Screening Subterranean Clover (Trifolium spp.) Germplasm for Resistance to Meloidogyne species

This study was conducted to identify lines of subterranean clover (Trifolium spp.) with resistance to Meloidogyne arenaria (Neal, 1989) Chitwood, 1949, race 1; M. incognita (Kofoid and White, 1919) Chitwood, 1949, race 3; and M. javanica (Treub, 1885) Chitwood, 1949. A collection of 134 subterranean clover lines was evaluated and all had intermediate to high susceptibility. Root galling was negatively correlated with both seed and dry matter yields. Soil fumigation significantly reduced the nematode population in the field. Results indicate there is limited genetic resistance to root-knot nematodes among subterranean clover lines. Alternative sources of variation for this trait should be investigated.     

Plant resistance, plant traits, and host plant choice of the leaf-folding sawfly on the arroyo willow

Abstract. 1. The hypotheses that genetic variation in host plant resistance of the arroyo willow affected leaf folder ( Phyllocolpa sp.) (Hymenoptera: Tenthredinidae) density and that genetic variation in shoot length and leaf length was correlated with resistance were tested.
2. Willows grown in pots and exposed to ovipojsition by the leaf folding sawfly in cages had significantly different densities among clones, indicating variation in resistance caused by genetic differences among conspecific host plants.
3. There was a general correspondence between leaf folder density on potted cuttings and on the plants in the field that were the sources of cuttings.
4. In behavioural choice experiments, susceptible clones (with highest leaf fold densities) had the highest oviposition activity of female leaf folders compared to clones that were resistant to the leaf folder.
5. Clones differed significantly in shoot length and leaf length among clones grown in pots, among clones in the field, and between shoots with galls and shoots without galls on clones in the field.
6. Leaf folder density was significantly positively correlated with mean shoot length on field clones in 1985 and 1986, but was not correlated with leaf length, although leaf length and shoot length were correlated.
7. Leaf length variation among willow clones accounted for a significant portion of the variation in resistance of potted willows, but shoot length was unimportant.     

Control of Mn status and infection rate by genotype of both host and pathogen in the wheat take-all interaction

Take-all is a world-wide root-rotting disease of cereals. The causal organism of take-all of wheat is the soil-borne fungus Gaeumannomyces graminis var tritici (Ggt). No resistance to take-all, worthy of inclusion in a plant breeding programme, has been discovered in wheat but the severity of take-all is increased in host plants whose tissues are deficient for manganese (Mn). Take-all of wheat will be decreased by all techniques which lift Mn concentrations in shoots and roots of Mn-deficient hosts to adequate levels. Wheat seedlings were grown in a Mn-deficient calcareous sand in small pots and inoculated with four field isolates of Ggt. Infection by three virulent isolates was increased under conditions which were Mn deficient for the wheat host but infection by a weakly virulent isolate, already low, was further decreased. Only the three virulent isolates caused visible oxidation of Mn in vitro. The sensitivity of Ggt isolates to manganous ions in vitro did not explain the extent of infection they caused on wheat hosts. In a similar experiment four Australian wheat genotypes were grown in the same Mn-deficient calcareous sand and inoculated with one virulent isolate of Ggt. Two genotypes were inefficient at taking up manganese and were very susceptible to take-all, one was very efficient at taking up manganese and was resistant to take-all, and the fourth genotype was intermediate for both characters. All genotypes were equally resistant under Mn-adequate conditions.     

Root respiration during grain filling in sunflower: The effects of water stress

Instantaneous rates of (soil + root) respiration were measured periodically during grain filling in sunflower crops that were i) irrigated at weekly intervals and ii) subjected to water stress for the last 25 days of the 40-day grain filling period. Daily (soil + root) respiration was calculated using instantaneous respiration rates, an empirically determined temperature response function, and diurnal records of soil temperature. Daily soil respiration was estimated using empirically determined functions linking soil respiration to soil temperature and water content. Between anthesis and maturity, daily root respiration of the irrigated crop dropped by about one half from ca. 1.8 g C m^-2 d^-1, exhibiting a strong association with daily crop gross photosynthesis. Water stress brought about a rapid decrease in root respiration, which fell to about 0.1 g C m^-2 d^-1 at maturity. Root respiration during grain filling was 46 and 30 g C m^-2 for irrigated and stressed crops, respectively.     

Frankia strains of soil underBetula pendula: Behaviour in soil and in pure culture

A K/Rb isotope dilution method was used to determine the uptake of K from undisturbed subsoils. Rb was applied to the topsoil (0–30 cm) to trace the K taken up from the topsoil by crops. The K/Rb ratio in the crops increases when roots contact the Rb-free subsoil. This change in the K/Rb ratio enables the calculation of the uptake of K from the subsoil. Results of 34 field experiments on loess-parabrown soils in N. Germany showed that the subsoil (>30 cm) supplied, on average, 34% of the total K uptake by spring wheat (range 9–70%). The range between the experimental sites is considered in relation to the contents of K in the top and subsoils (as extracted by 0.025 N CaCl₂ solution), the proportion of the total root length in the subsoils, and competition for K between roots in the top and subsoil. In subsoils with similar K contents, uptake from the subsoil decreased significantly from 65 to 21% of total K uptake, as K contents in the topsoils increased from 4 to 8 mg K/100 g. On sites with the same K contents in topsoils (9 mg K/100 g), the subsoil supplied 12 to 61% of total K uptake as the K contents of the subsoil increased from 2 to 27 mg K/100 g. The contribution of uptake of K from the subsoil increased with the development of the crop, from 8% at first node stage to 35% at ear emergence, as the proportion of total root length in the subsoil increased. High root length densities in the topsoil (9 cm/cm³) resulted in competition for K between roots and increased uptake of K from the subsoil.     

The relationship between phosphate status and cyanide-resistant respiration in bean roots

Bean ( Phaseolus vulgaris L.) seedlings were cultured on complete or phosphate-deficient nutrient medium. After 14 days of culture on phosphate-deficient medium the visible symptoms of P_i deficiency were observed only in the shoot, the fresh and dry weights of the roots were slightly higher than in control plants. The decreased P_i content in the roots had little effect on total respiration rate but had an effect on the level of inhibition of respiration by cyanide. The high resistance of respiration to cyanide observed in P_i-deficient roots was the result of the suppression of cytochrome path activity and an increased participation of the alternative, cyanide-resistant pathway. The cytochrome pathway activity increased when inorganic phosphate was supplied to P_i-deficient roots for 1 or 3.5 h. It is speculated that the suppression of cytochrome pathway in P_i-deficient roots may result from restriction of the phosphorylating capacity or a partial inhibition of cytochrome oxidase activity.     

Effect of zeatin on the growth and indolyl-3-acetic acid and abscisic acid levels in maize roots

Elongation, indolyl-3-acetic acid (IAA) and abscisic acid (ABA) levels, – gas chromatography-mass spectrometry quantification –, in the elongating zone were analysed for maize ( Zea mays L., Cv. LG11) roots immersed in buffer solution with or without zeatin (Z). The effect of Z depends on the initial extension rate of roots. The slower growing roots are more strongly inhibited by Z (10⁻⁷−10₋₅ M ) and they show a greater increase in IAA and ABA content. When compared to the rapidly growing roots, the larger reactivity of the 'slow'ones cannot be attributed to a higher Z uptake as shown when using [¹⁴C]-Z. It is suggested that Z could regulate root elongation by acting on the IAA and/or ABA level. The comparative action of these two hormones is discussed.     

Net Mg2+ uptake in relation to the amount of exchangeable Mg2+ in the Donnan free space of ryegrass roots

Ammonium acetate and BaCl₂-triethanolamine were used to desorb Mg²⁺ from the root Donnan free space (DFS) of 23-d-old ryegrass (Lolium multiflorum Lam. cvs. Gulf and Wilo). Amounts of desorbed Mg²⁺ increased with the increase in Mg²⁺ activity of the nutrient solution. Slightly less Mg²⁺ was desorbed by Ba²⁺ than by NH₄ ⁺. Previously published data on short-term net Mg²⁺ uptake by intact 23-d-old ryegrass plants of the two cultivars were linearly related to the amount of exchangeable Mg⁺ desorbed from the root DFS (r²=0.90 and 0.81 for the desorption by NH₄ ⁺ and Ba²⁺, respectively). A sward of Mg²⁺ ions attracted to the negative charges of the cell surface is suggested to represent a part of a pool of Mg²⁺ available for active transport through the plasmalemma.     

Comparative effects of organic and inorganic nitrogen sources applied to a flooded soil on rice yield and availability of N

A pot experiment was conducted to study the effect of organic and inorganic nitrogen (N) sources on the yield and N uptake of rice from applied and native soil-N. The residual effect of these N sources on a succeeding wheat crop was also studied. Organic N was applied in the form of ¹⁵N-labelled Sesbania aculeata L., a legume, and inorganic N in the form of ¹⁵N-labelled ammonium sulphate. The two sources were applied to the soil separately or together at the time of transplanting rice. Recovery of N by rice from both the applied sources was quite low but both sources caused significant increases in biomass and N yield of rice. Maximum increase was recorded in soil treated with organic N. The residual value of the two materials as source of N for wheat was not significant; the wheat took up only a small fraction of the N initially applied. Loss of N occurred from both applied N sources, the losses being more from inorganic N. Both applied N sources caused a substantial increase in the availability of soil-N to rice and wheat; most of this increase was due to organic N and was attributed to the so-called ‘priming’ effect or ANI (added nitrogen interaction) of the applied material.     

Genotypical differences among graminaceous species in release of phytosiderophores and uptake of iron phytosiderophores

Graminaceous species can enhance iron (Fe) acquisition from sparingly soluble inorganic Fe(III) compounds by release of phytosiderophores (PS) which mobilize Fe(III) by chelation. In most graminaceous species Fe deficiency increases the rate of PS release from roots by a factor of 10–20, but in some species, for example sorghum, this increase is much less. The chemical nature of PS can differ between species and even cultivars.The various PS are similarly effective as the microbial siderophore Desferal (ferrioxamine B methane sulfonate) in mobilizing Fe(III) from a calcareous soil. Under the same conditions the synthetic chelator DTPA (diaethylenetriamine pentaacetic acid) is ineffective.The rate of Fe(III)PS uptake by roots of graminaceous species increases by a factor of about 5 under Fe deficiency. In contrast, uptake of Fe from both synthetic and microbial Fe(III) chelates is much lower and not affected by the Fe nutritional status of the plants. This indicates that in graminaceous species under Fe deficiency a specific uptake system for FePS is activated. In contrast, the specific uptake system for FePS is absent in dicots. In a given graminaceous species the uptake rates of the various FePS are similar, but vary between species by a factor of upto 3. In sorghum, despite the low rate of PS release, the rate of FePS uptake is particularly high.The results indicate that release of PS and subsequent uptake of FePS are under different genetic control. The high susceptibility of sorghum to Fe deficiency (lime-chlorosis) is most probably caused by low rates of PS release in the early seedling stage. Therefore in sorghum, and presumably other graminaceous species also, an increase in resistance to lime chlorosis could be best achieved by breeding for cultivars with high rates of PS release. In corresponding screening procedures attention should be paid to the effects of iron nutritional status and daytime on PS release as well as on rapid microbial degradation of PS.