P. J. Hocking

Plant mechanisms to optimise access to soil phosphorus.

Phosphorus (P) is an important nutrient required for plant growth and its management in soil is critical to ensure sustainable and profitable agriculture that has minimal impact on the environment. Although soils may contain a large amount of total P, only a small proportion is immediately available to plants. Australian soils often have low availability of P for plant growth and P-based fertilisers are, therefore, commonly used to correct P deficiency and to maintain productivity. For many soils, the sustained use of P fertiliser has resulted in an accumulation of total P, a proportion of which is in forms that are poorly available to most plants. The efficiency with which different P fertilisers are used in agricultural systems depends on their capacity to supply P in a soluble form that is available for plant uptake (i.e. as orthophosphate anions). In addition to fertiliser source, the availability of P in soil is influenced to a large extent by physico-chemical and biological properties of the soil. Plant access to soil P is further affected by root characteristics (e.g. rate of growth, specific root length, and density and length of root hairs) and biochemical processes that occur at the soil–root interface. The ability of roots to effectively explore soil, the release of exudates (e.g. organic anions and phosphatases) from roots that influence soil P availability, and the association of roots with soil microorganisms such as mycorrhizal fungi are particularly important. These processes occur as a natural response of plants to P deficiency and, through better understanding, may provide opportunities for improving plant access to soil and fertiliser P in conventional and organic agricultural systems.

Comparison of canola, Indian mustard and Linola in two contrasting environments. I. Effects of nitrogen fertilizer on dry-matter production, seed yield and seed quality

Abstract The potential for extending the production of winter oilseed crops into the drier region of the cereal belt of eastern Australia was examined by comparing canola, Indian mustard and Linola in field experiments in the contrasting environments of Junee and Condobolin, NSW. Junee is in the region where canola production is concentrated, and Condodolin is in the drier part of the cereal belt currently considered marginal for canola. Different rates of nitrogen (N) fertilizer were applied in all experiments, and there were two times of sowing at Condobolin. Wheat was grown for comparison in all experiments. Crop performance was evaluated over two seasons by measuring dry-matter production, grain yield, water-use efficiency, and seed oil and protein contents. Maximum grain yields of all crops at Junee were higher than at Condobolin, and yield responses to N fertilizer in both environments were greater in the wetter 1992 season than in the drier 1991 season. Canola and wheat showed the largest yield responses to N fertilizer, but Linola generally had the lowest yield responses because of poor seedling emergence at high N rates, intolerance of drought during grain filling (1991) and lodging (1992). Based on the overall growth and grain yield responses to N fertilizer, and on N (protein) removal in grain, it is suggested that about 25% more N be applied to canola than to wheat, that Indian mustard receive about the same N fertilizer rate as wheat, and that Linola requires about 20% less N than wheat. Sowing late at Condobolin, when there was below-average rainfall during grain filling (1991) reduced the yield of canola and Linola, but not Indian mustard. There was no yield penalty for sowing the crops late in 1992 when conditions were favourable during grain filling. Grain harvest indices based either on dry matter or the biosynthetic cost of seed production were higher for wheat and canola than for Indian mustard and Linola, and were largely unaffected by the N treatments. A potential transpiration efficiency value of 12.5 kg seed ha −1 mm −1 was derived for the oilseeds based on the biosynthetic costs of seed production. Oil concentrations were reduced by dry conditions during seed filling, by late sowing and by N fertilizer. Oil concentrations were higher at Junee than Condobolin, due mainly to differences in ambient temperature as there was a 2.7% decrease in oil concentration for each 1°C rise in mean temperature during seed filling. Oilseed meal and wheat grain protein levels were more responsive to N fertilizer when conditions were dry during grain filling. Protein levels in oilseed meals were higher but wheat grain protein was lower when conditions were favourable during grain filling. The study indicated that there are prospects for extending the production of canola and Indian mustard into drier regions of the cereal belt because, when sown early, they have similar water-use efficiencies to wheat based on the biosynthetic costs of grain production. While the yield of Indian mustard is limited by a low harvest index, its yield stability under dry post-anthesis conditions indicates greater potential than current canola cultivars for more marginal areas. Linola showed the least potential as an oilseed for the drier parts of the cereal belt because of shallow rooting and susceptibility to drought.

The response of dryland canola to nitrogen fertilizer: partitioning and mobilization of dry matter and nitrogen, and nitrogen effects on yield components

Abstract Canola (Brassica napus) was grown under dryland conditions in field experiments at Greenethorpe (1988) and Canowindra (1989) in the cereal belt of New South Wales to determine (1) the response of the crop to nitrogen (N) fertilizer when grown late in a cropping sequence; (2) the seasonal course of dry-matter production and N accumulation; (3) the distribution of dry matter and N among plant parts, including shed leaves; and (4) the apparent mobilization of dry matter and N from stems and leaves to seeds. At both sites, maximum dry-matter production and seed yields occurred at 75 kg applied N ha−1. Seed yields increased from 2.3 to 3.5 t ha−1 at Greenethorpe, and from 0.85 to 2.5 t ha−1 at Canowindra. Topdressing with a single application of N at the 5–6 leaf rosette stage, flower buds visible or the start of flowering resulted in 70–90% of the seed yields obtained when the equivalent amount of N was applied pre-sowing. At maximum seed yield, canola accumulated 165 kg N at Greenethorpe and 110 kg N ha−1 at Canowindra. Averaged over both seasons and all N treatments, 52% of the N content of the mature plants accumulated before flowering and 50% of the dry-matter content of the mature plants accumulated during flowering. Maximum dry-matter and N contents for leaves occurred at the start of flowering, and for the stem at the end of flowering. Averaged over all N treatments at Greenethorpe, about 20% of the dry matter and 60–65% of the N was apparently mobilized from the stem and leaves, after flowering. The combined mobilization from the stem + leaves could have contributed to about 17% of the dry matter and 55% of the N accumulated by seeds. Amounts of dry matter and N lost in shed leaves ranged from 1-1.75 t and 10–30 kg ha−1, and N removal in seed ranged from 63–112 and 27–96 kg ha−1 at Greenethorpe and Canowindra, respectively. N concentrations in whole shoots and vegetative organs declined during the season, irrespective of the rate of N fertilizer applied. N fertilizer increased pod number per plant but had little effect on seed number per pod or 1000 seed weights. Seed oil concentrations were unaffected by the N rate at which maximum seed yield was obtained. N fertilizer rate had no effect on dry-matter harvest indices (mean both sites 30%) which, when expressed on the basis of the biosynthetic costs for straw and seed production, were comparable to those reported for wheat (37%). N harvest indices (mean both sites 76%) were reduced only at the highest N rates at Greenethorpe. Indices of N fertilizer use-efficiency generally decreased with increasing N fertilizer rate, and were similar to values reported for wheat when differences in the biosynthetic cost of grain production were taken into account.

Transgenic barley (Hordeum vulgare L.) expressing the wheat aluminium resistance gene (TaALMT1) shows enhanced phosphorus nutrition and grain production when grown on an acid soil

Barley (Hordeum vulgare L.), genetically modified with the Al(3+) resistance gene of wheat (TaALMT1), was compared with a non-transformed sibling line when grown on an acidic and highly phosphate-fixing ferrosol supplied with a range of phosphorus concentrations. In short-term pot trials (26 days), transgenic barley expressing TaALMT1 (GP-ALMT1) was more efficient than a non-transformed sibling line (GP) at taking up phosphorus on acid soil, but the genotypes did not differ when the soil was limed. Differences in phosphorus uptake efficiency on acid soil could be attributed not only to the differential effects of aluminium toxicity on root growth between the genotypes, but also to differences in phosphorus uptake per unit root length. Although GP-ALMT1 out-performed GP on acid soil, it was still not as efficient at taking up phosphorus as plants grown on limed soil. GP-ALMT1 plants grown in acid soil possessed substantially smaller rhizosheaths than those grown in limed soil, suggesting that root hairs were shorter. This is a probable reason for the lower phosphorus uptake efficiency. When grown to maturity in large pots, GP-ALMT1 plants produced more than twice the grain as GP plants grown on acid soil and 80% of the grain produced by limed controls. Expression of TaALMT1 in barley was not associated with a penalty in either total shoot or grain production in the absence of Al(3+), with both genotypes showing equivalent yields in limed soil. These findings demonstrate that an important crop species can be genetically engineered to successfully increase grain production on an acid soil.

Effects of sowing time and nitrogen fertiliser on canola and wheat, and nitrogen fertiliser on Indian mustard. I. Dry matter production, grain yield, and yield components

Canola, Indian mustard, and wheat were grown under dryland conditions at Ariah Park and Cowra (canola only) in the cropping belt of New South Wales, Australia, to determine the effects of sowing time (canola and wheat) and nitrogen (N) fertiliser on the growth, grain yield, and yield components of the crops. Compared with an April sowing, the grain yield of canola at Ariah Park was reduced by 35% for a May sowing and by 67% for a July sowing. Canola yield at Cowra was reduced by 45% between early and late May sowings. Wheat yield declined by 35% between the May and July sowings at Ariah Park. Grain yields of canola and wheat at Ariah Park responded to N fertiliser in the April and May sowings, but not in the July sowing. Indian mustard had a higher yield than the comparable sowing of canola. Canola yields at Cowra were more responsive to N fertiliser than at Ariah Park, and increased from 0.5 to 2.9 t/ha with 100 kg N/ha. For each day that sowing canola was delayed at both sites after April-early May, anthesis was delayed on average by 0.52 days. For Dollarbird wheat, the delay in anthesis was 0.39 days per day sowing was delayed. Dry matter accumulation by the oilseeds was greatest during flowering, but before anthesis for wheat. Late sowing had little effect on the proportions of dry matter accumulated in a particular growth period. Irrespective of sowing time, grain yields and dry-matter harvest indices of the oilseeds were similar to values for wheat when differences in the biosynthetic costs of grain and straw production were taken into account. Late sowing usually resulted in a greater reduction in canola oil concentration than high N fertiliser rates. Canola oil concentration was reduced by 1.7 percentage points per 1°C increase in mean temperature during grain filling as a result of sowing late. It was concluded that N fertiliser could not compensate for the yield reduction in canola and wheat due to sowing late. Early sowing was essential to achieve high oil levels in canola. Additional keywords: Brassica juncea, Brassica napus, dry-matter harvest indices, oil concentration, oilseed rape. AR011 Poctapp Pck S C, wan musr

Comparison of canola, Indian mustard and Linola in two contrasting environments. II. Break-crop and nitrogen effects on subsequent wheat crops

Abstract The main canola-growing region in Australia is southern New South Wales where previous studies showed higher yield and grain protein of wheat growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oilseeds in two field experiments, one in this region and the other in central western New South Wales which is generally drier during the growing season, warmer throughout the year and is currently considered marginal for oilseeds. The effect of nitrogen (N) fertilizer on the response of wheat to previous crops was also investigated by considering both the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conducted over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseline levels of soil mineral-N and average levels of root-disease inoculum. At the drier site the inoculum of wheat leaf and root pathogens remained during both phases but there were no break-crop effects. The effect of previous crops on the yield and protein of a subsequent wheat crop could be explained by the amount of residual soil mineral N. At the wetter site, wheat responses to previous crops could be explained by the amounts of residual soil mineral N in one phase of the experiment when there was no root disease of wheat. Under these conditions, yield and grain protein generally increased in response to increasing levels of soil mineral N. The exception was the yield of wheat after Linola which decreased when an excessive amount of residual N resulted in greater vegetative biomass, rapid depletion of soil water and decreased yield. In the other phase, when root disease was present, break crops increased yield of a subsequent wheat crop by 30% and grain protein by 1.3% compared to wheat growing after wheat, and among the oilseeds the brassicas gave a greater break-crop benefit than Linola. Application of fertilizer N to wheat growing after wheat failed to compensate for the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to the second successive wheat crop for the phase in which root diseases were present, with increases of 13% in grain yield. The break-crop effect at the wetter site confirms previous observations of this benefit of oilseeds in general, and brassicas in particular, in southern New South Wales. The absence of any break-crop effect at the drier site suggests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs.

Organic acids exuded from roots in phosphorus uptake and aluminum tolerance of plants in acid soils

Abstract Acid soils comprise about 30%of the world s arable land,and aluminum (Al) toxicity and phosphorus (P) fixation to soil minerals are major problems for crop production on these soils.However,applying even moderate rates of P fertilizer and lime to improve acid soils is not economic in many countries.Plants that can access fixed P and are Al tolerant have an important role in sustaining crop production on these soils. It is now clear that organic acids exuded from roots can bene fit the P nutrition of plants and protect roots by detoxifying Al in the rhizosphere.Studies with 32 P-labeled soil have shown unequivocally that species which exude organic acids can access fixed inorganic P that is unavailable to other plants.Organic acids appear to be important in enabling plants to access phytate,a form of organic P which is also fixed to soil minerals.Organic acid anions such as citrate, malonate, oxalate, and tartrate are commonly implicated in enhancing access to fixed soil P, and citrate, malate,and oxalate are implicated in enhancing aluminum tolerance. Intercropping of an organic acid-secreting crop with a nonsecreting crop can improve the P nutrition of the nonsecreting species.An organic acid-secreting crop may also improve the P nutrition of a following crop in the rotation. Genetic engineering has the potential to increase organic acid effiux from roots. Most work has tried to increase citrate effiux by overexpressing citrate synthase,but the results are contradictory.Other approaches include overexpressing enzymes to increase substrate supply for citrate synthesis,or antisensing production of enzymes that metabolize citrate.Mechanisms by which P deficiency and Al activate or induce the effiux of specific organic acids from roots need to be identified.Anion channels in the plasma membrane are likely to have a major regulatory role in the transport of organic acids from roots,and genes that encode for these channels will be key targets for genetic engineering.

Nitrogen nutrition of sunflower (Helianthus annuus L.): Yield components, the timing of their establishment and seed characteristics in response to nitrogen supply

Abstract The components of yield of Helianthus annuus L. are seed number per plant, single seed weight and oil % seed dry weight. Glasshouse experiments with cultivar Hysun 30 using a range of nitrogen supply levels identified seed number as the major yield component. Changing nitrogen supply levels at the end of floret initiation and at anthesis showed that seed number was determined by N supply before floret initiation, i.e. by the number of florets produced. Single seed weight was influenced by N supply throughout plant development but mainly between floret initiation and anthesis. Oil % dry weight was affected only by N supply after anthesis: the higher the N supply the lower % oil. Oil yield per plant, like seed number, was determined by N supply before floret initiation. Seed hull dry weight (mainly ovary wall) was determined before anthesis; seed kernel dry weight after anthesis. Seed N content and concentration were affected by N supply in all developmental phases. Concentrations of amino acids (free and protein amino acids) increased as N % seed dry weight increased. Arginine, glutamate and histidine increased proportionally more than did the total amino acids. The increased arginine and histidine were mainly in the free amino acid fraction, glutamate in the protein fraction. Nevertheless gel electrophoresis did not show any differences in the relative amounts of protein subunits present in any of the nitrogen treatments. The results suggest that management of N application to sunflower crops should ensure an adequate supply just before floret initiation to obtain large seed numbers. Later applications to support floret growth up to anthesis will ensure maximum single seed weights.

Comparison of the ability of different crop species to access poorly-available soil phosphorus

The crop species canola, narrow-leaf lupin, pigeon pea, soybean, sunflower, wheat and white lupin were grown in an Oxisol low in plant-available phosphorus (P), but high in total P, to investigate their capacities to access poorly-available soil P. The soil was labelled with 32P, and the plants were grown in a phytotron glasshouse. Values for specific 32P activity in plant tops were: white lupin > pigeon pea > canola, sunflower and wheat > narrow-leaf lupin and soybean. The results show that white lupin and, to a lesser extent, pigeon pea obtain P from a soil pool not available to the other species, particularly canola.

Genotypic variation in cadmium accumulation by seed of linseed, and comparison with seeds of some other crop species

The accumulation of cadmium (Cd) in plants is a health issue because a range of grain and vegetable crops can accumulate levels of Cd that are in excess of limits set by the World Health Organization and individual countries. Many Australian agricultural soils used to produce confectionery linseed have a history of intensive use of Cd-contaminated phosphatic fertilisers and this, combined with soil properties such as high chloride salinity, can result in enhanced availability of Cd to crops. We investigated genotypic variation in Cd accumulation in seed of 17 linseed and Linola (termed linseed) lines from Australia and elsewhere in a glasshouse study using a soil from southern Australia that had a history of the application of Cd-contaminated phosphatic fertiliser. Canola, Indian mustard, lupins, and wheat were also included in the study for comparison. Under the experimental conditions, Cd concentrations in seed of all but one of the linseed lines exceeded the maximum permitted concentration (MPC) of 250 µg/kg for confectionery linseed traded on the international market. There was a 2.3-fold variation in seed Cd concentrations between all the linseed lines (range, 233–545 µg/kg). Linseed lines from Australia and overseas were equally capable of accumulating Cd in seed. Brown-seeded and golden-seeded lines accumulated similar concentrations of Cd. Canola, Indian mustard, lupins, and wheat accumulated about 10-fold lower concentrations of Cd in seed than linseed, and did not exceed Australian or other MPCs. There was little difference in Cd concentrations between the seed and de-seeded capsules of linseed, but a large difference in Cd concentration between the seed and de-seeded fruit parts of the other crops. The mean seed to de-seeded fruit part Cd concentration ratio for linseed was 0.87 : 1 compared with a ratio of 0.35 : 1 for the other crops, suggesting that linseed has comparatively ineffective barriers discriminating against the transport of Cd to seed. Analysis of seed lots of confectionery linseed sampled from a grain receival depot showed that seed lots from farms in Victoria (range 140–560 µg Cd/kg) had 5-fold greater Cd concentrations than those from farms in New South Wales (range 20–160 µg/kg). This is presumably due to a more intensive history of the application of Cd-contaminated phosphatic fertiliser to pastures and crops in Victoria, as well as differences in environmental and soil conditions.