The ambient temperature of our planet has significantly increased over the past century (0.13 °C per decade in last 50 years) as part of current global climate change attributed to anthropogenic activities and natural factors (Rohde et al., 2013). Modern climate models have predicted that mean global temperature will rise by 1.1 °C by the end of this century and intense heat waves will also occur more frequently (IPCC, 2013). Heat stress, caused by elevated temperature, is a major threat for cereals, fruits, and vegetable production, especially in warm regions of the world (Farooq et al., 2011; Hasanuzzaman et al., 2013). Vegetables are a rich source of essential dietary micronutrients (Graham et al., 1999) but the effects of heat stress on the productivity of most vegetables have yet not been well-characterized (Fahad et al., 2017; Mahmood et al., 2017). It may be anticipated that cool-season vegetables, like carrot, are especially sensitive to heat stress.
Carrot (Daucus carota L., 2n = 2x = 18) is a biennial, cool-season, Apiaceous root vegetable that is a rich dietary source of provitamin A carotenoids (α- and β-carotene), which is vital for human development and health (Tanumihardjo, 2012). The optimal germination temperature of carrot is 20 to 30 °C (Corbineau et al., 1994) and, like most crops, abiotic stress reduces carrot yield (Bano et al., 2014, Kahouli et al., 2014). Heat, drought, and/or salinity-induced stress contributes to the much lower carrot yields in countries like Pakistan and most of South Asia relative to North America or Europe (FAO, 2017).
Previous studies have reported that temperatures higher than 27 °C can cause significant reduction in root mass, which accounts for the yield of a carrot (Rosenfeld, 2004). High temperature causes heat stress and is considered to be one of the major abiotic factors that limits carrot production because of its adverse effects on seed germination, seedling emergence, and plant growth (Landjeva et al., 2008). Heat tolerance is a complex trait that varies with the severity of stress and plant growth stage. Therefore, there is a need to identify heat-tolerant carrot germplasm with stable growth and yield under high temperature at various stages of growth. Seedling establishment, vegetative growth, and the reproductive phase of the life cycle in carrot are all critical developmental stages, but without the ability to germinate under heat stress, tolerance at later stages of growth is irrelevant. Heat stress tolerance in seeds is typically a multicomplex and polygenic trait (Senthil-Kumar et al., 2007) that varies from species to species and environmental conditions (intensity and exposure period of stress). It has been demonstrated in previous studies that the seed germination of various crops, including wheat (Balla et al., 2012), maize (Iloh et al., 2014; Riley, 1981), rice (Shah et al., 2011), chick pea (Sleimi et al., 2013), and spinach (Hum-Musser et al., 1999), is adversely affected as temperature increases.
Several previous studies demonstrated the negative effects of low temperature on carrot seed germination and stand establishment (Nascimento and Pereira, 2007; Pereira et al., 2007; Vieira et al., 2005), but relatively little has been reported about the effects of high temperatures. Vieira et al. (2005) demonstrated that high temperature inhibits carrot seed germination. High temperature affects some physiological and biochemical processes like oxygen requirement during seed imbibitions (Nascimento et al., 2008). There is no known mechanism for thermotolerance in carrot and insufficient information is available to characterize the influence of high temperature on carrot seed germination and biomass production. The most expansive carrot germplasm evaluation, to date, for heat tolerance at the germination stage, was done by Nascimento et al. (2008), who evaluated 34 commercial cultivars and 63 diverse germplasm accessions. They evaluated heat stress traits including ethylene production, time to first germination, and RHT under 35 °C. The current lack of information limits our understanding of heat tolerance mechanisms in carrot. To expand the information needed to better understand those mechanisms, the objectives of this study were to identify the optimal temperature conditions for evaluating heat tolerance in carrot; to evaluate a large carrot germplasm collection for heat tolerance traits, including HTI, II, and AD at the germination stage; to evaluate the relationship between domestication status and root color on heat tolerance; and to calculate broad-sense heritability for carrot.
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Carrot accession, root color, country of origin, domestication status (DS), mean percent germination without heat stress (Nonstress) ± se, mean percent germination with 35 °C heat stress (Stress) ± se, mean absolute decrease (AD), mean inhibition index (II), relative heat tolerance (RHT), mean heat tolerance index (HTI), hundred seed weight (HSW), and rank based on HTI.