Sensory analysis was performed on 3 advanced hybrid carrot (Daucus carota L.) selections, and the genetically nonbitter cucumber hybrid ‘County Fair’ (Cucumis sativus L.). The crops were grown in field plots of silt loam amended with 90 MT/ha of industrial or municipal sludge and a control with no sludge application. The 3 bed types used were a ground level bed, a 15-cm raised bed, and a 15-cm raised bed of unamended topsoil over prepared ground bed (overcover bed). Carrots produced in industrial sludge-amended soil and raised bed had the harshest, least desirable flavor. Cucumbers grown in raised and overcover beds produced the most bitter fruit.
Four methods for screening carrot (Daucus carota L.) germplasm for resistance to bacterial soft rot were compared. There were differences in resistance among strains, with most severe damage caused by Erwinia carotovora pv. carotovora SR 394 (L.R. Jones) Holland and Erwinia carotovora pv. atroseptica SR 159 (van Hall) Jennison. Inoculation of cross-sectional root slices with bacteria applied in suspension-soaked paper disks produced the most consistent response. The severity of disease damage was proportional to bacterial suspension concentration. With the development of a standard screening method, it may be possible for breeders to breed carrots with reduced susceptibility to soft rot.
Cavity spot of carrot, caused by several species of Pythium, is endemic in many carrot production areas of the world, including the Holland/Bradford Marsh region of Ontario, Canada. Field trials were conducted from 2002–04 to determine if carrots with different pigments varied in susceptibility to the disease. Carrots from the USDA breeding program at the University of Wisconsin were seeded in muck soil (pH 6.4, 60% organic matter) on 28, 30, and 27 May, harvested 22, 22, and 23 Oct., and assessed for disease on 5, 8, and 10 Dec. 2002, 2003, and 2004, respectively. The carrots were white (W 105-7), yellow (W 102-1), dark orange (W 101-23), red (W 104-3), and purple (W 106-3). Cultivar `Cellobunch' was included in 2003 and 2004. Twenty-five carrots of each of four replicate plots were assessed in 2002 and 2003, and 50 carrots were assessed in 2004, for disease incidence and severity [disease severity index (DSI), based on the size of the largest lesion per carrot]. Disease incidence was moderate in 2002 and 2003 (34%, 33%), and high in 2004 (60%). Consistent differences in susceptibility to cavity spot were identified over the three years of trials. The purple carrot had the lowest incidence (12%) and severity (7 DSI) of cavity spot, followed by the dark orange carrot (39%, 22 DSI) as compared to the susceptible yellow carrot (58%, 41 DSI). There was no difference in disease reaction between the yellow and white carrots. `Cellobunch' had the same reaction as the dark orange carrot. Studies are needed to determine whether the pigments themselves cause differences in the disease response.
Garlic is an asexually propagated crop in which the greatest yield losses are attributed to virus infection. Currently, virus-free garlic is produced through shoot tip culture, and there are no known naturally occurring resistant clones. This study evaluated garlic germplasm (propagated from typical bulbs, not shoot tips) for incidence of two viruses known to infect garlic (onion yellow dwarf virus, OYDV and leek yellow stripe virus, LYSV) using dot blot ELISA. Young leaf tissue was collected from 173 garlic clones. For 118 clones, plants grown in the field from typical bulbs only were evaluated. For 55 clones, plants grown in the greenhouse from both bulbs and topsets (bulbils) were evaluated. Topsets are small bulbs that are produced in the inflorescence of stalking garlic. Each clone was tested at least three times for incidence of both viruses. In field grown bulbs, we found that 70% were infected with OYDV and 85 % were infected with LYSV. In greenhouse grown samples, incidence of OYDV was generally higher in plants from topsets than those from bulbs while no differences were seen for LYSV. Three clones were negative for both viruses and might be a useful source of resistance that can be used in producing virus resistant lines.
A sample of 124 Daucus carota L. accessions, including cultivated carrot [D. carota ssp. sativus (Hoffm.) Arcangeli] and related wild subspecies, using a variety of molecular markers was examined. Represented within the samples were wild accessions from 18 countries, 14 of 16 major root types of European origin, and examples of major North American and Asian cultivated carrot types. Amplified fragment length polymorphism (AFLP) and inter-simple sequence repeat (ISSR) markers revealed extensive variation within D. carota. Although cultivated carrot and wild D. carota subspecies can cross freely, cultivated and wild carrots clustered separately, supporting the possibility that human selection for desirable horticultural traits has artificially reduced gene flow between cultivated and wild forms. Our analyses support the likelihood that North American D. carota populations arose due to introduction of weedy materials rather than escape of cultivated forms. With the exception of wild vs. cultivated types, no genetic alliances were evident in dendrogram topology. Furthermore, between and even within nonmapped marker classes, dendrogram topology predictions were not consistent. Generally poor correlations among root types, geographic origin, mitochondrial, plastid, and specific nuclear diversity and AFLP/ISSR data were also observed. We concluded that genetic diversity in carrot is extensive and relatively nonstructured in nature.
Drought is one of the major environmental challenges constraining the production of agricultural crops, including carrot. Seed germination is the initial and most critical stage of crop establishment, and it is very sensitive to drought stress because water scarcity affects the enzymatic solubilization of stored metabolites in seeds that provide energy for the growth of germinating embryo. Few studies evaluating the effect of drought stress on carrot seed germination of more than a few cultivars grown under stress have been published. Therefore, the present study was designed to define the appropriate osmotic potential for evaluating drought tolerance of carrot, evaluate the response of diverse carrot germplasm to drought stress during seed germination to identify drought-tolerant accessions that may be used by plant breeders, and evaluate the relation between amylase activity and germination rate of drought-tolerant and drought-sensitive accessions. To identify an appropriate screening osmotic potential, two commercial cultivars and two United States Department of Agriculture inbreeds were evaluated at six osmotic potentials (00, −0.30, −0.51, −0.58, −0.80, and −1.05 MPa); −0.58 MPa was identified as the optimal osmotic potential for screening the drought tolerance of carrot seed. Cultivated and wild carrot plant introductions (PIs) (n = 200 and n = 50, respectively) from the National Plant Germplasm System were evaluated for drought tolerance under nonstress and simulated drought stress conditions (00 MPa and −0.58 MPa, respectively) by calculating the absolute decrease (AD) in percent germination, inhibition index (II), relative drought tolerance (RDT), and drought tolerance index (DTI). All measurements of drought tolerance identified significant differences among accessions; the AD in seed germination ranged from 0.0% to 69.3%, II ranged from 0.0% to 80.2%, RDT ranged from 0.2 to 1.0, and DTI ranged from 0.13 to 1.47. All wild carrot accessions displayed low levels of drought tolerance, but PI 652387 and PI 177381 (both from Turkey) and PI 274297 (Pakistan) were most drought-sensitive, whereas cultivated accessions PI 643114 (United States), PI 652208 (China), and PI 502347 (Uzbekistan) were most drought-tolerant. Tolerant accessions displayed much higher α-amylase activity under nonstress conditions than sensitive accessions, and α-amylase activity of tolerant accessions was also reduced less with seed germination under increasing osmotic potential (range, 0.0 to −1.05 MPa) than sensitive accessions over 24, 48, and 72 hours of seed germination. This is the first evaluation of drought stress tolerance during seed germination and the enzymatic response of diverse carrot germplasms under simulated drought stress.
Carrot production is constrained by high levels of heat stress during the germination stage in many global regions. Few studies have been published evaluating the effect of heat stress on carrot seed germination or screening for genetic heat stress tolerance. The objectives of this study were to evaluate the response of diverse carrot germplasm to heat stress, identify heat-tolerant germplasm that may be used by plant breeders, and define the appropriate temperature for assessing heat tolerance in germinating carrot seed. To identify an appropriate screening temperature, three commercial hybrids and an open pollinated variety were evaluated at five temperatures (24, 32.5, 35, 37.5, and 40 °C). In preliminary studies, 35 °C was identified as the optimal temperature for screening heat tolerance of carrot seed. Cultivated and wild carrot plant introductions (PIs) (n = 270) from the U.S. Department of Agriculture (USDA) National Plant Germplasm System (NPGS) representing 41 countries, inbred lines from the USDA Agricultural Research Service (n = 15), and widely grown commercial hybrids (n = 8) were evaluated for heat tolerance under heat stress and nonstress conditions (35 °C and 24 °C, respectively) by calculating absolute decrease in percent germination (AD), inhibition index (II), relative heat tolerance (RHT), and heat tolerance index (HTI). All measurements of heat tolerance identified significant differences among accessions; AD ranged from −13.0% to 86.7%, II ranged from 35.7% to 100.0%, RHT ranged from 0 to 1.36, and HTI ranged from 0.0 to 1.45. The broad-sense heritability (H2) calculations ranged from 0.64 to 0.86 for different traits, indicating a moderately strong genetic contribution to the phenotypic variation. Several wild carrot accessions and inbred lines displayed low levels of heat tolerance, whereas cultivated accessions PI 643114 (United States), PI 652400 and PI 652403 (Turkey), PI 652208 (China), and PI 652403 (Russia) were most heat tolerant. This is the first evaluation of heritability for heat stress tolerance during carrot seed germination, the first measure of HTI, and the first correlation calculation between heat and salt tolerance during germination in carrot.
Heat waves occur with more regularity and they adversely affect the yield of cool season crops including carrot (Daucus carota L.). Heat stress influences various biochemical and physiological processes including cell membrane permeability. Ion leakage and increase in cell permeability are indicators of cell membrane stability and have been used to evaluate the stress tolerance response in numerous crops and inform plant breeders for improving heat tolerance. No study has been published about the effects of heat stress on cell membrane stability and relative cell injury of carrot. Therefore, the present study was designed to estimate these stress indicators in response to heat stress at the early and late seedling developmental stages of 215 diverse accessions of wild and cultivated carrot germplasm. The article identifies the relationship between early and late stages of seedling tolerance across carrot genotypes and identifies heat-tolerant genotypes for further genetic analysis. Significant genetic variation among these stress indicators was identified with cell membrane stability and relative cell injury ranging from 6.3% to 97.3% and 2.8% to 76.6% at the early seedling stage, respectively; whereas cell membrane stability and relative cell injury ranged from 2.0% to 94.0% and 2.5% to 78.5%, respectively, at the late seedling stage under heat stress. Broad-sense heritability ranged from 0.64 to 0.91 for traits of interest under study, which indicates a relatively strong contribution of genetic factors in phenotypic variation among accessions. Heat tolerance varied widely among both wild and cultivated accessions, but the incidence of tolerance was higher in cultivated carrots than in wild carrots. The cultivated carrot accessions PI 326009 (Uzbekistan), PI 451754 (Netherlands), L2450 (USA), and PI 502654 (Pakistan) were identified as the most heat-tolerant accessions with highest cell membrane stability. This is the first evaluation of cell membrane stability and relative cell injury in response to heat stress during carrot development.