THE DISTRIBUTION AND ORIGIN OF GENES FOR RACE-SPECIFIC RESISTANCE TO MELAMPSORA LINI IN LINUM MARGINALE

The genetic basis of resistance in wild flax (Linum marginale) to its host-specific pathogen Melampsora lini was investigated in seven lines collected from a single population growing at Kiandra, New South Wales and in an additional ten lines collected more widely across southeastern Australia. All lines showed different phenotypic patterns of resistance and susceptibility. Genetic analyses indicated the presence of single dominant genes for race-specific resistance in all but one of these lines. That particular line appeared to carry two linked dominant genes for resistance. Intercrosses between lines in each of these groups of L. marginale detected substantially more linkage between the resistance genes in the Kiandra population sample than between those in the broader geographic collection. This result is interpreted to indicate a possible mechanism whereby resistance genes are generated in natural populations.     

A linkage map of diploid Avena based on RFLP loci and a locus conferring resistance to nine isolates of Puccinia coronata var. ‘avenae’

An F₂ oat population was produced by crossing the diploid (n=7) species Avena strigosa (CI 3815) with A. wiestii (CI 1994), resistant and susceptible, respectively, to 40 isolates of Puccinia coronata, the causal agent of crown rust. Eighty-eight F₂ individuals were used to construct an RFLP linkage map representing the A genome of cultivated hexaploid oat. Two hundred and eight RFLP loci have been placed into 10 linkage groups. This map covers 2416 cM, with an average of 12 cM between RFLP loci. Eighty-eight F₃ lines, derived from F₂ individuals used to construct the map, were screened for resistance to 9 isolates of P. coronata. One locus, Pca, was found to confer a dominant resistance phenotype to isolates 203, 258, 263, 264B, 290, 298, 325A, and 345. Pca also conferred resistance to isolate 276; however, an unlinked second gene may also be involved.Journal Paper No. 15143 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 3134 and 2447     

Inheritance of black sigatoka disease resistance in plantain-banana (Musa spp.) hybrids

Black sigatoka (Mycosphaerella fijiensis Morelet), an airborne fungal leaf-spot disease, is a major constraint to plantain and banana (Musa spp.) production world-wide. Gaining further knowledge of the genetics of host-plant resistance will enhance the development of resistant cultivars, which is considered to be the most appropriate means to achieve stable production. Genetic analysis was conducted on 101 euploid (2x, 3x and 4x) progenies, obtained from crossing two susceptible triploid plantain cultivars with the resistant wild diploid banana Calcutta 4. Segregating progenies, and a susceptible reference plantain cultivar, were evaluated over 2 consecutive years. Three distinct levels of host response to black sigatoka were defined as follows: susceptible (< 8 leaves without spots), less susceptible (8–10) and partially resistant (> 10). Segregation ratios for resistance at the 2x level fitted a genetic model having one major recessive resistance allele (bs ₁) and two independent alleles with additive effects (bsr ₂ and bsr ₃). A similar model explains the results at the 4x level assuming that the favourable resistance alleles have a dosage effect when four copies of them are present in their respective loci (bs _i ⁴ ). The proposed model was further validated by segregation data of S ₁ progenies. Mechanisms of black sigatoka resistance are discussed in relation to the genetic model.     

零下低温对杂交杨树皮层膜脂组成的影响

以不耐寒的美洲黑杨（Ｐｏｐｕｌｕｓｄｅｌｔｏｉｄｅｓｃｖ．“Ｌｕｘ”Ｉ－６９／５５,父本）和耐寒性较强的欧美杨（Ｐ．ｅｕｒａｍｅｒｉｃａｎａｃｌｃｖ．Ｉ－４５／５１,母本）的４个杂交Ｆ＿１代无性系（９５杨、５５９杨、６００杨和１３８１杨）为材料,分析了零下低温寒潮前后枝条皮层的脂质组成。结果表明,寒潮影响下,皮层中磷脂含量增加而组成基本不变,膜脂脂肪酸组成的变化规律是：寒潮前脂肪酸不饱和指数（ＩＵＦＡ）值大的无性系,寒潮前后的ＩＵＦＡ值变化量小;寒潮前ＩＵＦＡ值较小的无性系,寒潮前后ＩＵＦＡ值变化量较大。本文借用力学概念,提出相对抗性概念,给出杨树无性系的相对抗性序列。序列表明Ｆ＿１代无性系的耐寒性已较不耐寒的父本提高,这与田间观察基本一致。     

An In Vitro Test for Temperature Sensitivity and Resistance to Meloidogyne incognita in Tomato

An in vitro root explant tissue culture technique is described for determining susceptibility of tomato (Lycopersicon esculentum Mill.) breeding lines and cultivars to the root-knot nematode Meloidogyne incognita. Root explants were taken from 2-day-old seedlings cultured for 30 days at 28 C on Gamborg''s B-5 medium with or without nematode inoculum. The remaining portion of the root and stem from the excised root explants was transferred to soil in pots and grown to maturity in the greenhouse. In vitro root explants were evaluated for growth and occurrence of juveniles, adults, and egg masses. The regenerated plants were used to produce more seed, The proposed technique is simple, reliable, and adapted to routine screening of large numbers of F₁ and F₂ samples, and it utilizes less space than tests performed on intact plants in the greenhouse or growth chamber. Evidence is presented also on the breakdown of resistance to M. incognita under high temperature stress using this in vitro root explant technique.     

Influence of A1 cytoplasmic substitution on the downy-mildew incidence of pearl millet

Large-scale cultivation of pearl millet [Pennisetum glaucum (L.) R. Br. F₁ hybrids in India has led to increased incidence of downy-mildew (Sclerospora graminicola). There is concern that the A₁ male-sterile cytoplasm used in all the hybrids released so far is responsible for this increase. The influence of A₁ malesterile cytoplasm on downy-mildew incidence in pearl millet was studied by comparing the disease reaction of 40 pairs of F₁ hybrids, each pair carrying respectively a₁ male-sterile and normal B cytoplasm. Mean downy-mildew incidence was similar in the hybrids carrying either A₁ male-sterile or B cytoplasm. The general combining ability of lines with and without A₁ cytoplasm was found to be similar for downy-mildew incidence. These results indicated that in pearl millet A₁ cytoplasm is not associated with increased downymildew incidence. The possible danger of using only one source of cytoplasm has been briefly discussed.     

The need for micrometeorological research of the response of the energy balance of vegetated surfaces to CO2 enrichment

A Penman-Monteith equation has been used to evaluate a change in canopy resistance on the evapotranspiration of a savannah and agricultural area in Botswana. After a short introduction, some problems concerning the K-theory or first order closure are indicated when one uses it for transport modelling within and above a canopy. The Penman-Monteith equation was used to calculate the canopy resistance for a savannah vegetation and sorghum under the same environmental conditions. After that, by changing the stomatal resistance due to an increase of the CO₂ content, the change in the evapotranspiration was estimated.Finally some recommendations for future research are given and an outline of a proposed FACE experiment is presented.     

Interpretive value of symptoms of infection by Gaeumannomyces graminis in wheat seedlings grown in sand culture

Symptoms of infection by the fungal pathogen Gaeumannomyces graminis were studied in roots of wheat seedlings grown in sand culture in order to assess resistance. Three sets of wheat were studied, 10 cultivars previously thought to differ in infection, and 44 and 54 segregating families from two crosses between four of these wheats, Aus1080 × Condor and Termu89–72 × Bayonet respectively. The isolates of G. graminis employed had previously been selected for differences in virulence. The interpretive value of symptoms of infection were considered, by comparison with direct measurement of the spread of hyphae from a fixed point of inoculation, and by repeatability of measurements over experiments. Two symptoms of infection, cortical browning and stelar blackening, are thought to be host mechanisms to resist hyphal invasion and were suppressed by rapid colonisation of root tissues. A third symptom, general discolouring of tissue, appeared to be a necrotic symptom of aggressive colonisation. Since only root segments distal to the point of infection were studied, stelar blackening appeared the least flexible measure of severity of infection. Cortical browning appeared to be influenced by nutritive status of seedlings, and was negatively associated with the incidence of necrotic discolorations. Wheats differed consistently and mostly in cortical browning, and while also differing in extent of necrotic discoloration, difficulty in quantifying the latter character limited its value in assessing resistance to G. graminis in this study.