Inheritance of partial resistance to the leaf aphids Macrosiphum euphorbiae and Uroleucon sonchi in lettuce

The genetics of partial resistance (PR) of lettuce to the leaf aphid species Macrosiphum euphorbiae and Uroleucon sonchi was studied in F₁ and F₂ generations of six crosses between four cultivars. Partial resistance inherited mainly additively. With M. euphorbiae the estimates of broad-sense F₂ heritability were low and non-significant in crosses between parents of the same resistance level (S x S and PR x PR), and high (0.69-0.76) in crosses between parents of different resistance level (PR x S). The level of resistance in lettuce cultivars ‘Charan’ and ‘Marbello’ led to a marked reduction in percentage of plants with colonies of M. euphorbiae in a field experiment. With U. sonchi the highest estimates of F₂ heritability were found in ‘Charan x ‘Avoncrisp’ (0.51) and ‘Charan’ x ‘Chou de Naples' (0.48). In the cross ‘Charan’ x ‘Marbello’ (PR x PR) no significant segregation for resistance to M. euphorbiae was found, indicating that these cultivars may have the same genetic constitution for resistance to M. euphorbiae.‘Charan’ has additional genes for resistance to U. sonchi, that are not, or only weakly linked to the gene(s) for resistance to M. euphorbiae. The heritability estimates for the partial resistance to M. euphorbiae and U. sonchi indicate that it is possible to effectively select for these traits in plant breeding programmes.     

Chemical Composition and Allelopathic Potential of Essential Oils from Tipuana tipu (Benth.) Kuntze Cultivated in Tunisia

In Tunisia, Tipuana tipu (Benth .) Kuntze is an exotic tree, which was introduced many years ago and planted as ornamental street, garden, and park tree. The present work reported, for the first time, the chemical composition and evaluates the allelopathic effect of the hydrodistilled essential oils of the different parts of this tree, viz., roots, stems, leaves, flowers, and pods gathered in the area of Sousse, a coastal region, in the East of Tunisia. In total, 86 compounds representing 89.9 – 94.9% of the whole oil composition, were identified in these oils by GC‐FID and GC/MS analyses. The root essential oil was clearly distinguished for its high content in sesquiterpene hydrocarbons (β‐caryophyllene, 1 (44); 24.1% and germacrene D, 2 (53); 20.0%), while those obtained from pods, leaves, stems, and flowers were dominated by non‐terpene hydrocarbons. The most important ones were n‐tetradecane (41, 16.3%, pod oil), 1,7‐dimethylnaphthalene (43, 15.6%, leaf oil), and n‐octadecane (77, 13.1%, stem oil). The leaf oil was rich in the apocarotene (E)‐β‐ionone ( 4 (54); 33.8%), and the oil obtained from flowers was characterized by hexahydrofarnesylacetone ( 5 (81); 19.9%) and methyl hexadecanoate (83, 10.2%). Principal component and hierarchical cluster analyses separated the five essential oils into three groups and two subgroups, each characterized by the major oil constituents. Contact tests showed that the germination of lettuce seeds was totally inhibited by the root essential oil tested at 1 mg/ml. The inhibitory effect on the shoot and root elongation varied from ?1.6% to ?32.4%, and from ?2.5% to ?64.4%, respectively.     

Response of lettuce to pre- and post-transplant phosphorus and pre-transplant inoculation with a VA-mycorrhizal fungus

Pre-transplant inoculation of lettuce (Lactuca sativa L.) seedlings with the vesicular-arbuscular mycorrhizal fungusGlomus aggregatum (Smith and Schenck emend. Koske) increased P uptake and dry matter yields after transplanting into soil when the concentration of P in the soil solution was 0.02 mg L^–1 but had little affect in soil with 0.30 mg L^–1 solution P. Tissue P concentrations and dry matter yields after transplanting were increased by supplying adequate P prior to transplanting. Adequate levels of pre-transplant P appeared to be more important than maximum mycorrhizal infection of transplants for promoting post-transplant growth of the fast maturing lettuce crop.Journal Series No. 0000 of the Hawaii Institute of Tropical Agriculture and Human Resources.     

Regulation of phenylalanine ammonia-lyase activity and growth in lettuce by light and gibberellic acid

Abstract The effects of light and gibberellic acid (GA3) on growth and phenylalanine ammonia-lyase (PAL) activity were studied in seedlings of lettuce (Lactuca sativa L.). Using an in vivo assay for PAL it was shown that wounding caused by excising hypocotyls results in an increase in PAL activity with time that can mask the effect of light on the activity of this enzyme. When hypocotyl sections were excised from light-treated seedlings immediately prior to the in vivo assay of PAL, light was shown to cause a marked increase in PAL activity. Experiments with an inhibitor of PAL activity, α-aminooxy-β-phenylpropionic acid (AOPP), confirmed that the volatile radioactive products measured in the in vivo assay resulted from the activity of PAL. Gibberellic acid suppresses the light-induced increase in PAL activity and there is an inverse relationship between GA₃-induced growth and the activity of PAL. Over a wide range of GA₃ concentrations, the activity of PAL is also inversely correlated with growth rate along the length of the hypocotyl section; the upper halves of sections elongate more rapidly and have lower levels of PAL than the lower halves. Despite the strong correlation between growth and PAL activity, experiments with AOPP and t-cinnamic acid show that it is unlikely that elongation is regulated directly by products of PAL activity.     

C and N utilization of two lettuce genotypes during growth under non-varying light conditions and after changing the light intensity

Growth and N-incorporation in two lettuce genotypes ( Lactuca sativa L. cv. Deci minor and cv. Grosse brune), which differ significantly in nitrate accumulation, were studied. Under constant environmental conditions cv. Deci minor produced more fresh and dry weight than cv. Grosse brune. Cultivar Deci minor also produced more fresh weight per mmol N absorbed than cv. Grosse brune, and contained less organic nitrogen in the dry matter, but accumulated more nitrate. As cv. Deci minor showed a higher fresh and dry weight production per mmol N absorbed than cv. Grosse brune, it used its nitrogen more efficiently.
When the light intensity was decreased, the growth of both cultivars decreased, and the fresh weight production per mmol N absorbed increased. After reduction of the light intensity, cv. Deci minor maintained a higher fresh weight production per N absorbed than before, whereas cv. Grosse brune returned to its original level. After decrease of the light intensity, an increased nitrate concentration in the cell sap was accompanied by a decreased concentration of organic compounds in both cultivars. The organic nitrogen level in the dry matter remained constant after the higher intensity was reduced. However, due to the decreased dry weight percentage, the demand for nitrogen for protein synthesis decreased on fresh weight basis.
It can be concluded that the two cultivars differ in their partition of C and N between dry matter and cell sap. Nitrate accumulation in preference to accumulation of organic compounds does not automatically result from a shortage of organic compounds. The high accumulation of nitrate of cv. Deci minor enables it to use more carbohydrates for structural growth than cv. Grosse brune.     

Molecular diversity at the major cluster of disease resistance genes in cultivated and wild Lactuca spp.

Diversity was analyzed in wild and cultivated Lactuca germplasm using molecular markers derived from resistance genes of the NBS-LRR type. Three molecular markers, one microsatellite marker and two SCAR markers that amplified LRR-encoding regions, were developed from sequences of resistance gene homologs at the main resistance gene cluster in lettuce. Variation for these markers were assessed in germplasm including accessions of cultivated lettuce, Lactuca sativa L. and three wild Lactuca spp., L. serriola L., L. saligna and L. virosa L. Diversity was also studied within and between natural populations of L. serriola from Israel and California; the former is close to the center of diversity for Lactuca spp. while the latter is an area of more recent colonization. Large numbers of haplotypes were detected indicating the presence of numerous resistance genes in wild species. The diversity in haplotypes provided evidence for gene duplication and unequal crossing-over during the evolution of this cluster of resistance genes. However, there was no evidence for duplications and deletions within the LRR-encoding regions studied. The three markers were highly correlated with resistance phenotypes in L. sativa. They were able to discriminate between accessions that had previously been shown to be resistant to all known isolates of Bremia lactucae. Therefore, these markers will be highly informative for the establishment of core collections and marker-aided selection. A hierarchical analysis of the population structure of L. serriola showed that countries, as well as locations, were significantly differentiated. These differences may reflect local founder effects and/or divergent selection. Received: 7 March 1999 / Accepted: 25 March 1999     

Rooting characteristics of lettuce grown in irrigated sand beds

To avoid the current water pollution from intensive glasshouse horticulture, closed systems have to be developed with recirculating drainage water. For crops with a high planting density, such as letuuce, shallow beds of coarse sand may be used if water and nutrient supply can be regulated adequately. The aim of the present study was to determine the rooting characteristics and root distribution of lettuce in sand beds, as affected by substrate depth, the distance to a drain, drip lines and drip points, and the excess of nutrient solution applied. The hypothesis was tested that a small excess and a large distance between drip points leads to local salt accumulations in the root environment and thus to a less homogeneous root distribution.The data confirmed both parts of the hypothesis: spatial patterns in salt distribution were found. Detailed measurements in a sand bed with only one drip line per two crop rows and an amount of fertigation solution added of 2 times the estimated evapotranspiration, showed that root length density was negatively correlated with salt content when comparisons were made within the same layer. Crop yield per row was influenced in the extreme treatment, i.e. one drip line per two crop rows and an amount of fertigation solution added of 1.3 times the estimated evapotranspiration, but yield per bed was still unaffected. The increased heterogeneity of the crop will cause problems at harvest and indicates that the most extreme treatment included in the comparison is just beyond the limit of acceptable heterogeneity in the root medium. Lettuce can be grown on sand beds with a recirculating nutrient solution provided that drip lines are well distributed in the bed and the daily nutrient solution excess is more than 30% of demand.     

Transactivation of Ds elements in plants of lettuce (Lactuca sativa)

The maize transposable element, Activator (Ac), is being used to develop a transposon mutagenesis system in lettuce, Lactuca sativa. In this paper, we describe somatic and germinal transactivation of Ds by chimeric transposase genes in whole plants. Constructs containing either the Ds element or the Ac transposase open reading frame (ORF) were introduced into lettue. The Ds element was located between either the 35S or the Nos promoter and a chimeric spectinomycin resistance gene (which included a transit peptide), preventing expression of spectinomycin resistance. The genomic coding region of the Ac transposase was expressed from the 35S promoter. Crosses were made between 104 independent R₁ plants containing Ds and three independent R₁ plants expressing transposase. The excision of Ds in F₁ progenies was monitored using a phenotypic assay on spectinomycin-containing medium. Green sectors in one-third of the F₁ families indicated transactivation of Ds by the transposase at different developmental stages and at different frequencies in lettuce plants. Excision was confirmed using PCR and by Southern analysis. The lack of green sectors in the majority of F₁ families suggests that the majority of T-DNA insertion sites are not conducive to excision. In subsequent experiments, the F₁ plants containing both Ds and the transposase were grown to maturity and the F₂ seeds screened on medium containing spectinomycin. Somatic excision was again observed in several F₂ progeny; however, evidence for germinal excision was observed in only one F₂ family.     

Production and detoxification of H2O2 in lettuce plants exposed to selenium

Selenium is considered an essential element for animals. Despite that it has not been demonstrated to be essential for higher plants, it has been attributed with a protective role against reactive oxygen species in plants subjected to stress. In this study, lettuce plants ( Lactuca sativa cv. Philipus) received different application rates (5, 10, 20, 40, 60, 80 and 120 μM) of selenite or selenate, with the aim of testing the effect of Se on the production and detoxification of H₂O₂ in non-stressed plants. The results indicate that the form selenate is less toxic than selenite; that is, the plants tolerated and responded positively to this element, and even increasing in growth up to a rate of 40 μM for the form selenate. On the contrary, the application of selenite triggered a higher foliar concentration of H₂O₂ and a higher induction of lipid peroxidation [malondialdehyde content and lipoxygenase activity] in comparison to that observed after the selenate application. Also, the plants treated with selenate induced higher increases in enzymes that detoxify H₂O₂, especially ascorbate peroxidase and glutathione (GSH) peroxidase, as well as an increase in the foliar concentration of antioxidant compounds such as ascorbate and GSH. These data indicate that an application of selenate at low rates can be used to prevent the induction in plants of the antioxidant system, thereby improving stress resistance.