Corrigendum to: Impact of crop load on nitrogen uptake and reserve mobilisation in Vitis vinifera

Nitrogen deficit affects both crop production and composition, particularly in crops requiring an optimal fruit N content for aroma development. The adaptation of cultural practices to improve N use efficiency (NUE) (i.e. N uptake, assimilation and partitioning) is a priority for the sustainable production of high-quality crops. A trial was set on potted grapevines (Vitis vinifera L. cv. Chasselas) to investigate the potential of crop limitation (via bunch thinning) to control plant NUE and ultimately fruit N composition at harvest. A large crop load gradient was imposed by bunch thinning (0.5–2.5 kg m–2) and N traceability in the plant was realised with an isotope-labelling method (10 atom % 15N foliar urea). The results indicate that the mobilisation of root reserves plays a major role in the balance of fruit N content. Fertiliser N uptake and assimilation appeared to be strongly stimulated by high-yielding conditions. Fertilisation largely contributed to fulfilling the high fruit N demand while limiting the mobilisation of root reserves under high yield conditions. Plants were able to modulate root N reserve mobilisation and fertiliser N uptake in function of the crop load, thus maintaining a uniform N concentration in fruits. However, the fruit free amino N profile was modified, which potentially altered the fruit aromas. These findings highlight the great capacity of plants to adapt their N metabolism to constraints, crop thinning in this case. This confirms the possibility of monitoring NUE by adapting cultural practices.

Impact of crop load on nitrogen uptake and reserve mobilisation in Vitis vinifera

Nitrogen deficit affects both crop production and composition, particularly in crops requiring an optimal fruit N content for aroma development. The adaptation of cultural practices to improve N use efficiency (NUE) (i.e. N uptake, assimilation and partitioning) is a priority for the sustainable production of high-quality crops. A trial was set on potted grapevines (Vitis vinifera L. cv. Chasselas) to investigate the potential of crop limitation (via bunch thinning) to control plant NUE and ultimately fruit N composition at harvest. A large crop load gradient was imposed by bunch thinning (0.5–2.5 kg m–2) and N traceability in the plant was realised with an isotope-labelling method (10 atom % 15N foliar urea). The results indicate that the mobilisation of root reserves plays a major role in the balance of fruit N content. Fertiliser N uptake and assimilation appeared to be strongly stimulated by high-yielding conditions. Fertilisation largely contributed to fulfilling the high fruit N demand while limiting the mobilisation of root reserves under high yield conditions. Plants were able to modulate root N reserve mobilisation and fertiliser N uptake in function of the crop load, thus maintaining a uniform N concentration in fruits. However, the fruit free amino N profile was modified, which potentially altered the fruit aromas. These findings highlight the great capacity of plants to adapt their N metabolism to constraints, crop thinning in this case. This confirms the possibility of monitoring NUE by adapting cultural practices.

The effect of the soil compaction on the contents of alfalfa root reserve nutrients in relation to the stand density and the amount of root biomass

The reserve root nutrients influence the overwintering, regrowth, yield, and persistence of alfalfa plants. The total amount of the root reserves is considered more important than their concentration. One of the factors which can affect the reserve content can be the soil compaction. The aim of this study is to clarify the effect of the soil compaction on the reserve root nutrients in relation to the stand density and the amount of the root biomass. In this experiment, the stand density ranged from 28 to 112 plants per m². The average soil bulk density in the uncompacted and compacted variants was found to be 1.38 and 1.52 g/cm³, respectively. In spring and autumn periods, the root samples were taken from an area of 0.25 m² (the depth 150 mm) in four replications. The number of plants, the root weight, and the concentrations of starch, saccharose, fructose, and crude protein were assessed in each plot. The total amount of the root reserves was calculated from the determined concentrations and the weights of roots of each sample. A higher soil compaction reduced significantly the stand density, root weight, total amount of all nutrients as well as the starch and crude protein concentrations. The concentration of the soluble non-structural saccharides was identical to or increased over that in the compacted variant. The negative significant effect of a higher soil compaction on the root weight and, consequently, on the total amount of all reserve root nutrients was explained by the changes in the stand density. When the root weight effect was excluded, the compacted variant provided a significantly lower density and crude protein amount and concentration. The significant effect of density on the reserve nutrients was explained by changes in the root weight.

Factors influencing herbicide efficacy when removing lucerne prior to cropping

Farmers often experience inconsistent responses when using herbicides to terminate an established lucerne pasture prior to cropping. In an attempt to redress this problem, a series of field experiments were conducted between 1999 and 2002 at various locations in southern and northern New South Wales, the Australian Capital Territory, and south Western Australia that aimed to identify management guidelines that improved the efficacy of herbicide mixtures commonly used to remove lucerne. Collectively, these studies indicated that herbicides were generally less effective when applied either early (less than 2 weeks) or late (6 weeks or more) in the regrowth cycle of lucerne after defoliation. Herbicide efficacy tended to be greatest if applied to regrowth 3–5 weeks after defoliation, which corresponds to a time when the lucerne crown and root reserves are likely to be in the process of being replenished by photoassimilates transported from the shoot. The impact of timing of herbicide application in relation to season was compared at a number of locations. Across all the sites and years, spring herbicide applications were generally the most effective, removing on average 87% of the lucerne (range 53–100%) compared with 72% in summer (24–100%) and 60% in autumn (7–92%). Spring applications were also more consistent in their effect, removing >80% of the lucerne plants in 9 out of 12 experiments, whereas similar rates of removal occurred on 4 occasions in 9 summer applications and only twice in 8 autumn applications. Some of the seasonal variation could be explained by differences in the amount of rainfall prior to herbicide applications. It was assumed that the relationship between rainfall and herbicide efficacy reflected the stimulation of lucerne shoot and root growth by the additional soil moisture before herbicide treatment. Herbicide mixtures that contained ingredients such as picloram that retain residual activity in the soil tended to be more effective and were less influenced by lucerne growth and season than those herbicides with little or no residual activity. However, such chemicals could potentially restrict which crops can subsequently be grown after a lucerne pasture has been removed. It was concluded that >80% of lucerne plants were likely to be removed using herbicides provided that the herbicide treatment was applied to actively growing lucerne 3–5 weeks after defoliation, and when greater than 70–95 mm rain had fallen in the 6–8 weeks prior to application.

Simulating lucerne growth and water use on diverse soil types in a Mediterranean-type environment

The performance of the Agricultural Production Systems Simulator (APSIM) lucerne (Medicago sativa) model was assessed by comparing calculations from APSIM with measured (or observed) data from 9 sites in Western Australia. This comparison was also to obtain new insights into lucerne production and the effect of lucerne on the water balance in this environment. APSIM accounted adequately for the temporal change in above-ground biomass production and the plant-available water (PAW) for most of the sites. The root mean square deviation (RMSD) for biomass was 1.3 t/ha for the mean observed biomass of 4.17 t/ha. The RMSD for PAW was 16 mm to a depth of 1.6–2.1 m, with the mean observed PAW of 50 mm. The good prediction of PAW was partly because critical soil parameters used in APSIM were derived from the soil water content measurements. APSIM also adequately (within the standard error) estimated evapotranspiration (Et) and drainage below the root-zone, which was measured at 2 of the sites. The analysis supports previous findings with lucerne that increased storage of carbohydrates in root reserves occurs in autumn and winter. Given that APSIM performed adequately when calibrated, it was used to simulate Et and drainage for the 7 sites in which measurements were not taken. Et in all 3 years of lucerne and drainage in Year 1 were related to the amount of rainfall. Fifty-one percent of rainfall above 230 mm was lost as water excess in Year 1 (R2 = 0.68). Drainage in Year 3 was less than drainage in Year 1, confirming previous studies that established lucerne can reduce drainage.

Crop growth and development affect seasonal priorities for lucerne management

Successful lucerne stand management requires balancing animal and plant requirements to produce crops of high quality and yield at times of high animal demand. Understanding the impact of environmental signals on crop growth and development can aid management decisions throughout the season. In spring, crops remobilise reserves from the roots to shoots and expand nodes accumulated through the winter, producing rapid stem extension and canopy closure as temperatures increase. The timing of spring defoliation should be based on crop yield and animal requirements rather than any specific developmental stage. Through spring and summer, crops should be rotationally grazed, with highest lamb live-weights achieved from 6-8 weeks grazing solely on lucerne. Summer crop production is dependent on rainfall and the plant available water content. During summer, grazing at the appearance of open flowers or basal buds is recommended as a compromise between maximum yield and quality. In autumn, the priority of assimilates allocation in the crop changes from above to below ground growth. To enhance the recharge of root reserves, an extended period of flowering is recommended in February or March. The time of flowering is dependent on the accumulation of thermal time and increases as photoperiod shortens. In periods of prolonged drought, lucerne herbage should be hard grazed and then spelled until the end of late autumn regrowth. A final hard grazing in June or early July, to remove overwintering aphids, should be followed by spraying 7-14 days later. Crops continue to develop nodes through the winter, and stands should be spelled until spring to ensure nodes are not removed by grazing, as this delays regrowth and reduces dry matter production. Key words: Flowering, grazing, herbicide, Medicago sativa, nodes, photoperiod, phyllochron, root reserves

Pasture and stock management of Californian thistle

A combination of mowing and grazing by sheep has been shown to successfully control Californian thistle (Cirsium arvense) in small plot and paddock scale trials. Between 1994 and 2000 the technique was applied to 15 paddocks on nine farms in Otago, Southland and Mid- Canterbury. Thistle-infested pastures were mown as low as possible, usually in December after weaning. Thistles at this time were up to 1m tall at the mid to late bud stage. Mobs of at least 350 ewes/ha were introduced at mowing or a day or two before, with all mown herbage eaten within 3 days. Pastures were re-grazed with this or similar sized mobs at approximately threeweek intervals to remove re-growth thistles which had emerged since the previous grazing and before their spines hardened. Each grazing forced the plant's root system to produce a new crop of aerial shoots and this ultimately used up the root reserves causing the thistles to die. The farmers involved were responsible for implementing recommendations on mob size, time and length of grazing. Three or four repeat grazings markedly reduced thistle numbers the following summer, resulting in almost complete elimination by autumn in year two after the programme was repeated. With fewer grazings in year one, control took longer. The grazing regime had no apparent effect on ewe health and resulted in improved pasture quality. Keywords: Californian thistles, mob size, repeat grazing, thistle numbers

Weed biology: importance to weed management

Knowledge of weed biology is essential for development of both economically and environmentally acceptable weed management systems. Weed biology relates to plant attributes such as morphology, seed dormancy and germination, physiology of growth, competitive ability, and reproductive biology. Concepts of population biology such as seedbank dynamics for annuals and root reserves, dormancy, and longevity of vegetative propagules for perennials can be used to predict weed infestations better and to evaluate sustainable management strategies. Integrated approaches that give priority to depletion of root reserves or seedbanks through interfering with dormancy or germination requirements have great potential to enhance weed management strategies in the future.