Storage root formation of sweetpotato (Ipomoea batatas (L.) Lam) is a complex developmental process relating to the activity of cambium. Little information is available on the relationship between nitrogen (N) application levels and the initiation and development of sweetpotato storage roots (SRs). This study aims to examine how N application rates promoted/inhibited the formation and development of storage roots (SRs) for sweetpotato cultivar ‘Orleans’ during the first 8 weeks after planting. Cuttings were grown in coarse river sand culture supplied with modified Hoagland nutrient solution at four different rates (0 (N0), 50 (N50), 100 (N100) or 200 (N200) mg L−1) of N. The results showed that N100 treatment promoted the formation of primary and secondary cambium, resulting in a significant higher rate of SR formation between 21 and 56 days after transplanting (DAT). Due to the higher N demand after formation of SRs, N200 treatment displayed faster growth, higher N acquisition and the highest efficiency of N use after 35 DAT, but the SR formation rate and SR number per plant remained insignificantly lower than N100 when differentiation of adventitious root was mostly completed (49 DAT), suggesting irreversible an effect of N rate during SR initiation, which eventually affects SR number. The results suggested that the optimal substrate N level for sweetpotato SR initiation is lower than that for following SR growth, which should be considered in the fertilisation scheme.

A key driver of Australian sweetpotato productivity improvements and consumer demand has been industry adoption of disease-free planting material systems. On a farm isolated from main Australian sweetpotato areas, virus-free germplasm is annually multiplied, with subsequent 'pathogen-tested' (PT) sweetpotato roots shipped to commercial Australian sweetpotato growers. They in turn plant their PT roots into specially designated plant beds, commencing in late winter. From these beds, they cut sprouts as the basis for their commercial fields. Along with other intense agronomic practices, this system enables Australian producers to achieve world's highest commercial yields (per hectare) of premium sweetpotatoes. Their industry organisation, ASPG (Australian Sweetpotato Growers Inc.), has identified productivity of mother plant beds as a key driver of crop performance. Growers and scientists are currently collaborating to investigate issues such as catastrophic plant beds losses; optimisation of irrigation and nutrient addition; rapidity and uniformity of initial plant bed harvests; optimal plant bed harvest techniques; virus re-infection of plant beds; and practical longevity of plant beds. A survey of 50 sweetpotato growers in Queensland and New South Wales identified a substantial diversity in current plant bed systems, apparently influenced by growing district, scale of operation, time of planting, and machinery/labour availability. Growers identified key areas for plant bed research as: optimising the size and grading specifications of PT roots supplied for the plant beds; change in sprout density, vigour and performance through sequential cuttings of the plant bed; optimal height above ground level to cut sprouts to maximise commercial crop and plant bed performance; and use of structures and soil amendments in plant bed systems. Our ongoing multi-disciplinary research program integrates detailed agronomic experiments, grower adaptive learning sites, product quality and consumer research, to enhance industry capacity for inspired innovation and commercial, sustainable practice change.

The Australian Sweetpotato Growers Association partnered researchers from Agri-Science Queensland (with co-funding from Horticulture Australia Limited) to identify new, high performing sweetpotato cultivars with diverse colours and tastes. The project evaluated a mix of purple, red, orange and white skin and flesh, tailored for Australian growers and consumers. Australia's sweetpotato market currently relies on one gold cultivar for 90% of national production. Major retailers were requesting a reliable supply of quality sweetpotatoes in emerging categories such as red or white skin or purple flesh. To identify suitable cultivars, over 40 new sweetpotato cultivars were virus tested, and extensively evaluated in multiple experiments in Queensland and northern New South Wales. Larger-scale plantings by growers, using standard agronomy, provided additional performance feedback under commercial conditions. In partnership with growers and wholesalers, cultivars were evaluated in field and laboratory for desired characteristics such as shape, size range, skin and flesh colour, estimates of productivity and suitability for commercial production, cooking characteristics and taste. New high performing gold cultivars had better soil insect and nematode tolerance than the current cultivars. The new colours offered diverse health-related opportunities for consumers, more anthocyanins in purple-fleshed cultivars; higher beta carotene content in new gold fleshed cultivars; and potentially lower GI in white-fleshed cultivars. To enhance adoption, the industry/research partnership will tailor agronomic guidelines to maximise on-farm performance and identify niche marketing pathways for each of the cultivars. Increased consumption of this versatile vegetable will drive sweetpotato industry development and expansion into the future.

In Australia, sweetpotato production has grown remarkably (1700%) in the last 16 years. Growers currently market 75 000 t per annum, worth $ 80-90 million at farm gate. The orange-fleshed cultivars are the most familiar to consumers, but other cultivars with varying flesh colour and properties also have potential for the consumer market. Given that Australian sweetpotato growers desire alternative cultivars to promote market demand, it is important to articulate the characteristics of sweetpotatoes that are most and least desirable for consumers. Research indicates that consumer acceptability of the new cultivar 'Evangeline' may assist sweetpotato growers and marketers in understanding the impact of both sensory properties, such as colour and the importance of flavour and texture of sweetpotatoes, and an awareness of the potential health benefits of sweetpotato consumption. In addition, whilst consumer preferences (regarding size, colour, texture, skin tone) and nutritional knowledge of sweet potato (regarding glycaemic index) is increasing, there is limited research investigating consumers understanding of health messages of sweetpotato attributes. This industry and consumer research review highlights the potential for promoting innovative strategies to improve adoption of new cultivars in the marketplace.

Australian sweetpotato production has grown remarkably (1700%) in the last 16 years. Growers currently market 75000 t per annum, worth $80-90 million at farm gate. Gold-fleshed dessert types dominate (90% of total production), almost exclusively cultivar 'Beauregard', bred at Louisiana State University Agricultural Center (LSU AgCenter) in the USA. Australian sweetpotato growers desire alternative cultivars, to reduce risks associated with relying on one genotype. They also wish to expand demand, by offering diverse products. Recent research identified 'Evangeline', another LSU AgCenter cultivar, as an alternate gold sweetpotato possessing attributes desired by consumers (regular, smooth shape; highly coloured skin and flesh). In experimental and grower evaluations across key Australian growing regions in Queensland and New South Wales, 'Evangeline' produced marketable yields similar to 'Beauregard'. 'Evangeline' had a high proportion of premium small-medium sweetpotatoes. In sandy locations, 'Evangeline' also demonstrated superior root-knot nematode resistance to 'Beauregard'. However, compared to ` Beauregard', 'Evangeline' had greater risks of splitting, or over-purple skin colouration, at harvest, particularly when dug in Winter/Spring. Initial evidence suggested splitting was more common with increased fertiliser nitrogen inputs. Split roots or off-specification colours are unmarketable; the associated risks are currently substantial impediments to adoption of 'Evangeline' by Australian sweetpotato growers. Scientists from AgriScience Queensland and Central Queensland University are currently partnering with Australian Sweetpotato Growers (Inc.) to develop strategies for maximising performance and mitigating risks of growing 'Evangeline' in Australian conditions. This will enhance industry profitability and resilience; improve understanding of sweetpotato physiology; and increase diversity and quality of sweetpotato products available to consumers.

Ongoing uncertainty in irrigation water supply is a problem facing many of Australia's horticultural producers, whether it is due to drought, groundwater depletion, increased water regulation or community expectation for natural resource conservation. To respond to these pressures, improved irrigation management options are required. Simulation analyses provide the ability to explore the impacts of management options under these changing conditions but to date appropriate tools have not been available for horticulture. APSIM is a component-based modelling environment that has a long history of usage in crop, pasture and forestry systems analysis. Horticultural crop models are now being included into the suite of crop models available within APSIM. APSIM-Broccoli is one such model.APSIM-Broccoli calculates plant growth, development and water use on a daily time step. Predictions of phenological development emerge from calculations of various growth processes. For example, time to floral initiation is calculated from thermal time adjusted for accumulated vernalisation from germination or transplanting, and time to buttoning is dependant upon the thermal time required for the appearance of all leaves initiated prior to floral initiation. Photosynthesis is calculated using a light use efficiency which is affected by temperature, water and nitrogen stresses. A simple phytomer approach is used for canopy development where each successive leaf on the main stem is defined in terms of the length of its growth, lag and senescent phases. Assimilate is partitioned to individual leaves based upon daily growth rates determined by temperature-dependant leaf expansion processes. Canopy water demand is calculated using a Penman-Monteith formulation within the APSIM Micromet module (Snow and Huth, 2004). Extraction of soil moisture to satisfy this demand is calculated using the approach of Meinke et al. (1993).Testing of the model was carried out using two datasets chosen to highlight different areas within the model. The data of Tan et al (2000) includes leaf and phenological observations for several cultivars over a broad range of sowing dates for two locations in South East Queensland. APSIM-Broccoli was able to describe observed crop responses in canopy development and floral initiation to climatic conditions. Data for Broccoli growth and water use from the Gatton Research Station (27.55 degrees S, 152.33 degrees E) has been used to test the capability of the model in simulating biomass production and yield across different seasonal condition and irrigation strategies.Other horticultural crop models are currently under development. These include sweet corn, green bean, lettuce and potato. Once complete, these models will be used to explore management decisions at the field and farm level for landholders facing complex irrigation management decisions.

We investigated potential post-emergence herbicides for managing broadleaf weeds in broccoli, Chinese cabbage, cabbage, and cauliflower, as no products are currently registered for these uses in Australia. Subsequent to spraying clopyralid, picloram, or pyridate 5 weeks after direct-sowing broccoli or Chinese cabbage, or transplanting cabbage or cauliflower seedlings, we recorded crop phytotoxicity symptoms, measured marketable crop yields, and assessed weed control achieved. Neither maximum application rates of 90 g clopyralid/ha, nor 45 g clopyralid/ ha mixed with 30 g picloram/ha, adversely affected vegetable yields. Spraying 60 g/ha clopyralid controlled burr medic (Medicago polymorpha), and suppressed common sowthistle (Sonchus oleraceus). Applying 90 g clopyralid/ ha, or mixing 22.5 g clopyralid/ ha with 15 g picloram/ha, controlled both burr medic and common sowthistle. At the rates tested, neither clopyralid nor picloram affected deadnettle (Lamium amplexicaule). Applying 450 g pyridate/ha caused chlorotic spotting of the sprayed vegetable leaves, but did not affect marketable yields of broccoli, cabbage or cauliflower. This rate controlled deadnettle, reduced sowthistle growth by only 30-50% compared with an unweeded control, and had no impact on burr medic. Spraying 900 g pyridate/ha increased the severity and persistence of chlorotic spotting, and resulted in lower broccoli and Chinese cabbage yields than obtained in the best treatments in the respective experiments. Cabbage and cauliflower yields were unaffected by spraying 900 g pyridate/ha. This rate improved sowthistle control to a commercially acceptable level. Our studies suggest that both clopyralid and pyridate could be successfully utilised in Australian vegetable brassica production, providing issues of the residual activity of clopyralid on following crops, and optimal application rates and timing for pyridate, were resolved.

The phytotoxicity to green beans (Phaseolus vulgaris) of the herbicides metolachlor, pendimethalin, cyanazine, acifluorfen, diflufenican, bentazone, metribuzin, prometryn, terbutryn, methabenzthiazuron, and oxyfluorfen was investigated in 4 experiments on a black earth soil (clay content 40-60%) at Gatton Research Station in southern Queensland during 1990-91. Metolachlor was applied post-sowing and pre-emergence; up to 4 kg a.i./ha did not significantly (P>0.05) affect growth or yields, indicating a considerable safety margin for this herbicide when used at commercial rates. Pendimethalin did not cause significant crop damage when applied in the same manner at rates up to 2.7 kg a.i./ha. Acifluorfen and diflufenican were each applied at 3 or 4 weeks after sowing in 3 experiments. Sensitivity of the bean crop to acifluorfen varied: 0.5 kg a.i./ha did not significantly reduce bean growth or yield in 2 experiments, but 0.11 kg a.i./ha reduced yields by 20% in a third experiment. Application of 0.1-0.12 kg a.i./ha of diflufenican had no adverse effect on beans in 2 experiments, although significant damage was observed in an initial screening experiment. Bentazone applied 3 weeks after sowing had no significant effect on bean yield or growth in 1 experiment; in another, the maximum label rate of 0.96 kg a.i./ha significantly reduced bean growth and yield. Post-emergence application of cyanazine, metribuzin, prometryn, terbutryn, methabenzthiazuron, or oxyfluorfen at rates required for acceptable weed control either killed the bean plants within a few days or resulted in complete yield loss. Levels of damage from these herbicides preclude their use in green beans. Although green beans showed some tolerance to postsowing, pre-emergence application of cyanazine, low rates of 0.75-1 kg a.i./ha reduced yields by 35%. Both metolachlor and pendimethalin appear suitable for pre-emergence use in green beans. Further work on factors affecting phytotoxicity of acifluorfen, diflufenican, and bentazone to green beans is required.

The phytotoxicity of the pre-emergence herbicides metolachlor, pendimethalin, and propachlor to heading lettuce was investigated in 5 experiments conducted in southern Queensland during 1990-91.Metolachlor was applied before transplanting lettuce seedlings in 2 experiments. On a krasnozem soil at Toowoomba, spraying >1.75 kg a.i./ha reduced lettuce growth and the number of marketable lettuce heads. When applied on a black earth at Gatton, 1.44 kg a.i./ha stunted the lettuce and reduced the number of marketable heads by 30% compared with hand-weeded treatments. On both soils, 2.2-2.9 kg a.i./ha of metolachlor is recommended for broadleaf weed control in other crops. The risks of phytotoxicity from such rates preclude its use in transplanted lettuce in this environment.Lettuce were severely stunted from application of 1 kg a.i./ha of pendimethalin after transplanting. Spraying at the same rate before transplanting initially reduced lettuce width in 1 of 4 experiments, but the lettuce recovered. Lettuce yields from areas treated with 1 kg a.i./ha before transplanting were unaffected. The number and size of harvested heads tended to decline where >1.3 kg a.i./ha was applied. Higher rates (up to 1.5 kg a.i./ha) may be acceptable on krasnozem soils. Use of pendimethalin at 1-1.3 kg a.i./ha would control or suppress many important broadleaf weeds in lettuce, as well as a range of grasses.Propachlor was less phytotoxic when applied immediately after transplanting the lettuce seedlings than when sprayed before transplanting. Yields were unaffected from application of 2-2.5 kg a.i./ha after transplanting on black earth and sandy soils. When sprayed before transplanting, >2 kg a.i./ha caused significant yield reductions in several experiments, although lettuce grown on a krasnozem soil appeared more tolerant. Use of 2 kg a.i./ha of propachlor would give a suppressive effect against several broadleaf weeds, but another herbicide would also be required to achieve effective weed control.A spraying strategy involving pendimethalin and propachlor herbicides in transplanted lettuce has the potential to reduce weed control costs by up to 80%.

The effects of soil compaction and deep ripping on the growth and yield of crop (wheat, barley, oats, triticale, narrow leaf lupins and field peas) and pasture species (barrel medic and subterranean clover) were investigated for deep, sandy soils near Geraldton, Western Australia.In 1984 (an average rainfall season), growth and yields of all species were substantially reduced by soil compaction. Lupins were not included in the experiment. Barley, wheat and pea yields were reduced by around 45%; oat and triticale yields by 30%; and spring biomass of both pasture species was reduced by about 30%. The differences in response between the species were not significant (P > 0.05).In 1985 (a very dry year), amelioration of soil compaction by deep ripping increased the dry matter at flowering of all the species, including lupins, by about 30%. Severe water stress in the cereals after flowering prevented the conversion of this dry matter advantage into grain yield differences. Because of late flowering and slow maturation, the yield of the oats was significantly (P < 0.05) lower on the deep-ripped areas, compared with the yield on the compacted soil. Deep ripping increased the yield of peas by 60% and lupins by 20%, probably because flowering and seed filling were completed before the onset of severe water stress.For deep, sandy soils in mediterranean environments, maximum economic benefit would normally accrue from deep ripping prior to the cereal phase, given that current agronomic principles generally preclude the growing of peas on these soils. Selection of cereal species mainly depends on relative yields and prices; however, the results suggest that use of cultivars with rapid early growth, early flowering and quick maturity would maximise the response to compaction amelioration and minimise the risk of poor grain filling.

The relationships between soil penetration resistance and the growth and yield of wheat were examined for a range of tillage and compaction experiments conducted on earthy sands near Geraldton, W.A. Overall, a single index of penetration resistance explained around 50% of the growth and yield variation, across sites and seasons. Equations using this index showed good potential for predicting the impact of various tillage and traffic practices on wheat yield.

The effects of the density of lupin (Lupinus angustifolius L. cv. Illyarrie) taproots on a following wheat crop (Tritium aestivum L. cv. Gutha) were investigated on a compacted, earthy sand soil near Geraldton, Western Australia. In 1985, plots were sown to lupins at densities ranging from 35 to 220 plants m-2. Because peak lupin biomass varied by less than 25%, and 100 kg N ha-1 of mineral fertiliser was supplied to the wheat, the effects of variation in residual N from the lupins were considered to be minimal. In 1986, all plots were split for shallow or deep tillage and wheat sown. There were no effects of tillage or lupin density on stored soil water at sowing. Growth and yield of wheat on the shallow tilled plots increased linearly with lupin plant density in the previous year, to equal or better wheat yields on the deep tilled plots, which were unaffected by lupin density. At normal farm lupin plant densities, the biological plough effect was estimated to improve wheat yields by 100 kg ha-1, substantially less than the benefits from nitrogen fixation and breaks in disease cycles. Nevertheless, it still suggests that lupin stand densities should be maintained or increased in crop rotations on compacted sands.

Quantitative models to predict the effects of soil compaction on wheat yields are being developed for the northern sandplains of Western Australia. An understanding of the relationships between soil water content (W), bulk density (p), compactibility and soil penetration resistance (P) is required. Thirteen subsoils from W.A. sandplain soils were tested for compactibility. As the amounts of very coarse sand or clay in the soil increased, the maximum density (max.) achieved with a standard compactive effort also increased, while the critical soil water content (Wcrit,.) for maximum compactibility declined. The effects of p and W on P were investigated for five of the soils. The value of P was only slightly affected as W was reduced to less than 70% of the field capacity water content. As the soils were dried further, P increased exponentially. At all water contents, an increase in p was found to markedly increase P. Particle size distribution could be used to predict max. and Wcrit., but could not be related to the effects of changes in p and W on P. The implications for the measurement and effects of soil compaction in the field are discussed.

SUMMARYReduction of leaf area in sorghum without tillers (uniculm sorghum) might result in conservation of water at early stages of growth and hence in stability of grain yield under dry conditions. In two experiments in south-east Queensland, Australia, tillers were removed by hand to examine the growth of uniculm sorghum. Tiller removal promoted root development at the flag leaf stage but significantly reduced shoot dry matter and lowered grain yield by about 20% in a wet season. The saving in soil water as a result of tiller removal was relatively small but statistically significant.