commercialisation of S. formosa as a pot plant is its inability to produce flowers under light conditions. The possibility of using exogenous sugar application as a replacement for high light intensity conditions is considered with the objective of evaluating the effect of exogenous sugar application on the in vitro flowering of S. formosa. Media containing different sugars (sucrose. fructose and glucose) and concentrations (1.5, 3.0. 4.5 or 6.0% w/v) were used in both high and low light.

Conventional soil management systems (SMS) use synthetic inputs to maximize crop productivity, which leads to environmental degradation. Organic SMS is an alternative that is claimed to prevent or mitigate such negative environmental impacts. Vegetable production systems rely on frequent tillage to prepare beds and manage weeds, and are also characterized by little crop residue input. The use of crop residues and organic fertilizers may counteract the negative impacts of intensive vegetable production. To test this hypothesis, we evaluated the effect of sweet corn ('Zea mays' L. var. 'rugosa') residue incorporation in a corn-cabbage ('Brassica oleracea' L.) rotation on crop yields, nutrient uptake, weed biomass and soil nutrients for organic and conventional SMS in two contrasting soil types (a Chromosol and a Vertosol). Yields of corn and cabbage under the organic SMS were not lower than the conventional SMS, possibly due to the equivalent N, P and K nutrients applied. Macro-nutrient uptake between the organic and conventional SMS did not differ for cabbage heads. Corn residue incorporation reduced the average in-crop weed biomass in cabbage crops by 22% in 2010 and by 47% in 2011. Corn residue-induced inhibitions on weed biomass may be exploited as a supplementary tool to mechanical weed control for the organic SMS, potentially reducing the negative impacts of cultivation on soil organic carbon. Residue incorporation and the organic SMS increased the average total soil N by 7 and 4% compared with the treatments without residue and the conventional SMS, respectively, indicating the longer-term fertility gains of these treatments. Exchangeable K, but not Colwell P, in the soil was significantly increased by residue incorporation. The clayey Vertosol conserved higher levels of nutrients than the sandy Chromosol. Yields under organic SMS can match that of conventional SMS. Residue incorporation in soil improved soil nutrients and reduced weed biomass.