Increased food production and enhanced sustainability depend on improving nitrogen-use efficiency (NUE) of crops. Breeding for enhanced NUE can take advantage of doubled-haploid populations derived from parents differing in the trait. This study evaluated variation in photosynthetic parameters at various growth stages in 43 wheat genotypes (parents of the existing doubled-haploid mapping populations) under optimal and low (one-quarter of the optimal) N supply. For relative chlorophyll content, the genotype × N treatment interaction was significant at tillering, booting, pre-anthesis and anthesis. Genotypes with small differences in relative chlorophyll content between the two N supplies were CD87 at tillering and pre-anthesis, and Batavia at anthesis. Potential photochemical activity (Fv/Fm) was measured at tillering and anthesis. The genotype × N treatment interaction was significant in both growth stages. Based on net photosynthesis, stomatal conductance and intrinsic water usez efficiency, there was variable potential of the genotypes to cope with low N supply; significant differences were found among genotypes at ambient CO₂ and between N treatments at elevated CO₂ concentration (2000 µmol mol^–1) for all three parameters. Based on all studied parameters, a dissimilarity matrix was constructed, separating the 43 genotypes into four groups. Group 2 comprised 15 of the genotypes (Batavia, Beaver, Calingiri, CD87, Frame, Krichauff, Neepawa, Soissons, Spear, Stiletto, WAWHT2036, WAWHT2074, Westonia, Wilgoyne, Yitpi), characterised by small differences in relative chlorophyll content and Fv/Fm caused by different N supply at tillering and anthesis. These genotypes therefore appear to have relative tolerance to low N supply and a potential to be used in discerning the molecular basis of tolerance to low N supply.

The aim of this study was to investigate the impact of elevated concentration of carbon dioxide (CO₂), as expected over coming decades, on yield and quality of winter bread wheat (Triticum aestivum L.). Plants (cv. Bologna) were grown by using the free-air CO₂ enrichment (FACE) system at Fiorenzuola d’Arda under ambient (control) and elevated (570 ppm, e[CO₂]) CO₂ concentrations for two growing seasons. We addressed whether there would be a response of wheat grains to elevated CO₂ concentration in terms of the contents of nitrogen (N), micro- and macronutrients, proteins and free amino acids. Under e[CO₂], total wheat biomass and grain yield increased in both years of the study. Grain N percentage was reduced under e[CO₂], but grain N yield (kg ha^–1) was increased. Among macro- and micronutrients, a decrease in zinc concentration was observed. The proteome pattern was significantly different in grains grown at the two different CO₂ levels, but the observed changes were highly dependent on interactions with prevailing environmental conditions. Finally, a negative trend was observed in the early germination rates of seeds from plants grown under e[CO₂] compared with the controls. The results suggest that the expected increase in CO₂ levels and their interactive effects with environmental variables may influence agronomic performance by increasing yield and negatively affecting quality.

Shortage of fresh water and drought stress are important factors limiting crop productivity in semi-arid and arid regions. Irrigation management needs to be optimised to improve irrigation water-use efficiency (IWUE), and thus, agricultural sustainability in these climates. A field experiment was conducted at two semi-arid locations in Iran to evaluate the impact of irrigation regime (applied after crop evapotranspiration of 70 mm (I₇₀), 100 mm (I₁₀₀) or 130 mm (I₁₃₀)) and irrigation method (applied to every furrow (EFI) or variable alternate furrow (AFI)) on yield and IWUE of maize (Zea mays L.). Yield response to irrigation rate was quadratic. Kernel yield was 8476 kg ha^–1 under I₇₀, and this reduced by 12.3% under I₁₀₀ and 27.7% under I₁₃₀. Yield reduction due to water stress was attributed to decline in both kernel number and kernel weight. Implementation of AFI resulted in a significant saving in irrigation water. At I₇₀, 31% less water was used with AFI than with EFI. Irrigation water saving was even greater under I₁₀₀ and I₁₃₀ when AFI was implemented. Regardless of irrigation regime, IWUE under AFI was always greater than under EFI (1.32 vs 1.03 kg m^–3 for grain and 3.30 vs 2.47 kg m^–3 for biomass production). In addition, plants were shorter with a longer root system under AFI, and the role of carbohydrate remobilisation in kernel filling was greater under AFI consistently among irrigation regimes. The results indicated good potential of AFI for development of water-saving strategies for maize production in semi-arid climates.

The main aim of this research was to verify whether mineral nitrogen (N) continuously released by organic fertilisers during the field bean growth cycle may be sufficiently high to enhance plant growth and seed yield but sufficiently low that it does not negatively affect nodulation and symbiotic N₂ fixation. Plants were grown without N fertilisation, and with mineral and organic N (biosolids) fertilisation. All plant parts were collected and dry matter, N content, %Ndfa, and N₂ fixed were measured at 8th node, flowering, and maturity stages. Nodule specific activity, N derived from soil, and N remobilisation were estimated. The nitrate concentration of soil was also determined. Biosolids reduced nodule growth, nodule fixation activity, and N₂ fixation during the vegetative but not the reproductive phase. During seed filling, nodule fixation activity increased and N₂ fixation was roughly twice that of the Control plants. Biosolids increased seed yield by removing the imbalance between N demand and N supply for pod growth. This may be related to an increase in nodule-specific activity due to the reduction in mineral N in the soil.

Cover crops grown during fallows can increase organic matter inputs, improve soil surface cover to reduce erosion risk, and enhance rainfall infiltration. An experiment compared a chemical fallow control with six different cover crops terminated at either 60 or 90 days after sowing. The commercial choice of millet (Echinochloa esculenta) was compared with two summer legumes (lablab (Lablab purpureus) and soybean (Glycine max)), and three winter legumes (field pea (Pisum sativum), faba bean (Vicia faba) and common vetch (Vicia sativa)). Cover crop biomass growth, atmospheric nitrogen (N) fixation, surface residue cover, and soil water and mineral N dynamics during the growth period and subsequent fallow were measured. Soil water and N availability and yield of wheat crops following the experimental treatments were simulated over a 100-year climate record using APSIM. Both experiments and simulations found the legumes inferior to millet as spring-sown cover crops, because they were slower to accumulate biomass, required later termination and provided groundcover that was less persistent, resulting in lower soil water at the end of the fallow. After 90 days of growth, the summer legumes, lablab and soybean, produced the most biomass and fixed more N (up to 25 kg N/ha) but also extracted the most soil water and mineral N. Legume N fixation was low because of high soil mineral N status (>100 kg N/ha) and occurred only when this had been depleted. At the end of the subsequent fallow in April, soil water was 30–60 mm less and soil mineral N 80–100 kg/ha less after both millet and 90-day terminated summer legume cover crops than the chemical fallow control. Simulations predicted soil-water deficits following legume cover crops to be >50 mm in the majority of years, but soil mineral N was predicted to be lower (median 80 kg N/ha) after millet cover crops. In conclusion, monoculture legume cover crops did not provide advantages over the current commercial standard of millet, owing to less effective provision of groundcover, low N fixation and possibly delayed release of N from residues. Further work could explore how legumes might be more effectively used as cover crops to provide N inputs and soil protection in subtropical farming systems.

Hedging is used to control tree size in macadamia orchards, but the effects on subsequent shoot growth and floral initiation may impair fruit production. Four-year-old grafted macadamia (Macadamia integrifolia Maiden & Betche) trees were subjected to pruning of the most recent seasonal shoot in autumn and spring. Factorial combinations of treatments included girdling or not girdling near the base of the previous season’s (parent) shoot; removal of all except two, four or six leaves from the parent shoot; and defoliation or no defoliation of the shoot that appeared after pruning. Initial numbers and dimensions of flush-shoot internodes were similar for all branch treatments in autumn and for girdled branches in spring, and were smaller than half those for non-girdled stems in spring. Later flush-shoot growth depended on the number of leaves retained on the parent shoot, the presence or absence of a connection to the tree below the parent shoot, and on the season, declining with limiting growing degree-days in winter and accelerating with increasing growing degree-days in summer. In both seasons, reserves beyond the parent (previous season’s) shoot contributed the major source of carbohydrate for continuing flush-shoot growth, and particularly the growth of leaves. The combinations of seasonal conditions, shoot parameters before the commencement of flush-shoot growth, and flush-shoot morphology permit the inference of allometric relationships that can be applied to the quantitative modelling of vegetative shoot morphology and growth in macadamia.

Glyphosate-tolerant (GT) cotton offers a multitude of benefits such as broad-spectrum and cost-effective weed control, simple weed management, and reduced impact on the environment. However, high adoption rates of GT cotton have led to overreliance on glyphosate in weed management and have decreased the use of other herbicide options and non-chemical weed-management strategies, possibly leading to the emergence of many resistant weeds. Previous surveys in 2006 and 2011 in the cotton-growing regions of New South Wales (NSW) and Queensland, Australia, indicated changes in weed populations over the period and increased prevalence of several weeds. These two surveys indicated increased dominance of Conyza bonariensis, Echinochloa colona, and Chloris virgata in these regions. Periodic weed surveys are necessary to assess weed population dynamics and shifts due to overreliance on glyphosate for weed management. A survey was carried out in the cotton-growing regions of NSW and Queensland in 2014–15, covering 135 fields. Survey results indicated the emergence of volunteer GT cotton as the most common weed present across all of the cotton-growing regions, occurring in 85% of fields, followed by E. colona (67% of fields surveyed), and C. bonariensis and Sonchus oleraceus, which were present in 51% of fields. The most prevalent grass weed after E. colona was C. virgata (37%). Broadleaf weeds Ipomoea lonchophylla and Amaranthus mitchellii were present in 40% and 37% of fields, respectively. Regional-level analysis indicated greater prevalence of Sesbania cannabina and Parthenium hysterophorus in Emerald region of Queensland. Lolium rigidum was present in the Griffith and Warren area of NSW during summer, even though it is a winter weed. The results of this study indicate integration of diversified weed-management options and inclusion of both non-chemical and chemical options because many major weeds observed in this study are tolerant to glyphosate and have already evolved resistance to glyphosate.

Perennial grasses have production and environmental benefits in areas of southern Australia typified by the mixed farming zone of southern New South Wales (NSW). The perennial grass phalaris (Phalaris aquatica L.) is widely used in southern Australia; however, it would find more use in the mixed farming zone if its persistence in marginal rainfall areas (450–500 mm average annual rainfall) were improved. We evaluated a range of germplasm (n = 29) including wild accessions, lines bred from these, and existing cultivars for persistence and production at three sites in a summer-dry area of southern NSW with 430–460-mm average annual rainfall. Two sites were used over 4 years and the third site over 5 years. Summer dormancy, maturity time and seedling growth were also assessed. Analysis of genotype × environment interaction employing factor analytic models and accounting for spatial and temporal correlations indicated that changes in persistence occurred mainly over time rather than between sites. Ranking changes occurred in the dry establishment phase of the experiment and during a severe final summer drought, with few changes occurring in the intervening high-rainfall years. Lines that survived the establishment phase best had vigorous seedlings and earlier maturity, whereas those surviving the final summer best were earlier maturing and higher in summer dormancy with high winter-growth activity. Some later maturing lines within the higher summer dormancy group were less persistent. Some accessions from North Africa were the most persistent; also, populations bred from these and other more persistent accessions generally persisted and produced better than cultivars used presently. However, present cultivars were capable of high yield in the higher rainfall years. We suggest that persistence of higher summer dormancy cultivars over very dry years could be improved by selecting for earlier maturity time.

This study investigated the ability of several plant species commonly occurring as weeds in Australian cropping systems to produce root exudates that inhibit nitrification via biological nitrification inhibition (BNI). Seedlings of wild radish (Raphanus raphanistrum), great brome grass (Bromus diandrus), wild oats (Avena fatua), annual ryegrass (Lolium rigidum) and Brachiaria humidicola (BNI-positive control) were grown in hydroponics, and the impact of their root exudates on NO₃^– production by Nitrosomonas europaea was measured in a pure-culture assay. A pot study (soil-based assay) was then conducted to confirm the ability of the weeds to inhibit nitrification in whole soils. All of the tested weeds slowed NO₃^– production by N. europaea in the pure-culture assay and significantly inhibited potential nitrification rates in soil-based assays. Root exudates produced by wild radish were the most inhibitory, slowing NO₃^– production by the pure culture of N. europaea by 53 ± 6.1% and completely inhibiting nitrification in the soil-based assay. The other weed species all had BNI capacities comparable to that of B. humidicola and significantly higher than that previously reported for wheat cv. Janz. This study demonstrates that several commonly occurring weed species have BNI capacity. By altering the N cycle, and retaining NH₄ in the soils in which they grow, these weeds may gain a competitive advantage over species (including crops) that prefer NO₃^–. Increasing our understanding of how weeds compete with crops for N may open avenues for novel weed-management strategies.