Jairo A. Palta

Early vigorous growth is a major factor influencing nitrogen uptake in wheat

A field trial, a lysimeter system study and a nutrient solution experiment were conducted to determine the genotypic differences in nitrogen (N) uptake among wheat (Triticum aestivum L.) genotypes differing in vigour of early growth. Plant growth and N uptake of Vigour 18, a breeding line with early vigour, and the commercial cultivars Westonia, Tincurrin, Camm and Janz were compared. Shoot biomass of Vigour 18 was higher than that of the other genotypes, except for Westonia at booting when 50 kg N ha-1 was applied 3 d after wheat emergence. Vigour 18 had significantly higher efficiency of fertiliser-N uptake than the other four cultivars at tillering when 50kg N ha-1 was applied. Fertiliser-N uptake efficiency at booting was similar in Vigour 18 and Westonia, but significantly higher than in three other commercial cultivars. Vigour 18 had higher root dry matter, root-length density and root surface area than Janz when examined in columns of soil. The greater root growth of Vigour 18 occurred across all soil layers to a depth of 0.6 m. Differences in total N uptake between Vigour 18 and Janz were apparent from tillering (Z14,22) to booting (Z19,24,49). Vigour 18 also had significantly higher shoot biomass and N uptake than Janz when grown in nutrient culture. Nitrate reductase activity (NRA) expressed on a whole-plant basis was higher for Vigour 18 than for Janz, and was related to total N uptake. However, NRA expressed on a per-unit-fresh-weight basis was not significantly different across the cultivars tested. It is concluded that vigorous early root and shoot growth in Vigour 18 was the main driving force for higher N uptake.

Response of wheat growth, grain yield and water use to elevated CO2 under a Free-Air CO2 Enrichment (FACE) experiment and modelling in a semi-arid environment

Abstract The response of wheat crops to elevated CO 2 (eCO 2) was measured and modelled with the Australian Grains Free‐Air CO 2 Enrichment experiment, located at Horsham, Australia. Treatments included CO 2 by water, N and temperature. The location represents a semi‐arid environment with a seasonal VPD of around 0.5 kPa. Over 3 years, the observed mean biomass at anthesis and grain yield ranged from 4200 to 10 200 kg ha−1 and 1600 to 3900 kg ha−1, respectively, over various sowing times and irrigation regimes. The mean observed response to daytime eCO 2 (from 365 to 550 μmol mol−1 CO 2) was relatively consistent for biomass at stem elongation and at anthesis and LAI at anthesis and grain yield with 21%, 23%, 21% and 26%, respectively. Seasonal water use was decreased from 320 to 301 mm (P = 0.10) by eCO 2, increasing water use efficiency for biomass and yield, 36% and 31%, respectively. The performance of six models (APSIM‐Wheat, APSIM‐Nwheat, CAT‐Wheat, CROPSYST, OLEARY‐CONNOR and SALUS) in simulating crop responses to eCO 2 was similar and within or close to the experimental error for accumulated biomass, yield and water use response, despite some variations in early growth and LAI. The primary mechanism of biomass accumulation via radiation use efficiency (RUE) or transpiration efficiency (TE) was not critical to define the overall response to eCO 2. However, under irrigation, the effect of late sowing on response to eCO 2 to biomass accumulation at DC65 was substantial in the observed data (~40%), but the simulated response was smaller, ranging from 17% to 28%. Simulated response from all six models under no water or nitrogen stress showed similar response to eCO 2 under irrigation, but the differences compared to the dryland treatment were small. Further experimental work on the interactive effects of eCO 2, water and temperature is required to resolve these model discrepancies.

Can elevated CO2 combined with high temperature ameliorate the effect of terminal drought in wheat

Wheat (Triticum aestivum L.) production may be affected by the future climate, but the impact of the combined increases in atmospheric CO2 concentration, temperature and incidence of drought that are predicted has not been evaluated. The combined effect of elevated CO2, high temperature and terminal drought on biomass accumulation and grain yield was evaluated in vigorous (38-19) and nonvigorous (Janz) wheat genotypes grown under elevated CO2 (700µLL-1) combined with temperatures 2°C, 4°C and 6°C above the current ambient temperature. Terminal drought was induced in all combinations at anthesis in a split-plot design to test whether the effect of elevated CO2 combined with high temperature ameliorates the negative effects of terminal drought on biomass accumulation and grain yield. Biomass and grain yield were enhanced under elevated CO2 with 2°C above the ambient temperature, regardless of the watering regimen. The combinations of elevated CO2 plus 4°C or 6°C above the ambient temperature did not enhance biomass and grain yield, but tended to decrease them. The reductions in biomass and grain yield (45-50%) caused by terminal drought were less severe (21-28%) under elevated CO2 with 2°C above the ambient temperature. The amelioration resulted from a 63% increase in the rate of leaf net photosynthesis in 38-19 and a 39% increase in tillering and leaf area in Janz. The contrasting responses and phenological development of these two genotypes to the combination of elevated CO2, temperature and terminal drought, and the possible influences on their source-sink relationships are discussed.

Waterlogging affects the growth, development of tillers, and yield of wheat through a severe, but transient, N deficiency

Tiller production and survival are suppressed on soils prone to waterlogging. The tiller production and growth of wheat (Triticum aestivum cv. Wyalkatchem) was investigated in a glasshouse experiment during and after a transient waterlogging to examine its effect on grain yield. Wheat plants received either a high or low nitrogen (N) application at sowing and were waterlogged at 22 days after sowing for 14 days. Plants received a second either high or low N application after waterlogging was released. Waterlogging induced a transient N deficiency. The N concentration of the youngest expanded leaf on the mainstem and tillers declined markedly during waterlogging, but its recovery 14 days after the waterlogging was ended was independent of treatment, reaching a greater than the critical minimum concentration of 3.5%. The growth of primary tillers 1 and 2 was severely inhibited by waterlogging while the exsertion of new tillers was delayed by 9 days. Shoot dry weight of the waterlogged plants at final harvest was reduced by 37% compared with the non-waterlogged plants. During the recovery period, the waterlogged plants produced higher order tillers that produced late ears. As a result, the number of ears per plant was similar in plants in continuously drained or previously waterlogged soil. Waterlogging reduced the number of grains per ear on the mainstem and tillers, and consequently grain yield by 32%. High N application after waterlogging increased grain yield by ~20%, but high N applied at sowing had no effect on yield. This suggests that N application after waterlogging can reduce the detrimental effect of waterlogging on grain yields in areas prone to waterlogging.

The yield performance of lupin genotypes under terminal drought in a Mediterranean-type environment

With a view to identifying and understanding the genotypic differences in yield under terminal drought, a range of lupin genotypes representing narrow-leafed lupin (Lupinus angustifolius L.) and yellow lupin (Lupinus luteus L.) was studied in field experiments in the low rainfall Mediterranean environment of Western Australia over 3 seasons. In each year Merrit, the most common commercial cultivar in Western Australia, was used as the reference to which the yield of other genotypes was compared. In the first and third year, 5 or 6 genotypes were grown with and without irrigation from the start of pod set. In the second year, 9 genotypes were grown with irrigation and under a rainout shelter from the start of pod set. Detailed measurements were made of plant water status, leaf area and biomass production, flowering and podding date, and seed yield and its components. The timing and intensity of the terminal drought varied from average in 1998 and 1999 to extreme in 2000. Post-podding leaf water potential (Ψleaf) under rainfed conditions decreased to -2 MPa in 1998 and 1999 and below -2.5 MPa in 2000, whereas under supplementary irrigation it was maintained at -1.2 MPa in 1998 and 1999 and at -1.5 MPa in 2002. The seed yield of all genotypes under terminal drought varied from 24 to 66% of that with supplementary irrigation. In each year, the seed yield under rainfed conditions showed genotypic differences consistent with the timing and intensity of the development of terminal drought. Under conditions of terminal drought the seed yields of the narrow-leafed lupin cultivars Belara and Tallerack, and of the breeding line WALAN 2049, were higher than of Merrit by 29% in 1998. Tanjil, Belara, and Quilinock out-yielded Merrit by 33-53% in 1999 and Belara and Quilinock out-yielded Merrit by 80% in 2000. Harvest index was higher in Belara and Quilinock than in Merrit. Under both terminal drought conditions and supplemental irrigation, Belara and Quilinock had high seed yields that were associated with a greater number of seeds per pod and larger seed size. It is argued that early flowering and podding in Belara and Quilinock allowed more seeds to develop and fill before the terminal drought became more severe.R Luper dr J. P et al

Wheat genotypes with high early vigour accumulate more nitrogen and have higher photosynthetic nitrogen use efficiency during early growth

Genotypic differences in early growth and nitrogen (N) uptake among 24 wheat (Triticum aestivum L.) genotypes were assessed in a field trial. At late tillering, large genetic variation was observed for shoot biomass (23-56gm-2 ground area) and N uptake (1.1-1.8gm-2 ground area). A strong correlation between aboveground biomass and N uptake was observed. Variation around this relationship was also found, with some genotypes having similar N uptake but large differences in aboveground biomass. A controlled environment experiment was conducted to investigate the underlying mechanisms for this variation in aboveground biomass using three vigorous genotypes (38-19, 92-11 and CV97) and a non-vigorous commercial cultivar (Janz). Vigorous genotypes had lower specific leaf N in the youngest fully expanded leaf than Janz. However, there was no difference in chlorophyll content, maximum Rubisco activity or the rate of electron transport per unit area. This suggests that Janz invested more N in non-photosynthetic components than the vigorous lines, which could explain the higher photosynthetic N use efficiency of the vigorous genotypes. The results suggest that the utilisation of wheat genotypes with high early vigour could improve the efficiency of N use for biomass production in addition to improving N uptake during early growth.

Non-invasive pressure probes magnetically clamped to leaves to monitor the water status of wheat

Background and aimsBeing able to monitor the hydration status of a plant would be useful to breeding programs and to providing insight into adaptation to water-limited environments, but most current methods are destructive or laborious. We evaluated novel non-invasive pressure probes (commercial name: ZIM-probe) for their potential in monitoring the water status of wheat (Triticum aestivum L.) leaves.MethodsThe probes consist of miniature pressure sensors that clamp to the leaves via magnets and detect relative changes in hydration status. Probes were clamped to leaves of six individual plants of the cultivar Wyalkatchem at the stem elongation stage and compared against traditional plant water relations measurements.ResultsOutput from the probes, called patch-pressure (Pp), correlated well with leaf water potential and transpiration of individual plants. Variation between plants in the original clamp pressure exerted by the magnets and leaf individual properties led to variations in the amplitude of the diurnal Pp profiles, but not in the kinetics of the curves where Pp responded simultaneously in all plants to changes in the ambient environment (light and temperature).ConclusionsDrying and rewatering cycles and analysis of the curve kinetics identified several methods that can be used to test comparisons of water status monitoring of wheat genotypes under water deficit.

Five decades of selection for yield reduced root length density and increased nitrogen uptake per unit root length in Australian wheat varieties

Background and aimsGiven the worldwide effort to improve the nitrogen (N) economy of crops, it is critical to understand the mechanisms of improved N uptake which have resulted from selection pressure for grain yield in Australian wheat (Triticum aestivum L.). Changes in root system traits and N uptake were examined in nine Australian wheat varieties released between 1958 and 2007.MethodsWheat varieties were grown in rhizo-boxes in a glasshouse. We measured nitrogen uptake and mapped root growth and proliferation to quantify root length density (RLD), root length per plant, root biomass, specific root length, and plant nitrogen uptake per unit root length.ResultsSelection for yield reduced total RLD and total root length, and increased N uptake per unit root length that overrode the reduction in root system size, effectively explaining the increase in N uptake. Importantly, N uptake in our experiment under controlled conditions matched field measurements, reinforcing the agronomic significance of the present study.ConclusionsWheat varieties released in Australia between 1958 and 2007 increased their N uptake, not because of increasing their root length and RLD, but for progressively increasing the efficiency of their root system in capturing N. Our collection of varieties is therefore an interesting model to probe for variation in the affinity of the root system for nitrate.

Foliar nitrogen applications increase the seed yield and protein content in chickpea (Cicer arietinum L.) subject to terminal drought

The effect of foliar application of isotopically labelled nitrogen ( 15 N-urea) at 4 stages during flowering and podding on the uptake and utilisation of nitrogen by chickpea (Cicer arietinum L.) under conditions of terminal drought was investigated in a glasshouse study. Five treatments were used to investigate the effect of timing of foliar application of urea, equivalent to 30 kg N/ha, on the uptake and utilisation of nitrogen for biomass, yield, seed protein content, and seed size: foliar application at (i) first flower, (ii) 50% flowering, (iii) 50% pod set, and (iv) the end of podding, and (v) an unsprayed control treatment. Terminal drought was induced from pod set onward, resulting in a rapid development of plant water deficits (-0.14 MPa/day) and a decrease in leaf photosynthesis irrespective of the timing of foliar urea application. Foliar applications of urea at first flower and at 50% flowering, before terminal drought was induced, increased yield and seed protein content. The increase in yield resulted from an increase in the number of pods with more than one seed rather than from increased pod number per plant or increased seed size, indicating greater seed survival under terminal drought. Also, the increase in the seed protein content resulted from increased nitrogen availability for seed filling. Foliar application of urea during flowering, before terminal drought was induced, resulted in 20% more biomass at maturity, suggesting that growth prior to the development of water shortage increased the carbon resources for sustained seed filling under conditions of terminal drought. Foliar applications of urea at 50% pod set and at the end of podding did not affect the yield or seed protein content, primarily because the uptake of nitrogen was limited by the leaf senescence that occurred with the development of terminal drought. The results indicate the potential to increase yields of chickpea by application of foliar nitrogen near flowering in environments in which terminal droughts reduce yield. Additional keywords: seed protein, seed survival, 15 N-urea, timing of foliar application, leaf photosynthesis.

Dryland wheat domestication changed the development of aboveground architecture for a well-structured canopy.

We examined three different-ploidy wheat species to elucidate the development of aboveground architecture and its domesticated mechanism under environment-controlled field conditions. Architecture parameters including leaf, stem, spike and canopy morphology were measured together with biomass allocation, leaf net photosynthetic rate and instantaneous water use efficiency (WUEi). Canopy biomass density was decreased from diploid to tetraploid wheat, but increased to maximum in hexaploid wheat. Population yield in hexaploid wheat was higher than in diploid wheat, but the population fitness and individual competition ability was higher in diploid wheats. Plant architecture was modified from a compact type in diploid wheats to an incompact type in tetraploid wheats, and then to a more compact type of hexaploid wheats. Biomass accumulation, population yield, harvest index and the seed to leaf ratio increased from diploid to tetraploid and hexaploid, associated with heavier specific internode weight and greater canopy biomass density in hexaploid and tetraploid than in diploid wheat. Leaf photosynthetic rate and WUEi were decreased from diploid to tetraploid and increased from tetraploid to hexaploid due to more compact leaf type in hexaploid and diploid than in tetraploid. Grain yield formation and WUEi were closely associated with spatial stance of leaves and stems. We conclude that the ideotype of dryland wheats could be based on spatial reconstruction of leaf type and further exertion of leaf photosynthetic rate.