Carrot crops are established by direct seeding, and poor stands may occur when sowing is done during extremely low or high temperatures. Many publications relate the negative effects of low temperature on carrot stand establishment (Cantliffe and Elballa, 1994; Corbineau et al., 1994; Nascimento and Pereira, 2007; Pereira et al., 2007; Vieira et al., 2005). However, high temperatures (35 to 40 °C) may also delay or inhibit carrot seed germination in the field and reduce uniformity and total stand establishment (Cantliffe and Elballa, 1994; Nascimento and Pereira, 2007). In tropical areas, carrot production is vulnerable to loss from thermal stress (heat) during stand establishment (Vieira et al., 2005), and most commercial carrot cultivars have reduced seed germination at high temperatures (Pereira et al., 2007). Carrot seeds germinate over a range from 10 to 35 °C (Rubatzky et al., 1999) with an optimal range of 25 to 30 °C (Corbineau et al., 1994). The Association of Official Seed Analysts recommends an alternating day/night regimen of 30 to 20 °C (8/16 h) as the standard protocol for carrot seed germination tests (Association of Official Seed Analysts, 1993).
Superior genetic resources are required to reduce the risk of loss from high temperature stress. Within the carrot collection of the USDA-ARS, located in Ames, IA, there are 1129 accessions in the NPGS Daucus collection with 893 accessions available for distribution (K. Reitsma, personal communication). Germplasm characterization is an important function of a gene bank, and knowing the morphological and agronomic traits as well as their reaction to biotic and abiotic stresses in the individual accessions increases the potential usefulness of the germplasm collection (Day-Rubenstein et al., 2006).
Carrot cultivars differ in their sensitivity to high temperatures during seed germination (Pereira et al., 2007). One approach to improve thermotolerance (ability of seed to germinate at high temperature) is to transfer superior alleles from intrinsically thermotolerant wild relatives to less tolerant commercial cultivars (Senthil-Kumar et al., 2007). However, the genetic variation in thermotolerance among cultivated carrot gene bank accessions has not been determined (P. Simon, personal communication).
Thermotolerance in seeds is typically a multigenic trait (Senthil-Kumar et al., 2007), and there is no known thermotolerance mechanism in carrot. In lettuce, quantitative trait loci analysis may provide a new approach for elucidating the physiological factors controlling the imposition and release of seed thermoinhibition (Argyris et al., 2005). Tolerance to high temperatures during germination would seem to require constitutive genetic effects, although the mother-plant environment during seed development and maturation can also affect carrot seed quality (Gray et al., 1988) and influence thermotolerance as observed in other species (Sung et al., 1998). Also, tolerance to high temperatures during seed germination and early seedling growth involves acclimation effects such as synthesis of heat-shock proteins (Vierling, 1991). Oxygen requirements for seed germination may also be modulated by temperature (Bradford et al., 2007), and high temperatures during carrot seed imbibition may affect sensitivity to low oxygen tensions (Corbineau et al., 1994).
High seed vigor is necessary for tolerance to environmental stress (Heydeker, 1972), including high temperatures. For example, improved seedling emergence and uniformity of preconditioned lettuce seeds at high temperature was related to high vigor (Perkins-Veazie and Cantliffe, 1984). Examining two lettuce genotypes, these authors found that priming prevented thermodormancy in unaged but not in aged lettuce seeds. Seed aging can lead to some physiological and biochemical changes such as reduced ethylene and endo-β-mannanase activity and thus lead to thermoinhibition in germination, as observed in lettuce seeds (Nascimento et al., 2005).
The role of ethylene in seed germination has been extensively studied in several species (Abeles et al., 1992). In lettuce, it has been suggested that ethylene is necessary for germination at supraoptimal temperatures (Kozareva et al., 2004, 2006; Nascimento et al., 2000, 2004). The involvement of ethylene in carrot seed germination, especially under stress conditions, has not been studied.
The objectives of this study were to characterize commercial carrot genotypes and screen germplasm to identify lines with greater tolerance to germinate at high temperatures and then to determine if there is a possible correlation between ethylene production and seed germination at high temperatures.
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