Tomato (Solanum lycopersicum L.) is influenced by some abiotic stresses that have a major impact on fruit quality and yield. Heat stress impacts the crop in several ways, including disruption of pollen development and viability, fertilization, enhancement of premature flower abortion, early fruit drop, and direct damage to fruits, affecting yield and quality. With growing evidence of climate change, increased average temperature, and extreme weather events, there is the potential for significant impacts on agriculture production (Sawicka et al., 2017; Wahid et al., 2007; Wang et al., 2017). Global climate change will impact agriculture production and food security throughout the world (Cheng et al., 2017; Hall, 2001; Meng et al., 2017; Zhou et al., 2017). The evidence of global warming from general circulation models (Intergovernmental Panel on Climate Change, 2014) has revived the interest in studying yield declines in crops, including tomato at physiologically critical temperatures (Peet et al., 1998; Wahid et al., 2007).
The impact of heat stress is complex; it is influenced by multiple factors, such as intensity, duration, and rate of temperature increase (Cheng et al., 2017; Meng et al., 2017; Zhou et al., 2017). Field-grown tomatoes around the world regularly experience heat stress conditions of 35 °C, whereas 40 to 45 °C temperatures are rare, resulting in annual crop losses resulting from a reduction in photosynthesis capacity and reduced fruit set (Peet et al., 1998; Zhang et al., 2012). In the United States, for instance, many of the leading tomato-growing regions regularly experience heat stress temperatures (Yilmaz and Tolunay, 2012). In California, Florida, and North Carolina, the majority of the major vegetable growing regions are exposed to a yearly average of 90 to 150 d, 150 to 210 d, and 60 to 120 d of ≥30 °C, respectively (American Horticultural Society, 1995). Optimum growing temperatures for tomato (Solanum lycopersicum L.) are less than 32 °C and 24 °C during the day and night, respectively. Therefore, most of the tomato production regions in these states are at risk of exposing the crop to heat stress temperatures for part or all of the summer production season. In addition, current U.S. temperature data indicate a trend of an increased frequency of rare high temperatures, especially of elevated nighttime lows (National Oceanic and Atmospheric Administration, 2016). These current trends are reminiscent of the heat waves experienced in the United States during the 1930s, which remain the worst on record (Environmental Protection Agency, 2016). These realities highlight the need for enhanced heat stress-tolerant varieties with desirable horticultural traits.
In tomato, heat stress causes reduced yields because of failure to set fruit and reduced photosynthetic capacity (Peet et al., 1998; Zhang et al., 2011, 2012). High-temperature stress has been reported to affect many physiologic traits, including fresh plant weight, dry weight, and leaf area in tomato (Shaheen et al., 2016; Zhou et al., 2017). Other vegetative effects include reduced photosynthetic efficiency (Bartsur et al., 1985; Criddle et al., 1997), reduced assimilate translocation, reduced mesophyll resistance, and enhanced disorganization of cellular organs (Chen et al., 1982).
The most damaging impact is on fruit yield. The yield reduction is related primarily to reduced fruit set, which may not occur for many reasons, including adverse effects on meiosis of ovules and pollen mother cells, reduced pollen shed resulting from impaired development of the endothecium in the anthers, stigma position (exerted under heat stress), number of pollen grains retained by the stigma, pollen germination, pollen tube growth, ovule viability, fertilization and postfertilization processes, and growth of the endosperm (Driedonks et al., 2016; Peet et al., 1997; Sato et al., 2002; Zhou et al., 2017). Other indirect yield-reducing effects of heat stress include fruit cracking, malformation of fruits (e.g., catfacing), and a malformed blossom-end scar. In sum, heat stress reduces the fruit number, quality, and marketable yield of tomato.
Heat stress tolerance has been analyzed in different plant species, including Arabidopsis thaliana and tomato, investigating the genetic mechanisms of adaptation. Heat shock protein analyses have been reported since the 1970s. In tomato, several studies have tried to identify genes conferring heat stress tolerance so they can be incorporated into breeding materials. Although some interesting and useful information was identified, its introgression into breeding materials has not progressed very well (Bita et al., 2011; Golam et al., 2012; Wahid et al., 2007).
For practical breeding purposes, phenotypic traits associated with heat stress tolerance are measured rather than the direct physiologic mechanisms. There are a few studies from various parts of the world investigating the genetic control of heat stress tolerance in tomato. However, those studies show inconsistent results in terms of genetic control. In genetic analyses, most of the heat stress-related traits were found to exhibit overdominance in inheritance analyses, whereas partial dominance was identified for days-to-flowering and the number of trusses per plant (Hazra and Ansary, 2008). Identification of heat tolerance in screening trials of tomatoes has been accomplished by evaluating them for flowering and fruit set traits because these two processes are sensitive to heat stress and are related directly to fruit yield and quality (Abdulbaki and Stommel, 1995; Berry and Uddin, 1988; Beshirelahmadi and Stevens, 1979).
The mechanisms of heat stress tolerance in tomatoes include many growth-related and morphological traits. Pollen viability (germination, tube growth, and fruit set) is negatively affected by heat stress (Zhou et al., 2015) and was found to be the significant factor affecting fruit set in another North Carolina breeding source, over and above heat stress effects on the female reproductive parts (Peet et al., 1998). Heat tolerance in tomato has also been linked to greater leaf pigment, retention of normal stomata (pore size and shape), chloroplasts (shape, lack of starch grain accumulation), and maintenance of the net photosynthesis rate (Zhou et al., 2015).
Increases in heat affect the efficiency of substrate carbon conversion, which drives growth and repair activities. The conversion efficiency in tomato at ≥30 °C is about 25% of the optimum (21 °C), which resembles the efficiency levels at the suboptimal temperature of 15 °C (Criddle et al., 1997).
In addition, heat stress conditions induce heat shock proteins in heat-tolerant genotypes (Neumann et al., 1987; Nover and Scharf, 1984). These proteins have been suggested to protect plants under heat stress. A tomato chaperone protein, DnaJ, is targeted to the chloroplasts and helps protect the activity of the Rubisco enzyme during heat stress (Wang et al., 2015). Elsewhere, it is reported that only the heat-susceptible lines exhibit the heat stress-induced physiologic perturbations during strong, acute heat stress, which suggests that heat-tolerant lines may also exhibit a greater sort of “buffering” capacity for heat before their systems are disrupted (Camejo et al., 2005, 2006, 2007). Selections from a wide genetic variation have been reported by Shaheen et al. (2016), and molecular characterization would provide further insight into the detailed investigation on the molecular mechanism of heat tolerance in tomato.
Heritability analysis is important to determine the appropriate method of genetic improvement. Heat stress tolerance was found to have low heritability (Hanson et al., 2002). We were interested in assessing the heritability of heat stress tolerance traits—number of flowers, number of fruit per cluster, and fruit set (measured as a percentage)—in two tomato populations derived from crosses between heat-tolerant and heat-sensitive breeding lines when grown under field conditions. Similar traits (number of flowers, number of fruit, and fruit set) have been used for heat stress tolerance studies in tomato (Abdulbaki, 1991; Hanson et al., 2002; Lin et al., 2010). We investigated heritability in a population derived from the line CLN2413, which was identified to be heat tolerant under extremely high-temperature conditions (Hazra and Ansary, 2008). Another heat tolerant line, 230 HS-1(99), is the result of the NCSU breeding program and can produce more flowers.
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