T Botwright Acuna

Genotypic differences in root penetration ability of wheat through thin wax layers in contrasting water regimes and in the field

A significant proportion of arable land in south-western Australia is highly susceptible to subsoil compaction, which limits access of roots of wheat to water and nutrients at depth. Genotypic variation in the ability of roots to penetrate a hardpan has been reported for other cereals, using a pot technique, where a thin wax-layer of paraffin wax and petroleum jelly is placed in a soil column to simulate a hardpan. Previously we have modified and validated this technique for measuring root penetration ability of wheat seedlings under contrasting water regimes. Here we report on a series of five experiments (runs), two in well-watered and three in drought stress conditions, which evaluated seminal and nodal root penetration ability through thin wax layers among 24 Australian wheat cultivars and breeding lines (entries). These results were compared with observations on their rooting depths in two contrasting soil types in field trials, including a sandy duplex that contained a hardpan and a red clay that increased in soil strength with depth. Nodal roots ceased growth early under soil water deficit, and water uptake was instead dependant on seminal roots under conditions imposed in the pots. Plants were then reliant on the ability of seminal roots to penetrate the wax layer. Eight entries had superior root penetration ability in both well-watered and drought stressed conditions. Roots of three other entries, which failed to penetrate the wax layers, died under drought stress conditions. In field trials, there was a significant interaction between site and entry for maximum root depth. Our results from the pot studies and field trials indicate that there exists genotypic variation in root traits that are required to penetrate uniformly hard soil, dry soil or soil containing a hardpan. As four of the eight superior entries also showed superior root penetration ability at both sites in the field, there was an overall consistency, but there were exceptions at individual field sites. Factors likely to result in such exceptions were discussed, and topics for further research identified.

Inheritance of coleoptile tiller appearance and size in wheat

Selection for rapid leaf area growth has the potential to increase wheat biomass, and both water-use efficiency andweedcompetitivenessearlyintheseason.Severalmorphologicalcomponentscontributetoincreasedseedlingleafarea, including rapid seedling emergence and production of longer, wider leaves. Early emergence of a large coleoptile tiller has also been demonstrated to increase plant leaf area and biomass in wheat and other grass seedlings. Yet little is known of the extent and nature of genotypic variation for coleoptile tiller growth in wheat. A random set of 35 wheat, barley, and triticale genotypeswasevaluatedinglasshouseandoutdoorstudiesforseedlingcharacteristics,includingcoleoptiletillergrowthand total plant leaf area. Coleoptile tillers were produced more reliably for seedlings grown outdoors and when supplied with additional soil nitrogen. Genotypic differences incoleoptile tiller frequencyand leaf areawere large, ranging from0 to78% and from 0.0 to 1.4cm 2 , respectively at very early growth stages. Australian commercial wheats tended to produce fewer coleoptiletillersofsmallersizethanoverseasgermplasmwherethecoleoptiletilleraccountedforupto12%oftotalseedling leaf area. This compared favourably with mainstem tiller leaf area, which ranged from 0 to 3.5cm 2 and accounted for up to 16% of plant leaf area. Broad-sense heritabilities were high for coleoptile tiller presence and size in favourable conditions (c.75%)butlow(c.40%)forseedlingsevaluatedacrossnitrogencontent-varyingsoils.Generationmeansanalysiswasused toinvestigategeneticcontrolforcoleoptiletillergrowthacrossmultiplepopulations.Significant(P <0.05)differenceswere observed among generations for coleoptile tiller frequency and growth (numbers of leaves, leaf area, and biomass). These differences reflected strong additive genetic control with little evidence for any gene actionyear interaction. Increases in coleoptile tiller frequency and mass were correlated with larger embryo size and wider seedling leaves to increase seedling leaf area (rg=0.89). Comparisons between reciprocal F1 and F2 generation means indicated strong maternal effects for coleoptiletillergrowthinsomebutnotallcrosses.Screeninginfavourableenvironmentswillincreaseheritabilityandaidin selectionforprogeniesproducinglargecoleoptiletillers.Evidenceforadditivegeneticcontrolshouldpermitearlygeneration selection but not without some progeny-testing for coleoptile tiller growth together with other early vigour components associated with increased plant leaf area.

Root penetration ability of wheat through thin wax-layers under drought and well-watered conditions

Sand over clay duplex soils and those compacted by heavy farm machinery restrict water infiltration and root growth because roots cannot penetrate hard soil. Under drought, restriction of roots to soil above the hard layer results in the early onset of plant water-deficit, unless roots can penetrate the hard layer to reach soil water and nutrients at depth. There is little to no information on the ability of roots of bread wheat (Triticum aestivum L.) to penetrate hardpans. Here we report on 3 experiments undertaken in a controlled environment in pots that validate and explore the use of thin Paraffin wax–Vaseline (WV) layers of different strengths to simulate a hardpan under contrasting water regimes. Seeds produced an average of 5 seminal roots, which all penetrated the low-impedance wax-layer (0.03WV), in such a way that seminal root dry matter (DM) was evenly distributed throughout the soil profile. The number and depth of penetrating seminal root axes declined as wax-layer strength increased, and a significant proportion of total length and DM of main seminal root axes was instead restricted to the 0–0.12-m soil layer above the wax layer. No roots penetrated the 0.60WV, which was equivalent to ~1.50 MPa penetrometer resistance. The distribution of seminal roots was less affected by water regime than nodal roots, which were severely reduced in number when drought was imposed at 14 days after sowing (DAS), compared with well-watered conditions. Growth of the seminal roots into soil beneath the wax-layer determined the pattern of stomatal conductance and volumetric soil water content (Jv) over the period of drought stress, as few nodal roots reached and penetrated the wax layer. Stomatal conductance declined suddenly at 19 days after the last irrigation, and partially recovered as water extraction increased in the 0.40–0.60-m soil depth. Reasons for this are discussed. The wax-layer technique requires validation for wheat in the field, but the technique offers promise for screening breeding lines for the ability to penetrate a hardpan.

Nitrogen contributions in a windmill grass (Chloris truncata): wheat (Triticum aestivum L.) system in south-western Australia

Chloris truncata, a perennial grass that is native to Australia, has potential as a short-lived summer pasture in rotation with wheat and other winter crops in the low to medium rainfall zone of south-western Australia. In this paper we examine the nitrogen contributions from a C. truncata-wheat system, with the expectation that C. truncata may take up nitrate which would otherwise be lost to leaching, for later release to the following wheat crop. In glasshouse experiments, residual soil nitrate in bare soil was available for uptake and growth of wheat, with a greater response when N was applied. In contrast, wheat grown on C. truncata stubble was mostly reliant on recently mineralised nitrogen, as the previous rotation had depleted the soil of nitrate. Shoot stubble of C. truncata provided sufficient mineralised nitrogen such that the uptake of nitrogen and biomass of wheat equalled those from bare soil. Wheat grown on root stubble of C. truncata had half the biomass production of that grown on either bare soil or shoot stubble, with root + shoot stubble intermediate. In a field trial undertaken at Bruce Rock in Western Australia, nitrogen release from C. truncata stubble at low to intermediate stubble densities increased tiller production, nitrogen uptake, and growth of wheat, but not at the highest N rate in this season, which received below-average rainfall in July. These results provide initial evidence concerning how a C. truncata-wheat system could improve the N balance of the farming system, by potentially reducing the leaching loss of nitrate in autumn, and then releasing mineralised N from stubble when needed by a following wheat crop. While these results require further confirmation, especially in the field, they raise exciting prospects for an improved agronomic system, with potential benefits to N balance, carrying capacity, yield stability, and groundwater discharge. The system requires further study to quantify these processes, and explore their implications.

INCREASES IN SEED DENSITY CAN IMPROVE PLANT STAND AND INCREASE SEEDLING VIGOUR FROM SMALL SEEDS OF WHEAT ( TRITICUM AESTIVUM )

Early vigour in wheat (Triticum aestivum) is an important physiological trait to improve water-use efficiency and grain yield, especially on light soils in Mediterranean-type climates. Potential interactions for plant stand and seedling vigour between seed density and various seed quality treatments were examined for wheat grown in two experiments, conducted under controlled and field environments in Western Australia. Seed lots were graded into seed size classes and seed density fractions using saturated solutions of ammonium sulphate or sodium polytungstate. Dense seed improved plant stands or produced seedlings with greater early seedling vigour than their low-density counterparts in all three field environments. Artificial ageing reduced germination and emergence in the controlled environment. When grown in the field at Merredin, Western Australia, on the sandy soil, plant development was delayed with aged seed, and total leaf area and dry weight of plants were reduced. Fungicide application diminished total plant dry weight in sandy soils, but had a much larger detrimental effect when applied to aged and low-density seeds than normal seeds, retarding development, total leaf area and total plant dry weight. Our results indicate that an increase in seed density, particularly in small seed, can potentially improve plant stand and seedling vigour independently of seed size, and may be especially important for wheat grown on sandy soils of poor fertility and low water-holding capacity. The results also suggest consistency in seedling vigour may benefit from combined screening against small seed size and low seed density, which may also reduce the likelihood of adverse reactions to seed-applied fungicides. More attention should be paid to seed density as a valuable trait for improved reliability in plant stand and seedling vigour.

Emergence, stand establishment and vigour of deep-sown Australian and CIMMYT wheats

The short coleoptile of the majority of released Australian wheat cultivars, is well-documented to increase time to emergence and reduce stand establishment. We examined the effect of seeding depth on coleoptile length and seedling characteristics of 36 CIMMYT wheats, bred for their ability to emerge from deep sowing, relative to 14 Australian cultivars in controlled and field environments. Coleoptile length of one of the CIMMYT lines, Berkut, exceeded that of Vigour 18 when grown in the dark in a controlled environment. In a second experiment in controlled conditions, this time with a regular day/night cycle, seedlings of 4 CIMMYT and 4 Australian cultivars emerged from 5 and 8 cm sowing depths, but only 50% of the Australian wheats emerged from 11 cm, compared with 100% for the CIMMYT wheats. In a third experiment, 6 cultivars were sown at depths of 5, 8 and 11 cm at 2 field sites with different soil types ( a sandy loam duplex and a red clay) at Kukerin, Western Australia. Field site and sowing depth interacted for emergence time, stand establishment and coleoptile length, with higher values of each at the clay site. The first internode elongated at both field sites, but to a lesser extent on the clay soil, where the response was only observed with deep sowing. CIMMYT wheats Parus/Pastor and Berkut were notable for rapid emergence, a long sub-crown internode and coleoptile, and above-average leaf area in controlled and field environments, in comparison with Australian cultivars. These lines hold promise as genetic sources of improved stand establishment and early vigour in wheats for use in Australia and similar environments.