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  • Author or Editor: Samuel Contreras x
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Daily and total elongation of spears, plus elongation rates, and the number of days to reach maximum height of spears prior to branching were measured on 5-year-old asparagus plants of cultivars Atlas, Ciprés, Gijnlim, Jersey Giant, JWC1, UC-157 F1, and UC-157 F2. Six measurement periods were carried out every other week during a spring harvest, measuring spears from emergence to maximum height before branching (5 mm of branch above budscale).Daily elongation varied widely through harvest, from less than 1 cm/day when average temperatures were below 10 °C, to more than 14 cm when daily temperatures reached a mean between 16 to 18 °C. Total spear length varied from an average of 50 cm, at the beginning of spring, to an average of 32 cm at the end of spring. Elongation rates varied between and within periods: less than 1cm/day at the beginning of the season to more than 15 cm/day at the end of the season; within a given period lower rates prevail at start of elongation. Days to reach maximum height varied from up to 13 days, at the beginning of the season, to 5 days at the end of the harvesting period. Cultivars showed somewhat similar elongation patterns; however, variations observed could have significant effects on quality and total commercial yield of spears.

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The main objective of this study was to evaluate some of the available seed vigor tests to predict lettuce (Lactuca sativa L.) seedling emergence. Eight lettuce seed lots were evaluated by the following laboratory vigor tests: germination (GT); accelerated aging (AA); saturated salt accelerated aging (SSAA); conductivity (EC); and seed vigor imaging system (SVIS). Also evaluated were the percentage (EP) and speed (ES) of the seed lots' emergence under three conditions: seedling trays (ST) filled with a mixture of peat (80%) and perlite (20%); boxes with a clay loam soil (BS); and boxes with a mixture of clay loam soil (66%) and sand [34% (BSS)]. Correlation coefficients among the laboratory and emergence test results were calculated and significant differences were found. The correlation coefficients between EC results and each emergence parameter were nonsignificant, while AA results were only significantly correlated with the emergence percentage for BSS. GT values and emergence results were significantly correlated in all cases. SSAA results were equally or more positively correlated than GT results with the EP and, for the three sowing conditions, they were more correlated with the ES than GT values. Vigor index results from the SVIS were significantly correlated with both emergence parameters, and its correlation with emergence on ST (EP and ES) was greater than GT values. The results of this study showed that SSAA and SVIS were the best laboratory tests for lettuce seed vigor evaluation, especially for seed lots to be used for plug seedling production.

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Germination in lettuce (Lactuca sativa) seeds can be inhibited by high temperatures (thermoinhibition) or darkness (positive photoblasticity). Priming is commonly used as a seed treatment to avoid these problems. However, this treatment is complicated, expensive and has detrimental effect on seed longevity. The objectives of this study were to 1) develop a simple alternative treatment to priming, based on red light irradiations and 2) to develop a treatment to extend seed longevity. Lettuce seeds from cultivars Tango, Milanesa, Ideal Cos, and Gallega de Invierno were partially hydrated in containers with 98% relative humidity (RH) and irradiated with red (R) light for 24 hours, or far-red (FR) light for 216 hours. Throughout the treatment, seeds reached 16% water content and, once finished, they were dried with air at 30 °C. Untreated (control) and primed seeds were also evaluated. Germination was evaluated in light and darkness at temperatures between 12 and 36 °C in a thermogradient table. Effects in longevity were estimated by evaluating germination after various aging periods at 45 °C and 75% RH. The R-light treatment improved germination in the dark at temperatures up to 25 °C, but did not have a significant effect on seed germination under light. Germination after accelerated aging showed that R- and FR- light treatments extended seed longevity when compared with control seeds. Therefore, this effect would not be associated to photoblasticity alleviation or imposition. Primed seeds deteriorated faster than the control. Compared with priming, the R-light treatment was simpler and improved seed longevity. However, priming effects on alleviation of seed photoblasticity and thermoinhibition at temperatures over 25 °C were greater.

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Lettuce seeds (Lactuca sativavar. acephalacv. Tango) were used with the objective of determining the effect of temperature, light, and their interactions in promoting germination. Under standard op-timal conditions (20 °C, light), the seed presented 100% germination (radicle emergence 5 d after sowing). Different treatments evaluated germination under dark conditions, with or without a red light break (LB, 28.8 mmol·m-2) 48 h after sowing, and with different combination of temperatures pre- (soaking temperature, ST) and post- (germination temperature, GT) the LB. Germination at constant 20 °C without LB was less than 5%, and with LB, it was around 30%. However, germination was close to 100% at GT of 20 °C when LB was applied after a ST of 10 °C, and around 50% under the same conditions, but without LB. When GT was 30 °C and LB was applied, germination was less than 3% with ST = 30 °C, less than 10% with ST = 20 °C, and around 100% when ST = 10 °C. With ST and GT of 10 °C and 30 °C, respectively, and no LB, germination was less than 5%. Germination at 10 °C constant, with and without LB, was around 90% and 0%, respectively. When ST was 40 °C and LB was applied, germination was around 40% at GT= 20 °C, but less that 3% with GT= 30 °C. In summary, a severe inhibition of germination was observed when seeds were germinated in dark, which was partially reversed by either a light treatment or soaking at 10 °C, and fully reversed when both treatments were applied together. Inhibition of lettuce germination at 30 °C was observed when this temperature was applied after a light treatment, but not when applied before. Possible implications of these results for the phytochrome mechanism of action are discussed.

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Among the factors affecting germinability of a seed lot are the environmental conditions under which the seeds are produced. The objective of this study was to determine the effects of temperature during seed development on seed quality of two Asteraceae species. Seeds of lettuce cv. Tango and Helianthus debilis cv. Vanilla Ice and sp. cucumerifolius were produced in a greenhouse under one of two treatments: i) hot (27, 40, and 20 °C temperatures average, max, and min, respectively), and ii) cool (23, 33, and 18 °C temperatures average, max, and min, respectively). In both species, heavier seeds were produced under the cool conditions and no differences were observed in standard germination. In lettuce, germination percentage and rate were both affected by increased levels of exogenous ABA concentrations and reduced water potential (PEG solutions), and, in both cases, seeds from cool treatments were more affected. Germination at 30 °C and constant light was higher for seeds from the hot treatment. Lettuce seed showed a strong light requirement for germination. However, seeds from the hot treatment gave better dark germination at 13 and 19 °C. Seeds of H. debilis did not required light for germination, and the germination percentage and rates were evaluated at 13, 21, and 29 °C. For both lines, seeds from each treatment behave similarly; however, the germination of H. debilis cv. Vanilla Ice at 29 °C was higher when seeds were produced in the hot conditions. The results showed that temperature during seed development affected aspects of seed quality that are not detectable by the standard germination, but by germination at suboptimal conditions. Within the Asteraceae family, differences varied among and within species.

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Lettuce (Lactuca sativa) is an important vegetable crop worldwide, and its seed is commercially produced mainly under irrigation in arid and semiarid regions. The objective of this study was to determine how water availability during seed development affects lettuce seed productivity and quality. Three experiments were performed in the greenhouse and growth chambers using lettuce (cv. Tango) cultivated in pots. When watering volume was restricted (dry treatment) from bolting to seed harvest to 54% of the well-watered control (wet treatment), plants were shorter, had reduced dry weight, and produced fewer and heavier seeds. Water productivity (seed yield/watering volume) was nearly 50% higher in the dry treatment. Seeds from the dry treatment had a modest improvement in seed vigor (assessed by seedling growth) and decreased germinability (higher sensitivity to exogenous abscisic acid and water potential) compared with the wet treatment. In another experiment, water stress was applied abruptly to well-hydrated lettuce plants with developing seeds. Seeds that were at one-third and two-thirds of physiological maturity when water was withheld had lower germinability and greater storability than seeds with no water restriction. These results provide information that may be used for improvement of irrigation practices for lettuce seed production.

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Seed germinability and storability are important aspects of seed quality determined by the genotype and environment of seed development. Lettuce (Lactuca sativa L.) is produced commercially in most temperate and subtropical areas of the world. The objective of this study was to determine how photoperiod and light quality of the mother plant environment affects lettuce seed quality. Seeds of cv. Tango were produced in growth chambers under one of two treatments: a) short day (SD), consisting of 8 hours of fluorescent light (≈310 μmol·m−2·s−1) plus 16 hours of darkness daily, and b) long day (LD), consisting of 4 hours of incandescent light (≈21 μmol·m−2·s−1), 8 hours of fluorescent light, 4 hours of incandescent light, and 8 hours of darkness daily. The red to far-red ratio was ≈6.8 and 1.0 for the fluorescent and incandescent light, respectively. In both treatments, the temperature was 23 °C. The LD treatment produced significantly heavier seeds; however, germination at optimal conditions (20 °C-light) was similar for both treatments. Germinability (percentage and rates) at suboptimal conditions (30 °C, 20 °C with different external ABA concentrations, negative osmotic potentials, or dark) was higher for seeds produced under the LD treatment. On the other hand, seeds produced under the LD treatment presented better storability (evaluated by the accelerated aging test and standard germination after storage at 30 °C and 74% RH). The critical period for light environment effects was also studied. Seed weight patterns were determined early in seed development, during the first 6 days after flowering. Conversely, light environment effects on seed germinability and storability were determined at the end of seed development, after physiological maturity, which occurred by 11 days after flowering. These results show that lettuce seed germinability and storability may be modified by management of light conditions during seed production and provide useful information for seed producers, seed companies, and seed conservation institutions.

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Grafting of seedlings is a technique used for watermelon (Citrullus lanatus) production in many countries. Because of higher costs involved, the use of grafted seedlings can only be recommended if it provides clear biological and economic benefits. Since rootstock performance is influenced by compatibility with the cultivar, the existing disease pressure, and climatic conditions, it is necessary to evaluate rootstocks with current cultivars to appraise possible benefits in a given area. Two experiments were carried out in two consecutive seasons with the objective of evaluating the benefits of grafting under Chilean conditions. The rootstocks used were ‘Marathon’ (Cucurbita maxima × Cucurbita moschata) and ‘Macis’ (Lagenaria siceraria) with different scions, including some seedless cultivars. In both experiments, grafted plants increased their yield compared with nongrafted plants (136% and 159% in Expts. 1 and 2, respectively). This effect was due to an increased number of fruit per plant (P < 0.01), and the weight gain of the fruit (P < 0.01). Plants presented with fusarium wilt [Fusarium oxysporum f. sp. niveum (FON)] in both experiments, which seemed to be the main limitation for nongrafted plant production. In the evaluation of quality attributes [soluble solid concentration (SCC), firmness, color, polar diameter, equatorial diameter, and rind thickness], positive effects were observed in the fruit of grafted plants. For the conditions of these experiments, the increase in yield of grafted plants would be associated with an economic benefit that exceeds its additional cost.

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Germinability and desiccation tolerance are important attributes that seeds acquire during their development. The timing in the expression of these characteristics is important to understand how environmental conditions affecting the mother plant influence seed quality. Lettuce plants (cv. Tango) were cultivated in the greenhouse. Seed germination, under light and darkness, was evaluated in fresh and dry seeds at 3, 5, 7, 9, 11, 13, 15, and 17 days after flowering (DAF). Desiccation was performed ≈1 h after harvest by placing the seeds at 25 °C and ≈53% RH. The seed moisture level after desiccation decreased from ∼14% for 3 DAF seed to ∼7% for 7 DAF seed, and then remained constant until the last sampling. Seeds achieved maximum dry weight (physiological maturity) at ∼13 DAF. Germination of fresh seeds increased from 0% at 3DAF to ∼80% at 5 DAF, reaching 100% at 7 DAF. Dry seeds did not germinate when they were 3 or 5 DAF. Seeds at 7 DAF had ∼10% germination and at 9 DAF ∼100%. When germinated in the dark, an increase in germination from 0% in fresh seeds at 3 DAF to 50% germination at 5 DAF was observed. However, seeds at 9 DAF had dark germination values that decreased to 0% and increased again to ∼70% germination at 13 DAF. Dry seeds had no dark germination until 7 DAF, with variable and low germination (below 20%) until 11 DAF, then germination reached a maximum of ∼55% at 13 DAF and decreased to below 10% at 17 DAF. According to these results, lettuce seed germinability and desiccation tolerance were reached sooner than physiological maturity. In the dark, germination of fresh seeds presented a curve with two peaks suggesting that, depending on the seed developmental stage, two different physiological mechanisms restrict dark germination.

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Thermoinhibition and photosensitivity are two characteristics of lettuce seed that frequently affect its stand. The main objective of this study was to evaluate the hypothesis that lettuce seed germinability and longevity are affected by the red to far-red light ratio (R:FR) under which seeds maturate. ‘Tango’ lettuce seeds were produced in growth chambers under one of two treatments: 1) red-rich light (R treatment) and 2) far-red-rich light (FR treatment). For both treatments, the percentage of normal seedlings germinated at 20 °C–light was ≈100%. When germinated under the light, seeds from the R treatment exhibited a higher germination percentage and a faster germination (under a broader range of temperatures) than seeds from the FR treatment. When germinated in the dark, seeds from the R treatment germinated 100% between 12 and 23 °C and over 50% at 30 °C, whereas seeds from the FR treatment germinated less than 35% between 12 and 23 °C and less than 5% at 30 °C. Seeds from the R treatment had lower abscisic acid (ABA) content and were better able to germinate when exposed to external ABA concentrations than seeds from the FR treatment. Seed longevity as assessed by the accelerated aging test was higher in seeds from the FR treatment, indicating that red-rich light was detrimental to longevity. In another experiment, lettuce seeds that developed under similar conditions were harvested at approximately the moment of maximum dry weight accumulation and desiccated in dark, far-red, red, or fluorescent + incandescent light. Seeds desiccated under red light exhibited higher dark germination than the other treatments; however, no differences were observed in thermoinhibition or longevity. These results suggest that lettuce seed produced in an environment with a high R:FR light ratio will exhibit reduced thermoinhibition and photosensitivity as compared with production in a lower R:FR light environment.

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