Inheritance of cabbage head formation in crosses of cabbage × ornamental cabbage and cabbage × kale

We investigated the inheritance of head formation in Brassica oleracea by using two crosses, cabbage × ornamental cabbage and cabbage × kale. The degree of head formation (DHF) was classified into nine grades ranging from non‐heading to full heading. DHF in the two F₂ populations showed a continuous distribution. The variance of F₃ offspring selected for full heading or non‐heading was large. The DHF distributions in the F₃ offspring selected for moderate heading or randomly selected F₃ populations were similar to those of the respective F₂ populations, but had smaller average values and variance. However, the realized heritability in F₃ offspring was similar in full‐heading and non‐heading selections. Our findings suggest that head formation is a quantitative trait controlled additively with low dominance effects. In comparisons of leaf developmental patterns among the parents, only cabbage showed a change in leaf shape becoming wider because of the shorter petiole length with increasing leaf position. These findings suggest that cabbage acquired the developmental changes in leaves required to form a head during the process of domestication.     

Identification of sources of resistance in lettuce to the lettuce root aphid, Pemphigus bursarius

In a series of glasshouse and laboratory experiments, lettuce varieties and wild Lactuca species were screened for resistance to the lettuce root aphid, Pemphigus bursarius (Ron.). Several new sources of resistance to P. bursarius development and survival were identified. Extremely high levels of resistance were found in the wild species L. saligna, L. perennis and L. virosa. Resistance was also identified in the lettuce variety Grand Rapids. This resistance is controlled by one or two genes. At least one of these genes is not allelic to the existing Lra gene. Grand Rapids therefore represents a new source of resistance to P. bursarius. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetics of nitrate content in lettuce

Summary Adverse effects on human health makes the high nitrate content frequently found in lettuce (Lactuca sativa L.) grown under low light conditions an undesirable trait. Efforts have been made to breed cultivars with a reduced capacity for nitrate accumulation. In this study components of variance for nitrate content were estimated in F₂ and F₃ generations of ten lettuce crosses. Additive genotypic variances (A) were estimated from F₃ variance components and from the covariance between F₂ plants and corresponding F₃ lines. Estimates of wide sense heritability of the F₂ from crosses between a high nitrate genotype and four low nitrate genotypes ranged from 0.44 to 0.74 and the estimates for A ranged from 0.25 to 0.40 g·l^-1. Estimated wide sense heritabilities of F₂'s from six crosses involving two low nitrate parents ranged from 0.15 to 0.52. The parents of four of the low nitrate crosses showed relatively large effects of genotype x environment (GE) interaction in successive experiments: the nitrate content of the parents reacted differently to environmental changes between experiments. Estimates of A for crosses between low nitrate genotypes without large effects of GE interaction ranged from 0 to 0.19 g·l^-1. The estimated probability of selecting transgressive low nitrate lines in the progeny of a cross between a high and a low nitrate genotype was low (P=0.002–0.039), indicating that large populations should be evaluated to combine the positive traits of modern high nitrate cultivars with low nitrate content from genotypes not adapted to modern cropping practices. In the progenies from crosses between two low nitrate genotypes without important GE effects, only low estimates of the probability of obtaining transgressive low nitrate lines were obtained (P=0.04–0.06). With the growth conditions used in this study, the probability of selecting lines with a nitrate content compatible under all winter conditions with the proposed future maximum permissible level of 2.5 g nitrate per kg fresh matter is low. Therefore the solution of this problem should be found in a combination of low nitrate cultivars and cultural measures that reduce the nitrate content of the crop.     

Vegetative propagation of selected lettuce plants in autumn and winter

Summary A method is described of propagating lettuce (Lactuca sativa L.) by cuttings, reducing losses of selected heads to a minimum. This method is successfully applied to heads selected under glass in autumn and winter.Three ways of propagation by cuttings are dealt with, viz. by head cuttings, shoot cuttings and leaf cuttings.     

The use of endosperm balance number for predicting gamete selection in complex polyploid interspecific Trifolium repens × T. nigrescens crosses

Endosperm balance number (EBN) was used to predict the outcome of crosses among a series of complex polyploid interspecific hybrids between Trifolium repens and T. nigrescens. Normal endosperm development, and hence seed‐set, requires a 2 : 1 maternal : paternal EBN ratio in endosperm. Based on their crossability, T. repens (2n = 4x = 32) and T. nigrescens (2n = 2x = 16) were assigned the same EBN of 4. This number was used to predict the relative success of different backcrosses and intercrosses. The method was further developed to successfully predict which gametes would effect fertilization when complex polyploid hybrids with uneven genomic constitutions (5x, 7x) produced mixtures of euploid and aneuploid gametes. This was achieved by allocating EBNs to all potential gametes of complex polyploids. Allocation of gamete EBNs enabled not only prediction of the success or failure of a cross, but also which gametes would be filtered to effect fertilization in complex crosses. This enhanced ability to understand the potential outcomes of complex crosses will increase the efficiency of plant breeding programmes that integrate secondary gene‐pools into economic species by interspecific hybridization.     

Analysis of the differences in growth between five lettuce cultivars marking the development in lettuce breeding for winter production

Summary Since 1950 growth of lettuce for winter production has been improved considerably by breeding. It has been investigated whether this improvement must be attributed to a more rapid leaf production, to larger leaves or to both. To this end the differences in growth between the cultivars Meikoningin. Proeftuin's Blackpool, Rapide, Deci-Minor and Valentine were analysed at a range of temperatures (10, 14, 17 and 20°C) under natural light conditions in autumn. The last four cultivars represent successive improvements in growth of lettuce for winter production.From the experiments it can be concluded that the improvement in growth of lettuce for winter production was initially brought about through increasing the rate of leaf production and there-after through increasing leaf size. At the same time the heat requirement for optimal growth has become higher.     

Resistance to <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> race 1 in the lettuce cultivars Grand Rapids and Salinas-88

The objective of this work was to check the possible allelism between two sources of resistance to the root-knot nematode Meloidogyne incognita race 1 in lettuce (‘Grand Rapids’ and ‘Salinas-88’). The experiments were carried out in greenhouses, in expanded 128-cell polystyrene trays filled with commercial substrate. Lettuce cultivars ‘Salinas 88’ and ‘Grand Rapids’ were tested along with the populations F₁ (‘Grand Rapids’ × ‘Salinas-88’), F₂ (‘Grand Rapids’ × ‘Salinas-88’), F₃ (‘Grand Rapids’ × ‘Salinas-88’), and with F₄ families derived from the latter population. Seedlings were inoculated 15 days after sowing with a nematode egg suspension equivalent to 30 eggs ml⁻¹ of substrate. Plants were evaluated for apparent gall incidence, gall scores, egg mass scores and extracted egg numbers 45 days after the inoculation date. There was evidence that two different genes are involved in control of resistance to M. incognita race 1 in lettuce cultivars Grand Rapids and Salinas-88. Lines with higher levels of nematode resistance than either Grand Rapids or Salinas-88 could be selected in the F4 generation of the cross between these resistant parental lines.     

Modelling concept of lettuce breeding for nutrient efficiency

Modern lettuce cultivars are bred for use under high levels of input of water and nutrients, and therefore less adapted to low-input or organic conditions in which nitrate availability varies over time and within the soil profile. To create robust cultivars it is necessary to assess which traits contribute to optimal resource capture and maximum resource use efficiency. We therefore revisited earlier published results on root growth, resource capture and resource use efficiency of lettuce exposed to localized drought and nitrate shortage in a pot experiment. Root growth in a soil profile with localized resource shortage depended on the resource that was in short supply. We conceptualized a model describing nitrogen uptake and use efficiency. We also investigated the genetic variation among 148 cultivars in resource capture over time and soil depth and in resource use efficiency in four (two locations × two planting dates) field experiments. Cultivars proved to be highly diverse in their ability to capture and use resources. This ability, however, was strongly affected by other sources of variance, stressing the need for an eco-physiological model capable of reducing the residual variance and improving the expression and evaluation of cultivar differences in relation to both resource capture and use efficiency in lettuce. We showed that genetic variation was best expressed under limiting conditions. To improve the conceptualized model further we identified issues requiring further analysis, e.g., the physiological reasons why certain cultivars are capable of quickly responding to changes in the environment to maintain optimal resource capture.     

Transplanting matured head type lettuce for seed production

Summary We obtained success in seed production by transplanting matured head lettuce plants into clay pots after (1) removing the head with a knife, leaving a few leaves on the stalk to allow for the development of the shoots, and (2) after removing all leaves covering the seedstalk so that it was free to bolt. The use of gibberellin A₃ (GA₃) to promote early seed production proved detrimental on poorly established transplants.     

Sources and inheritance of resistance to stemphylium leaf spot of lettuce

Summary None of the tested cultivars of lettuce was found resistant to Stemphylium leaf spot, a common disease in Israel. Within a Lactuca saligna population collected in wild lettuce in Israel, resistance was traced. Interspecific crosses of L. saligna x L. sativa were made and the mode of inheritance of resistance to this disease was studied. Resistance is apparently controlled by two genes: one dominant (Sm₁) and one recessive (sm₂).Contribution No. 1176-E 1984 series, from the ARO.