Oxygen isotope ratios of plant available phosphate in lowland tropical forest soils

Phosphorus (P) cycles rapidly in lowland tropical forest soils, but the process have been proven difficult to quantify. Recently it was demonstrated that valuable data on soil P transformations can be derived from the natural abundance of stable oxygen isotopes in phosphate (δ¹⁸O_P). Here, we measured the δ¹⁸O_P of soils that had received long-term nutrient additions (P, nitrogen, and potassium) or litter manipulations in lowland tropical forest in Panama and performed controlled incubations of fresh soils amended with a single pulse of P. To detect whether δ¹⁸O_P values measured in the incubations apply also for soils in the field, we examined the δ¹⁸O_P values after rewetting dry soils. In the incubations, resin-P δ¹⁸O_P values converged to ∼3.5‰ above the expected isotopic equilibrium with soil water. This contrasts with extra-tropical soils in which the δ¹⁸O_P of resin-P matches the expected equilibrium with soil water. Identical above-equilibrium resin-P δ¹⁸O_P values were also found in field soils that did not receive P additions or extra litter. We suggest that the 3.5‰ above-equilibrium δ¹⁸O_P values reflect a steady state between microbial uptake of phosphate (which enriches the remaining phosphate with the heavier isotopologues) and the release of isotopically equilibrated cell internal phosphate back to the soil. We also found that soil nutrient status affected the microbial turnover rate because in soils that had received chronic P addition, the original δ¹⁸O_P signature of the fertilizer was preserved for at least eight weeks, indicating that the off-equilibrium δ¹⁸O_P values produced during microbial phosphate turnover was not imprinted in these soils. Overall, our results demonstrate that ongoing microbial turnover of phosphate mediates its biological availability in lowland tropical soils.     

Differential effect of nematicide treatments on tuber yields in early- and late-maturing potato cultivars

A 3-year field study examined the effects of 1,3-dichloropropene or oxamyl on tuber yields in four early- and five late-maturing potato cultivars. The nematicide treatments increased total tuber yield by ≈ 12% for early-maturing cultivars, but by less than 2% for the late-maturing cultivars. The treatments reduced the numbers of root lesion nematodes (primarily Pratylenchus penetrans ), which were the dominant plant parasitic nematodes at the site. Covariance analysis indicated that treatment effects on total tuber yield were no longer significant after adjusting for root lesion nematodes in soil at harvest, confirming that these nematodes had a direct effect on the host. As expected, there were significant inverse relationships ( P  = 0·05) between total tuber yields and numbers of root lesion nematodes in soil and in roots. However, the application of nematicides in late-maturing cultivars was found to be of no practical value. Crop production systems designed to reduce or optimize pesticide applications can form one criterion in the development of more environmentally sensitive management strategies. The merit of utilizing chemicals for root lesion nematode control are considered with regard to cultivar maturity.     

Differences in sheep production between sites within a heterogeneous area

Two stocking rate experiments were performed within an area of 160 ha with (a) weaners (randomised blocks) and (b) breeding ewes (completely randomised design). Using a quadratic economic objective function to combine outputs of wool, lambs, etc., the range of estimates of optimum stocking rates was, for weaners, from 9 to more than 25 per hectare and, for ewes 8 to 20 per hectare. Analyses of variance showed that residual errors were large compared with the effects of stocking rate, management and years. Where attributes of production were relatively accurately measured and sampled, the between site-within stocking rate variance accounted for approximately 75% of the total residual error.Differences in soil depth, aspect, slope, soil moisture and nutrient characteristics are suggested as the fundamental causes of these block differences. However, the few comparisons available did not permit further analysis. Possible benefits to both farm management and experimentation from predicting site potential from these parameters are discussed.     

Endophytic bacterial communities in the periderm of potato tubers and their potential to improve resistance to soil-borne plant pathogens

To evaluate whether the location of bacterial endophyte communities contributes to disease resistance in potato tubers ( Solanum tuberosum ), the population density, biodiversity and antibiotic activity of endophytic bacteria was examined from the tuber peel (periderm plus top 3 mm of tissue) of four cultivars (Russet Burbank, Kennebec, Butte and Shepody). There were no significant differences for population density of bacteria among the layers of peel examined and no cultivar × peel layer interaction. Endophytic bacteria from several layers of peel were challenged in in vitro bioassays to the soil-borne plant pathogens Fusarium sambucinum , Fusarium avenaceum , Fusarium oxysporum and Phytophthora infestans (mating types A1 and A2). In general, antibiosis of bacterial endophytes against these pathogens was significantly higher ( P  = 0.01) in isolates recovered from the outermost layer of tuber peel and decreased progressively toward the centre of the tuber. Antibiosis against P. infestans was variable, with a progressive decrease in antibiotic activity from outer to inner layers of peel occurring in cvs Russet Burbank and Kennebec only. For antibiosis there were significant cultivar × peel, and cultivar × pathogen interactions ( P  = 0.01). In all cases the inhibitory activity of endophytic bacteria was significantly greater ( P  = 0.01) against the A1 than the A2 mating type of P. infestans . In four of seven cases, where the same species of bacteria were recovered from all three peel layers, antibiosis to pathogens decreased significantly ( P  = 0.01) with depth of recovery (from the periderm to inside the tuber), indicating that in certain communities of endophytic bacteria, defence against pathogens may be related to bacterial adaptation to location within a host and may be tissue-type and tissue-site specific.     

Gonadal Stage and Sex Steroid Correlations in Siberian Sturgeon,Acipenser baeri,Habituated to a Semitropical Environment

Stages of gonadal development, in association with plasma concentrations of the sex steroids 17β‐estradiol (E2), progesterone (P4), testosterone (T), and 11‐ketotestosterone (11‐KT), were investigated for a single time point during a natural breeding season in 7‐yr‐old Siberian sturgeon, Acipenser baeri Brandt, exposed lifelong to a warmwater environment. Among females, examination of gonadal tissue showed variation in ovarian stage, with 12.5, 47.5, 22.5, and 17.5% of females found at Stages 2 (previtellogenic), 3 (early vitellogenic), 4 (mid‐vitellogenic), and 5 (migratory nucleus), respectively. Although patterns varied among the hormones, plasma concentrations of E2, T, and 11‐KT became increasingly elevated in females as maturation progressed. On the basis of histological criteria, males were classified as either premeiotic (quiescent) or meiotic and 50% of the males sampled were found at each stage. Significant elevations in circulating concentrations of plasma E2 and T were observed in meiotic versus premeiotic males, and there was a rise in plasma 11‐KT concentration that approached significance (P = 0.056).     

The effect of timing and severity of water deficit on growth,development, yield accumulation and nitrogen fixation of mungbean

Mungbean (Vigna radiata L.), as a dryland grain legume, is exposed to varying timing and severity of water deficit, which results in variability in grain yield, nitrogen accumulation and grain quality. In this field study, mungbean crops were exposed to varying timing and severity of water deficit in order to examine: (1) contribution of the second flush of pods to final grain yield with variable timing of relief from water deficit, (2) the sensitivity to water deficit of the accumulation of biomass and nitrogen (N) and its partitioning to grain, and (3) how the timing of water deficit affects the pattern of harvest index (HI) increase through pod filling. The results showed that the contribution of the second flush to final yield is highly variable (1–56%) and can be considerable, especially where mid-season stress is relieved at early pod filling. The capacity to produce a second flush of pods did not compensate fully for yield reduction due to water stress. Relief from mid-season stress also resulted in continued leaf production, N₂ fixation and vegetative biomass accumulation during pod filling. Despite the wide variation in the degree of change in vegetative biomass and N during pod filling, there were strong relationships between grain yield and net-above-ground biomass at maturity, and grain N and above-ground N at maturity. Only in the extreme situations were HI and nitrogen HI affected noticeably. In those treatments where there was a large second flush of pods, there was a pronounced biphasic pattern to pod number production, with HI also progressing through two distinct phases of increase separated by a plateau. The proportion of grain yield contributed to by biomass produced before pod filling varied from 0 to 61% with the contribution greatest under terminal water deficit. There was a larger effect of water deficit on N accumulation, and hence N₂ fixation, than on biomass accumulation. The study confirmed the applicability of a number of long-standing physiological concepts to the analysis of the effect of water deficit on mungbean, but also highlighted the difficulty of accounting for timing effects of water deficit where second flushes of pods alter canopy development, biomass and yield accumulation, and N dynamics.     

The Rht13 dwarfing gene reduces peduncle length and plant height to increase grain number and yield of wheat

The green-revolution Rht-B1b and Rht-D1b dwarfing alleles are usually associated with increased wheat yields but are linked to reduced early growth and poor emergence if sowing conditions are unfavourable. Other dwarfing genes are available but not used in commercial breeding. The Rht13 bread wheat donor, Magnif M1, produces uniquely short peduncle and penultimate internodes to reduce plant height. A set of near-isogenic (NILs) and recombinant inbred (RILs) lines varying for height were developed from the cross of Magnif M1 and the Rht8c-containing Chuan-mai 18, and evaluated for a range of agronomic characteristics across favourable environments. Reductions in plant height were associated with increased grain number (r² = 0.35**) and harvest index (r² = 0.62**) in the NILs. Reduced-height RILs containing the Rht13-linked, Xgwm577_M microsatellite marker were significantly shorter, produced greater biomass, yield and harvest index, and increased spike and grain number than lines without the marker. Approximately 74 and 7% of the total phenotypic variance in plant height was accounted for by allelic differences in Xgwm577 and Rht8 loci, respectively. The peduncle and penultimate peduncle internodes of Rht13-containing lines were proportionately shorter than Rht8c-containing sibs and lines containing the Rht-B1b dwarfing allele. The unique height-reducing phenotype, increased grain number and yield associated with Rht13 indicate considerable potential for use of this dwarfing allele for improving wheat performance.     

Yield benefits of triticale traits for wheat under current and future climates

Triticale often out-yields wheat in both favourable and unfavourable growing conditions. Observed traits suggested for the higher yields in triticale include greater early vigour, a longer spike formation phase with same duration to flowering, reduced tillering, increased remobilization of carbohydrates to the grain, early vigorous root growth and higher transpiration use efficiency. To quantify the impact of these traits systematically across seasons and contrasting rainfall regions and soil types, these triticale traits were introduced into a wheat model (APSIM-Nwheat). The impact of each individual trait and their full combination was analysed in a simulation experiment for three Mediterranean growing environments, two contrasting soil types and long-term historical weather data. The simulated impact of these traits was compared with measured impacts from a range of field experiments across several environments. Simulated responses of various crop characteristics including yield, were in general similar to responses observed in wheat-triticale comparison field experiments across a large range of growing conditions. The simulation analysis indicated that the yield response to the incorporation of the triticale traits into wheat was positive, in both low and high yielding growing conditions, similar to measured differences, but the simulated benefit was on average lower than the range observed in data of triticale and wheat. This suggests that other traits might also be involved in higher-yielding triticale, or the magnitude of some of the traits may be underestimated in field experiments due to ‘trait by environment’ interactions. The simulation results suggest the highest yield benefit can be achieved from increasing transpiration use efficiency in wheat, but early vigour, remobilization of stem carbohydrates and early root growth also contribute positively to a yield increase in the different growing environments. The yield benefits from the triticale traits increased in the future climate change scenario in particular on soils with high water-holding capacity from contributions of increased early vigour, remobilization of stem carbohydrates and transpiration use efficiency, and remained stable on the lighter soils.     

Effect of defoliation by livestock on stem canker caused by Leptosphaeria maculans in Brassica napus

Brassica napus (canola, oilseed rape), an important break crop for cereals across the Australian wheat belt, is being rapidly adopted as a dual‐purpose (forage and grain) crop in mixed farming systems. Stem canker caused by the fungus Leptosphaeria maculans is the most important disease of B. napus in Australia. The primary source of inoculum is airborne ascospores released during autumn/winter which coincides with the grazing of dual‐purpose crops. Field experiments were defoliated by sheep to determine the effect of grazing on blackleg stem canker severity at plant maturity in B. napus cultivars differing in their resistance level and grazed at different times. One cultivar was sown on different dates to investigate the impact of grazing at the same time, but at different growth stages. Defoliation by mowing was compared to defoliation by livestock. Similar amounts of dry matter remained after defoliation by machinery (0·66 t ha^?1) or livestock (0·52 t ha^?1). However, stem canker severity was higher in the grazed (40% of crown cross‐section diseased) compared with the mown (25%) treatment, which was higher than the ungrazed control (9%). Stem canker severity generally increased with grazing, but the increase was eliminated or reduced in cultivars with good resistance. Grazing during vegetative plant growth minimized the increase in stem canker severity compared with grazing during reproductive growth. Currently, cultivars with good L. maculans resistance are recommended in high disease situations. To avoid excessive yield loss in dual‐purpose B. napus crops due to L. maculans it is recommended that such cultivars are grown even in low‐moderate disease situations.     

A second world-wide migration and population displacement of Phytophthora infestans?

The appearance of the A2 mating type (previously restricted to central Mexico) in Europe during the 1980s prompted an investigation of the genetic make-up of European populations using allozyme loci as genetic markers. The investigation shows that major genetic changes have occurred in populations of Phytophthora infestans in the Netherlands, Poland, and the British Isles. It now appears that a new type of population has been introduced into several locations, and has displaced or is displacing the original populations in these locations. The new and old population types are characterized by unique allozyme alleles and genotypes. The mechanism for displacement of the'old'by the'new'population is not yet known.