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.     

Total DNA 5-methylcytosine level of rice calli derived from protoplasts is independent of regeneration ability

To study the roles of m⁵C in the differentiation of rice calli derived from protoplasts (protoclones), the m⁵C level of the total DNA was analyzed using the³²P post-labeling method. The level of m⁵C in regenerable and nonregenerable protoclones was similar, as was in calli and leaves of plants regenerated from the same protoclones. Treatment with 0.5 mM 5-azacytidine caused significant reduction of the level of m⁵C and of the regeneration frequency of callus. Significantly increased m⁵C levels were observed during prolonged culture.     

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.     

Characterization of New Highly Virulent German Isolates of Bremia lactucae and Efficiency of Resistance in Wild Lactuca spp. Germplasm

Four German isolates (FS1, SR2, SAW1 and DEG2) of Bremia lactucae originating from lettuce cultivars with R‐factors R18 and Dm6 + R36 were used for detailed characterization of virulence factors (v‐factors) and for the study of the resistance efficiency in wild Lactuca spp. germplasm. The highest complexity of v‐phenotype was recognized in isolate DEG2, which overcomes resistance in cv. Mariska (R18) and line CS‐RL (L. serriola × L. sativa, R18 + ?), until now known as resistant to all known races of B. lactucae in Europe. However, some sparse sporulation also occurred on cv. Titan (Dm6 + R36). The isolates SR2 and SAW1 overcome the resistance based on the gene R36, but are avirulent to R18. The v‐phenotype of SR2 is highly complex with the most important v‐factors being present except for v14 and v18. The isolate FS1 is the first in Germany originating from a cultivar with R18 (cv. Samourai). The search for efficient sources of resistance in 64 accessions of 11 wild Lactuca spp. and primitive forms of L. sativa showed broad variation in accession–isolate interactions. Expression of race‐specific resistance in wild Lactuca spp. (L. serriola, L. viminea, L. virosa) was recorded frequently. Lactuca indica and L. saligna could be considered as the most efficient sources of resistance against isolates FS1, SR2 and SAW1. The isolate DEG2 showed the highest level of virulence. On seedlings of L. saligna, which is considered as a most important source of resistance against B. lactucae, there was frequently recorded limited sporulation, however this response cannot be considered as a susceptible. Except for some L. saligna accessions (CGN 05310 and CGN 05315), the resistance to all studied isolates was only observed in one accession of L. serriola (PI 253467).     

Inhibited Long-Distance Movement of Potato Leafroll Virus to Tubers in Potato Genotypes Expressing Combined Resistance to Infection, Virus Multiplication and Accumulation

Plants of two potato clones which, in preliminary greenhouse assessments, showed resistance to multiplication and accumulation of potato leafroll virus (PLRV) were graft or aphid inoculated with the virus and grown in the greenhouse; plants of a moderately susceptible cultivar were used for comparison in all experiments. A high concentration of aphid‐borne inoculum was used to ensure strong infection pressure. Clone M62759 appeared to be highly resistant to PLRV infection, whereas clone PS1706 was more susceptible. Both clones expressed a high level of resistance to virus multiplication, when primary or secondary infection was assayed by enzyme‐linked immunosorbent assay. Moreover, PLRV was detected in only few or none of the progeny plants of clone M62759, which thus strongly inhibited virus transport to tubers. The study on PLRV translocation from aphid‐inoculated shoots to uninoculated shoots sprouted from the same tubers showed that no specific mechanisms are likely to impair PLRV movement through the tubers of the resistant genotypes. These results indicate that three valuable components of the resistance to PLRV are probably closely linked in the genotype, a combination that seems to occur rather rarely in potato clones. Nevertheless, selecting potato genotypes for the complex resistance to PLRV may prove to be a worthwhile part of breeding programmes, provided that the genetic mechanisms governing particular types of resistance are better recognized.     

Resistance of pear psylla (Cacopsylla pyri L.; Hom., Psyllidae) to deltamethrin and synergism with piperonyl butoxide

Abstract: Topical application bioassays, conducted on French populations of Cacopsylla pyri , showed resistance rates to deltamethrin ranging from 31-fold at the adult stage to 135-fold at the last larval stage. Synergism studies between deltamethrin and piperonyl butoxide (PBO), applied either topically or by tarsal contact with PBO, showed (i) that the rates of synergism were more than 10-fold in all the populations and that they could increase to a thousand-fold in some populations, (ii) that the synergistic effect was present to a similar extent at both the larval and adult stages, (iii) that there was no correlation between the synergistic effect and the resistance rates, and (iv) that no correlation was observed between the dates of collection and the rates of synergism. The results makes it impossible to determine whether resistance to deltamethrin is of genetic origin or results only from the induction of monooxygenases by allelochemical substances present in the food ingested by the species.     

Characterization of quantitative trait loci (QTLs) in cultivated rice contributing to field resistance to sheath blight (Rhizoctonia solani)

Sheath blight, caused by Rhizoctonia solani, is one of the most important diseases of rice. Despite extensive searches of the rice germ plasm, the major gene(s) which give complete resistance to the fungus have not been identified. However, there is much variation in quantitatively inherited resistance to R. solani, and this type of resistance can offer adequate protection against the pathogen under field conditions. Using 255 F₄ bulked populations from a cross between the susceptible variety Lemont and the resistant variety Teqing, 2 years of field disease evaluation and 113 well-distributed RFLP markers, we identified six quantitative trait loci (QTLs) contributing to resistance to R. solani. These QTLs are located on 6 of the 12 rice chromosomes and collectively explain approximately 60% of the genotypic variation or 47% of the phenotypic variation in the LemontxTeqing cross. One of these resistance QTLs (QSbr4a), which accounted for 6% of the genotypic variation in resistance to R. solani, appeared to be independent of associated morphological traits. The remaining five putative resistance loci (QSbr2a, QSbr3a, QSbr8a, QSbr9a and QSbr12a) all mapped to chromosomal regions also associated with increased plant height, three of which were also associated with QTLs causing later heading. This was consistent with the observation that heading date and plant height accounted for 47% of the genotypic variation in resistance to R. solani in this population. There were also weak associations between resistance to R. solani and leaf width, which were likely due to linkage with a QTL for this trait rather than to a physiological relationship.