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Abstract
Analysis of tissues of lettuce, Lactuca sativa L., seedlings failed to reveal the presence of anthocyanin, pheophytin or phenolic compounds as the cause of red cotyledons (physiological necrosis). Electron microscope study showed that the cells in the necrotic tissues were completely disintegrated with only the mitochondria identifiable.
In greenhouse and field plantings, necrotic seedlings that survived produced heads equal in size and total yield to those from normal seedlings. Seedlings with slight spotting were a few days behind normal seedlings in establishment, whereas those which were badly necrotic were about 2 weeks delayed. After 8 weeks in the greenhouse or field there was no differences in plant sizes. Seedlings with only slight necrosis appeared to be satisfactory for field seeding but badly necrotic ones would probably not give a satisfactory stand.
With a high nitrate supply, and most frequently under low-light conditions, lettuce accumulates relatively large amounts of NO3-as a result of an excess of uptake over reduction. Different approaches, which are used to reduce leaf nitrate, often result in a yield loss. A computerized aeroponic system, which supplies different nitrate concentrations in accordance with the changeable light conditions (dynamic light-dependent application of nitrate), was used to reduce nitrate accumulation in lettuce (Lactuca sativa L.) var. Capitata cv. Vanity. Under unfavorable light conditions nitrate was supplied at limited rates (slight, medium, and strong reduction) to the plants. In response to given light conditions the nitrate supply was reduced close to one-half or one-fourth of the full nutrient solution (8 mmol·L-1 NO3-). Controlled nutrition resulted in efficient reduction in leaf nitrate. In the early-spring experiment the average nitrate content in outer leaves was decreased by 9%, 63%, and 92% and in the late-spring experiment the decrease was 23%, 58%, and 76% compared to control. At the same time, the controlled, light-dependent nitrate deprivation did not result in a loss of a lettuce yield (except in the treatment with strong nitrate reduction) and had limited effects on photosynthesis (P N-C i measurements) and photosynthetic pigments.
Abstract
Achene color and leaf-type genes are linked, with p = 0.34. Achene color and male-sterility genes are linked, with p = 0.39. Leaf-type and male-sterility genes are independently inherited. Achene color and virescent chlorophyll-deficiency genes are linked, with p = 0.03. Thirty-four other linkage comparisons showed independent inheritance.
Endive-type leaf and chlorophyll deficiency are each regulated by recessive alleles. Three separate recessive alleles control albinism.
Abstract
Each of 3 albino characters is governed by a single recessive gene. Virescent chlorophyll deficiency, pale yellow flower, golden flower, abaxial leaf hairs, and plump involucre shape are also regulated by single recessive genes. Male-sterility is inherited as a single dominant.
Six genes controlling flowering time or bolting time in Lactuca L. have been reported. Several crosses between parents differing in time to opening of first flower were made to ascertain the inheritance of additional flowering time traits in Lactuca species. The parents in the crosses were of five flowering classes: very late (VL), late (L), early (E), very early (VE), and very, very early (VVE). Segregation from a cross between C-2-1-1 (VL) (L. sativa L.) and `Vanguard 75' (L) confirmed that `Vanguard 75' flowering was controlled by the previously identified gene Ef-2ef-2. Mutant line 87-41M-7 (VVE) was crossed by D-3-22M (VE) and segregated 3VVE:1 VE, indicating a dominant allele, Ef-3, that decreased flowering time an additional 7 days. Cos-like line 796 (VE) was crossed to cultivars Salinas (VL) and Vanguard 75. Segregation indicated a gene Ef-4ef-4, with lateness dominant. PI 175735 (E) (L. serriola L.), crossed with C-2-1-1 produced an F2 population with a bimodal distribution, segregating 3 E:1 VL, indicating a single gene Ef-5ef-5. PI 236396 (E) and PI 250020 (E) were crossed to `Salinas' and `Vanguard 75'. Segregation and morphological similarity indicated the same gene in both PI lines, Ef-6ef-6, with earliness dominant.
Incidence of the disease lettuce drop caused by Sclerotinia minor is often high in California lettuce fields despite the use of cultural and chemical controls. Development of resistant lettuce cultivars has been hindered by the difficulty of evaluating resistance in field tests and the lack of a screening procedure that reliably predicts field performance. Several lettuce genotypes of diverse geographic origin and plant architecture including modern and heirloom cultivars, plant introduction accessions, and breeding lines, were evaluated for resistance to S. minor using several methods. Resistance was evaluated in fields that contained naturally occurring S. minor, in a field that contained both naturally occurring and manually incorporated S. minor inoculum, and in the greenhouse using two types of inocula. Many genotypes exhibited partial resistance to S. minor, with significantly reduced disease incidence relative to susceptible controls. The similarity of disease ratings observed in replicated field tests supports the conclusion that partial resistance is under genetic control. Ratings obtained in some greenhouse tests were highly correlated with field ratings, but this was not true for all tests. Therefore, although greenhouse evaluation with adequate replication and repetition can be used as a selection tool, field testing remains an essential component of S. minor resistance breeding programs.
Abstract
Lactuca sativa seed, cvs. ‘Imperial 847’, ‘Imperial 456’, ‘Imperial 44’, ‘Fulton’, and ‘Oswego’ were stored at a wide range of temperature and relative humidity, to determine their effect upon development of red cotyledons (physiological necrosis).
Red cotyledons did not develop equally (rate or amount) in seeds of all cultivars at any given storage condition. More red cotyledons developed at a low than at a high relative humidity at the same temperature. However, longer storage at the low relative humidity was required before red cotyledons developed.
Storage at −12°C/70% relative humidity prevented red cotyledon development in all cultivars tested for 210 weeks. Storage at 10/90, 10/70, 4/35 and −1/40 prevented red cotyledon development in all cultivars for 157 weeks and in some cultivars for 210 weeks.
Abstract
Leaf index (width/length) was measured in the parental lines, F1 and F2 of the cross ‘Gallega’ (rosette type) × ‘Minetto’ (crisphead). The minimum genetic requirement for head formation in the F2 was probably a combination of 3 genes, 1 dominant and 2 recessive. However, none of the heading segregates fully recovered the ‘Minetto’ phenotype. The heading genotype of ‘Minetto’ differed from that of ‘Gallega’ by a mimimum of 3, and probably 4 or more, predominantly recessive genes. The primary effect of “heading genes” was to increase the leaf index, i.e. produce leaves which were wide at the base.
Screening for lettuce (Lactuca sativa L.) big-vein resistance in the F2 generation is highly inefficient. Efficiency improves in the F3 and following generations with continued inbreeding. Traits useful in ascertaining resistance are 0% of plants showing symptoms and percentage of plants showing symptoms at a given date. Breeding lines identified as resistant in greenhouse screening have proved resistant under field conditions. Forty-nine cultivars have been identified in preliminary testing as potentially resistant. Of these, 11 have been confirmed as resistant in greenhouse and field tests.
Abstract
Using sprinkler irrigation for the entire season allowed a change in bed conformity for head lettuce. Growing 6 rows on an 82-inch bed introduced a maturity variable, with the 2 outside rows having larger heads or heads with greater numbers of leaves.
On a flat soil surface with plants on square spacings of 10 × 10, 12 × 12, 14 × 14, 16 × 16, and 24 × 24 inches, growth was slowed but total yields increased as density increased. Using ‘Climax’ and the present system of packaging 24 or 30 heads per carton, the 14 × 14-inch spacing produced the greatest number of cartons per acre. The greater yields at the 10 × 10-inch spacing (45 tons per acre) were small heads that would be acceptable only for shredding.