Variation in flowering times within genotype classes of the Early Flowering (Efef) gene in lettuce (Lactuca sativa L.) suggested that more than one gene may govern flowering time. Segregation from crosses between earlier and later plants in the early class indicated a second discrete flowering time gene. This gene was confirmed in F3 family analyses and in subsequent progeny comparisons. The first gene is renamed Early flowering-1 (Ef-lef-1) and the second is named Early flowering-2 (Ef-2ef-2). The consequences of the presence of the second gene in cultivars and in the backcross breeding procedure are discussed. Flowering time is influenced by daylength and temperature.
Three new lettuce (Lactuca sativa L.) genes were described and named: shiny green (Sgsg), salmon (Sasa), and apple green (Agag). Plump involucre and male sterile are linked, with p = 0.345. Plump involucre and Early flowering are linked closely, with p approaching zero. Twenty-five gene pairs tested for linkage were independently inherited. In achene color, the order of recessive epistasis is white > yellow > brown. The double-recessive combination vivicd-2cd-2 (virescent and chlorophyll deficient) is phenotypically virescent and partially lethal.
Lettuce mosaic has been a serious virus disease for lettuce in all locations worldwide where lettuce has been grown. Consequently, the disease and its virus have been well studied. Lettuce plants react to lettuce mosaic virus in a variety of ways. The most common susceptible reaction is an overall vein clearing and mottling, followed by leaf recurving, leaf distortion, and stunting. However, some susceptible types manifest a mild mottling with little additional distortion. Others develop a necrotic reaction, which may be severe, mild, or seasonal. Finally, there are at least three resistant reactions, most frequently appearing as a systemic infection manifested with restricted yellowish lesions. Research is ongoing to sort out the various reactions and their genetic bases. This report describes the inheritance of the severe necrotic reaction and its relationship to the resistant reaction conferred by the allele mo-1. Several previous crosses among necrotic types indicate that the same necrotic allele is operating except that found in `Bibb'. Several crosses were studied. The cross `Salinas' (mot.) × `Crisp As Ice' (nec.) showed that necrotic is due to a single dominant allele. The cross `Salinas 88' (res.) × `Maikonig' (nec.) produced three phenotypes in F2, indicating the action of two loci. The crosses PI 251245 (res.) × `Prizehead' (nec.) and `Vanguard 75' (res.) × `Prizehead' disclosed two recombinant phenotypes, mottled and resistant-necrotic. Necrotic is dominant to nonnecrotic in both susceptible and resistant phenotypes. The genes are inherited independently.
A major goal in our lettuce (Lactuca sativa L.) disease resistance breeding program has been the development of cultivars resistant to big vein. Big vein is expressed primarily as vein clearing and stiffening of outer leaves giving a bushy effect. It is caused by big vein agent, a virus-like entity, which is transmitted into the roots of the plant by a root-feeding fungus Olpidium brassicae (Wor.) Dang. Symptoms are most commonly expressed during periods of low air temperature and in heavy wet soils (4, 5). Consequently, in the United States it is a common problem in the West during the early spring growing period. ‘Sea Green’ is a new cultivar with big vein resistance.
Big vein has been a problem for growers of lettuce (Lactuca sativa L.), particularly in the western United States, for many years. The principal symptom, vein clearing, lends an unsightliness to the lettuce that may reduce its market value (7). During periods of low temperatures, big vein may delay or prevent head formation and thus reduce the harvest recovery (4).
The artichoke is a popular vegetable in Italy, Spain, and France. The American consumer, upon first encountering this bristly vegetable, may wonder why. Dealing with the artichoke, either as a cook or around the dinner table, may seem more like a confrontation than a culinary venture, and the average artichoke novice may be happy with a stand-off.
In an earlier study, two genes controlled flowering time in certain lettuce crosses. Early alleles (Ef-1, Ef-2) at both loci are partially dominant; flowering time in the greenhouse ranges about 100 days from double dominant to double recessive. Crosses between C-2-1-1, a double recessive line, and four cultivars were analyzed to study further the distribution of these genes. Empire and Salinas flower at the same time as C-2-1-1, while Vanguard 75 and Prizehead are earlier than C-2-1-1. Empire × C-2-1-1 and Salinas × C-2-1-1 showed unimodal distribution in F2, indicating segregation of several genes or environmental variation. A linear relationship between F2 plant values and F3 family means from both crosses suggested a polygenic basis. The F2 distribution of Prizehead × C-2-1-1 was similar but showed a greater range of flowering times. An additional major gene was indicated in Vanguard 75 × C-2-1-1, with earliness recessive.
Resistance to lettuce mosaic (LMV) in current cultivars of lettuce is due to a single recessive gene, momo, in one of two allelic forms. The nature of the resistance may be described as resistance to multiplication and spread in the plant. Resistance is systemically manifested as small irregular yellowish areas on the leaf. This compares to the usual expression of susceptibility: vein clearing, followed by mottling, leaf margin recurving, and later stunting and yellowing. A cos-like stem lettuce from Egypt, `Balady Aswan', is susceptible to LMV, but reaction to the virus is a milder one than the usual susceptible reaction. Segregating generations from crosses with normal susceptible and resistant lines were analyzed. The data suggest a single gene for reaction type, with mild dominant or partially dominant to severe. Selection of lines from crosses with the resistant type allows the breeder to select resistant and mild alleles together, which confers a higher level of resistance than momo alone. Plants with the combined reaction either show no symptom or show the resistant symptom very late.