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- Author or Editor: Daniel J. Cantliffe x
Lettuce (Lactuca sativa L.) seeds can fail to germinate at temperatures above 24 °C. The degree of thermotolerance is thought to be at least partly related to the environment under which the seed developed. In order to study the effects of temperature during seed development on subsequent germination, various lettuce genotypes were screened for their ability to germinate at temperatures ranging from 20 to 38 °C. Seeds of the selected genotypes `Dark Green Boston' and `Valmaine' (thermosensitive), `Floricos 83', `Everglades', and PI 251245 (thermotolerant) were produced at 20/10, 25/15, 30/20, and 35/25 °C day/night temperature regimes in plant growth chambers. Seeds were germinated on a thermogradient bar from 24 to 36 °C under 12 h light/dark cycles. As germination temperature increased, the number of seeds that failed to germinate increased. Above 27 °C, seeds matured at 20/10 or 25/15 °C exhibited a lower percent germination than seeds that matured at 30/20 or 35/25 °C. Seeds of `Dark Green Boston' and `Everglades' that matured at 30/20 °C exhibited improved thermotolerance over those that matured at lower temperatures. Seeds of `Valmaine' produced at 20/10 °C exhibited 40% germination at 30 °C, but seeds that matured at higher temperatures exhibited over 95% germination. Germination of `Valmaine' at temperatures above 30 °C was not affected by seed maturation temperature. The upper temperature limit for germination of lettuce seed could thus be modified by manipulating the temperature during seed production. The potential thermotolerance of seed thereby increased, wherein thermosensitive genotypes became thermotolerant and thermotolerant genotypes (e.g., PI251245) germinated fully at 36 °C. This information is useful for improving lettuce seed germination during periods of high soil temperature, and can be used to study the biology of thermotolerance in lettuce.
Abstract
Seed treatments and soil covers were used to assess stand establishment and uniformity of direct-seeded cabbage (Brassica oleracea L. var capitata) under high and low soil temperatures. Generally, primed seeds did not result in increased or more uniform seedling emergence compared to untreated seeds. Germinated seeds sown with a magnesium silicate gel (Laponite) or a starch-acrylamide-acrylate polymer gel (Liquagel) resulted in incomplete stands under heat stress, and stands for all plantings were generally lower when cabbage seeds were sown in a gel than when sown without a gel. Peat-vermiculite (Plug-mix) and calcined clay (GrowSorb) seed covers improved stands regardless of seed treatment when average soil temperatures were ≥30°C. Under normal (25°) to cooler soil conditions stands were not improved by seed treatment or seed cover.
Early plant growth, root quality, and yield from sweetpotato plants obtained from zygotic seed, somatic embryos, or cloned from stock plants (through micropropagation, rooted node explants, or nonrooted terminal vine cuttings) were compared in field plantings established in 1986, 1987, and 1988 in Gainesville and/or Homestead, Fla. At planting, transplants derived from somatic embryos had more nodes than the other propagules, while vine length per plant was greatest with nonrooted vine cuttings obtained from stock plants. The number of nodes (up to 253%) and vine growth (up to 517%) were greater when plants were derived from stock plants and zygotic embryos than from somatic embryos 4 weeks (1987) and 6 weeks (1988) after planting. Vegetative growth, larger-sized storage roots (>6 cm in diameter), and total yields (all root grades combined) were consistently reduced when plants were derived from somatic embryos compared with propagules of stock plant origin. Plants obtained from somatic embryos required more time for roots to bulk or size than the other propagule types. Root yield from plantlets derived from somatic embryos showed a 14-fold increase when harvest was delayed at least 53 more days. Root weight, regardless of harvest date, was greater when plants were derived from stock plants rather than from somatic embryos, while in most cases plants derived from somatic embryos yielded a greater number of roots than from stock plants. Plants obtained through somatic embryony and harvested at a later date typically had yields exceed 1.8 kg per plant. Morphology of plants obtained from somatic embryos was uniform and identical to plants derived from stock plants.
Commercially processed citrus seeds of Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L) Raf.], Swingle citrumelo (C. paradisi Macf. × P. trifoliata), Cleopatra mandarin (C. reticulate Blanco), and sour orange (C. aurantium L.) were used to test the effects of grading, hydrating, and priming on the rate of germination and seedling emergence. Sorting seed into groups by fresh weight or diameter did not generally improve seed performance. Seed fresh weight was highly correlated with maximum seed diameter; also, large seed weight and size were associated with a larger number of embryos. When seedlings from the extra embryos were removed, large seed produced the largest seedlings. Soaking seeds in aerated water significantly increased germination and emergence rates over unsoaked seeds. Soaking at 35C rather than 25C enhanced these differences. Priming seeds in one of three solutions of polyethylene glycol 6000 (—0.6 to—1.2 Mpa) was not successful> as germination and emergence Per centages were lower than in distilled water.
Commercial sweetpotato fields are established with asexually derived propagules as either cut or pulled plants. Micropropagation and somatic embryogenesis were/are being investigated to improve seed production. Micropropagation of sweetpotato is utilized in California as part of its seed production program. Several studies have been conducted or are ongoing in North Carolina to evaluate the utility of micropropagation as part of its certification program. In 1992, yield was increased in `Beauregard' with micropropagated plants compared with plants that were derived from the North Carolina Certified Seed Program. However, there were no yield increases in four years of comparisons when `Jewel' and `White Delight' were micropropagated. A trend towards early bulking has consistently been observed in micropropagated `Jewel', `White Delight' and Beauregard. Early plant growth and yields of plants (`White Star') obtained from somatic embryos, micropropagation or vine cuttings were compared. Plants derived from somatic embryos grew slower and yielded less root weight than cut plants; however, more storage roots were obtained from plants obtained from somatic embryos indicating high yield potential if root bulking is achieved.
Abstract
2-Chloro-9-hydroxyfluorene-9-carboxylic acid (chlorflurenol) 3 at 50 or 100 ppm increased fruit production of a gynoecious cucumber (Cucumis sativus L. cv. MSU 713-5) under field conditions of both normal and reduced pollination. A monoecious cultivar, ‘Wisconsin SMR 18’, treated with (2-chloroethyl) phosphonic acid (ethephon) and then chlorflurenol produced over twice as many fruit as the control when pollen was plentiful, and more than 4 ⨯ as many when pollen supply was limited. Ethephon sprays alone increased pistillate flower formation on the monoecious cultivar but did not increase fruit set. Chlorflurenol treatments increased the proportion of fruit in the smaller, more valuable size grades and appear advantageous for mechanically harvested pickling cucumbers.
The high temperature severely reduces seed germination, emergence, and seedling uniformity in celery (Apium graveolens L.). Celery seeds were primed via solid matrix priming (SMP) using water or 1% sodium hypochlorite (NaOCl) solution at 1, 2, 3 or 4 ml rate for 2, 4, 6, 8, 10, 12 and 14 days. Moisture content of the seeds was calculated for each priming treatment and time interval. After priming, the seeds were dried back to the original dry weight. The germination percentage was calculated for each treatment at two different temperatures (15 and 30C). The seed primed with NaOCl gained significantly less moisture during priming than the water treatment. Seeds primed with NaOCl had significantly greater germination at both 15 and 30C, compared to seeds primed with water. The germination percentage of non-primed seeds was 83% and 2% at 15C and 30C, respectively. The final germination percentage at 30C was increase to 85% when the seeds were primed with 3 ml of NaOCl for 14 days. The combination of SMP with NaOCl significantly reduced the negative effect of high temperature on celery seed germination.
Thermotolerance in lettuce seed at high temperature was investigated using primed and nonprimed seed or seeds matured at 20/10°C and 30/20°C. During seed germination at 36°C, the structural changes of the seed coverings in front of the radicle tip were observed in an anatomical study. In all seeds during imbibition, regardless of seed maturation temperature or priming, a crack appeared on one side of the cap tissue and the endosperm separated from the integument in front of the radicle tip. Additional changes took place during imbibition: the protein bodies in the vacuoles enlarged and were gradually depleted, large empty vacuoles formed, the cytoplasm condensed, the endosperm shrank, the endosperm cell wall dissolved and ruptured, then the radicle elongated toward this ruptured area. The findings suggested that the papery endosperm layer presented mechanical resistance to lettuce seed germination and the weakening of this layer was a prerequisite to radicle protrusion at high temperature. Seeds of `Dark Green Boston', `Everglades', and PI 251245 matured at 30/20°C had greater thermotolerance than those matured at 20/10°C. Results of the anatomical study indicated that the endosperm cell walls in front of the radicle of seeds matured at 30/20°C were more easily disrupted and ruptured during early imbibition than seeds matured at 20/10°C, suggesting that these seeds could germinate quickly at supra-optimal temperatures. From anatomical studies conducted to identify and characterize thermotolerance in lettuce seed germination, it was observed that genotype thermotolerance had the ability to reduce physical resistance of the endosperm by weakening the cell wall and by depleting stored reserves.
Galia-type muskmelon (Cucumis melo cv. Gal-152) was grown as a fall and spring crop to determine the effect of plant density (1.7, 2.5, 3.3, and 4.1 plants/m2) on yield, fruit quality, plant growth, and economic feasibility for producing the crop in a greenhouse. Plant density had no influence on the early or total number of fruit produced per plant. Marketable yields increased linearly from 11.0 to 20.0 kg·m−2 in fall and from 21.9 to 48.3 kg·m−2 in spring with increasing plant density. Mean fruit size was unaffected by plant density during fall (mean weight, 1.0 kg), but was reduced linearly during spring from 1.8 kg at 1.7 plants/m2 to 1.5 kg at 4.1 plants/m2. Soluble solids content was unaffected by plant density in either fall or spring and averaged 10.1% in both seasons. Number of leaves per plant was unaffected by plant density, but internode length was increased at 4.1 plants/m2 compared with plants from the other densities. Increasing the plant density of ‘Gal-152’ muskmelon grown under protected cultivation led to increased yields in both fall and spring without negatively impacting fruit quality. When the market price is $1.44/kg, increased yields at 3.3 plants/m2 can potentially increase net returns over yields of plants spaced at 2.5 plants/m2 by 25% and nearly double net returns from plants grown at 1.7 plants/m2.
Lettuce (Lactuca sativa L.) `South Bay' transplant growth and development were evaluated at 0, 30, 60, 90, and 120 mg·L–1 N fertigated at frequencies of every 1, 2, 3, or 4 days in a floatation production system to produce plants with optimum roots and shoots which easily pull from trays. Greenhouse experiments (four) were conducted to evaluate root and shoot weight, percent transplant pulling success, and leaf N content, 28 days after sowing (DAS). Field trials, using transplants produced in Greenhouse experiments 2 and 4, were conducted to evaluated subsequent yield, head quality characteristics, and leaf N content. Generally, as N concentrations increased, dry shoot weight and leaf N concentration increased, and root:shoot ratios decreased linearly or quadratically. Lettuce transplants grown in a floatation irrigation system fertigated every second to third day with 60 to 90 mg·L–1 N resulted in transplants with optimum root systems to achieve the highest pulling success rate from flats. Subsequent yields and head quality were optimum for pretransplant production fertigation N concentration of 60 to 90 mg·L–1, regardless of irrigation frequency.