Variable water cycles have a greater impact on wheat growth and soil nitrogen response than constant watering.

Current climate change models project that water availability will become more erratic in the future. With soil nitrogen (N) supply coupled to water availability, it is important to understand the combined effects of variable water and N supply on food crop plants (above- and below-ground). Here we present a study that precisely controls soil moisture and compares stable soil moisture contents with a controlled wetting-drying cycle. Our aim was to identify how changes in soil moisture and N concentration affect shoot-root biomass, N acquisition in wheat, and soil N cycling. Using a novel gravimetric platform allowing fine-scale control of soil moisture dynamics, a 3 × 3 factorial experiment was conducted on wheat plants subjected to three rates of N application (0, 25 and 75 mg N/kg soil) and three soil moisture regimes (two uniform treatments: 23.5 and 13% gravimetric moisture content (herein referred to as Well-watered and Reduced water, respectively), and a Variable treatment which cycled between the two). Plant biomass, soil N and microbial biomass carbon were measured at three developmental stages: tillering (Harvest 1), flowering (Harvest 2), and early grain milk development (Harvest 3). Reduced water supply encouraged root growth when combined with medium and high N. Plant growth was more responsive to N than the water treatments imposed, with a 15-fold increase in biomass between the high and no added N treatment plants. Both uniform soil water treatments resulted in similar plant biomass, while the Variable water treatment resulted in less biomass overall, suggesting wheat prefers consistency whether at a Well-watered or Reduced water level. Plants did not respond well to variable soil moisture, highlighting the need to understand plant adaptation and biomass allocation with resource limitation. This is particularly relevant to developing irrigation practices, but also in the design of water availability experiments.

Nitrate uptake and its regulation in relation to improving nitrogen use efficiency in cereals.

On average less than half of the applied N is captured by crops, thus there is scope and need to improve N uptake in cereals. With nitrate (NO3-) being the main form of N available to cereal crops there has been a significant global research effort to understand plant NO3- uptake. Despite this, our knowledge of the NO3- uptake system is not sufficient to easily target ways to improve NO3- uptake. Based on this there is an identified need to better understand the NO3- uptake system and the signalling molecules that modulate it. With strong transcriptional control governing the NO3- uptake system, we also need new leads for modulating transcription of NO3- transporter genes.

Transition from a maternal to external nitrogen source in maize seedlings.

Maximizing NO3- uptake during seedling development is important as it has a major influence on plant growth and yield. However, little is known about the processes leading to, and involved in, the initiation of root NO3- uptake capacity in developing seedlings. This study examines the physiological processes involved in root NO3- uptake and metabolism, to gain an understanding of how the NO3- uptake system responds to meet demand as maize seedlings transition from seed N use to external N capture. The concentrations of seed-derived free amino acids within root and shoot tissues are initially high, but decrease rapidly until stabilizing eight days after imbibition (DAI). Similarly, shoot N% decreases, but does not stabilize until 12-13 DAI. Following the decrease in free amino acid concentrations, root NO3- uptake capacity increases until shoot N% stabilizes. The increase in root NO3- uptake capacity corresponds with a rapid rise in transcript levels of putative NO3- transporters, ZmNRT2.1 and ZmNRT2.2. The processes underlying the increase in root NO3- uptake capacity to meet N demand provide an insight into the processes controlling N uptake.

Distribution and remobilization of iron and copper in wheat.

BACKGROUND AND AIMS: The amount of iron (Fe) and copper (Cu) that is loaded into grains of wheat (Triticum aestivum) depends on both the amount of nutrient taken up by the plant post-anthesis and the amount that is remobilized from vegetative organs as they senesce. Previous reports have shown that these two micronutrients behave quite differently in wheat in that Cu is readily remobilized to the grain whilst Fe shows poor remobilization. The object was to quantify the distribution of Fe and Cu in wheat and to show how this distribution changes from anthesis to grain maturity. METHODS: The uptake and distribution of both Fe and Cu were investigated in wheat grown at two levels, adequate and low, of both micronutrients. Plants were grown in sand culture and the main culms were harvested at anthesis, 18 days post-anthesis and at maturity. Plants were separated into various organs and analysed for Fe and Cu using ICP-OES. KEY RESULTS: There was good remobilization of Fe from the rest of the shoot to the grain with 77 % of the total shoot Fe in the grain at maturity. In the adequate-Cu treatment there was 62 % of the total plant Cu in the grain at maturity, whereas in the low-Cu treatment this was only 40 %. There was no net Fe taken up into the above-ground plant parts post-anthesis whilst for Cu there was. The remobilization evident for Fe and Cu was greater than that found for zinc and much greater than evident for manganese in the same material. CONCLUSIONS: The results reported here represent good evidence for the high reproductive mobility of both Fe and Cu in wheat.

Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes.

Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH4+ and NO3- fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane & Maiden) Maiden. In addition, NH4+ and H+ fluxes were measured simultaneously at a range of NH4+ concentrations, and NO3- and H+ fluxes were measured simultaneously at a range of NO3- concentrations. Nitrogen concentrations ranged from 10-250 µM, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH4+ and NO3- fluxes exhibited saturating Michaelis-Menten-style kinetics. The Km was 16 µM for NH4+ and 18 µM for NO3-. Values of Vmax were 53 nmol m-2 s-1 for NH4+ and 37 nmol m-2 s-1 for NO3-. Proton fluxes were highly correlated with NH4+ and NO3- fluxes, but the relationships were different. Proton efflux increased with increasing NH4+ concentration and mirrored the changing NH4+ fluxes. The ratio between NH4+ and H+ fluxes was 1 : -1.6. Proton influx was evident with initial exposure to NO3-, with the flux stoichiometry for NO3- : H+ being 1 : 1.4. Subsequent increases in NO3- concentration caused a gradual increase in H+ efflux such that the flux stoichiometry for NO3- : H+ became 1 : -0.8. The presence of 100 µM NH4+ greatly reduced NO3- fluxes and caused a large and constant H+ efflux. These results are evidence that E. nitens has a preference for NH4+ as a source of N, and that the fluxes of NH4+ and NO3- are quantitatively linked to H+ flux.