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.
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.
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.