5. THE ROLE OF TEMPERATURE IN seed germination
Dormancy is one of the most important factors inhibiting seed germination and hindering productivity of field crops (Bentsink & Koornneef, 2008; Yildiz et al., 2017). Survival of a dormant seed is ensured by showing apparent metabolic arrest, enduring unfavorable environmental conditions and timing germination for the correct season (Bentsink & Koornneef, 2008; Bewley, 1997; Roberts, 1972a). Even at a dormant state, seeds continuously deteriorate and are still susceptible to a wide range of environmental fluctuations, such as temperature, moisture content and oxygen pressure, lately influencing seed viability (Ellis & Roberts, 1980a, 1980b; Owen & Ashton, 1956; Roberts, 1972b). Specifically, seed exposure to warm temperatures for a long period of time may act as catalyzer for dehydration (Berjak & Pammenter, 2013; Silva, 1998; Syeda, Khan, & Mohmand, 2000). For instance, rice and wheat seeds exposed to warm temperatures, 40°C for 40 days and 37-50°C for 12 months, respectively, show decreased longevity when compared to the control (Ellis, Hong, & Jackson, 1993; Nasreen, 1999).
The capacity to trigger seed germination at the right time is a key aspect in crop management and this is correlated with the level of seed dormancy (Benech-Arnold, Rodriguez, & Batlla, 2013). Deeply dormant seeds result in delayed germination or no germination at all even under favorable conditions, in contrast, a shallow dormant seed might exhibit pre-germination or even germinate while still attached to the mother plant, known as preharvest sprouting (Rodríguez, Barrero, Corbineau, Gubler, & Benech-Arnold, 2015; Soppe & Bentsink, 2016)(Fang & Chu, 2008; S. Liu et al., 2015). Breaking dormancy and coordinating germination is also critical during malt production, in which barley seeds undergo germination triggering processes (K. Shu et al., 2015; Ullrich, Han, & Jones, 1997). Although preharvest sprouting is highly species and genotype-dependent, it is mostly observed after exposing plants to a rainfall or high moisture air conditions, but also occurs upon exposing the plants to high temperatures during the grain ontogenesis stage (Figure 4E ). During grain ontogenesis, low temperatures produce higher level of seed dormancy and better physiological mature seeds than control temperatures (Nyachiro, Clarke, DePauw, Knox, & Armstrong, 2002).
Among the genes extensively studied by having a role in germination, two of the most important players have their expression level regulated by temperature: MOTHER OF FT AND TFL1 (MFT ) and DELAY OF GERMINATION 1 (DOG1 ) (Bentsink, Jowett, Hanhart, & Koornneef, 2006; Xi, Liu, Hou, & Yu, 2010). MFT is suggested to be the causal gene for the wheat seed dormancy QTL varying across a temperature range (Nakamura et al., 2011). Grains from plants at seed-setting stage that were exposed to high temperatures regimes (25°C) show an abnormally low expression of MFT . In contrast, wheat grains matured under lower temperature (13°C) show an increased transcript level of MFT . In rice, OsMFT2 knock-out lines showed preharvest sprouting under high temperature and rainy weather, whereas wild type plants and overexpression lines did not (Song et al., 2020). In parallel, DOG1 transcript levels fluctuate according to both dormancy level and soil temperature in buried seeds, with low temperatures showing a higher level of DOG1 than high temperatures (Footitt, Douterelo-Soler, Clay, & Finch-Savage, 2011; Graeber et al., 2014; Nakabayashi et al., 2012). DOG1 is suggested to play a role in balancing ABA and GA levels by promoting ABA biosynthesis and GA catabolism, hormones known for promoting and breaking dormancy, respectively (P. Li, Ni, Ying, Wei, & hu, 2019; A. Yan & Chen, 2017).
In short, dormancy level and establishment during the seed maturing stages are tightly correlated to the temperature plants or seeds are exposed to. Generally, lower temperatures result in increased dormancy depth; in contrast, plants or seeds exposed to supra-optimum temperature regimes show shallow dormancy, and therefore may drive occurrence of undesirable events such as preharvest sprouting. For crops, the domestication process already resulted in selection of less dormant cultivars than the ones found in the nature (Benech-Arnold et al., 2013); however, with the warming trend observed and predicted for climate changes, seed dormancy level might decrease even more, leading to more common productivity losses and decrease in grain quality.
After dormancy is broken by exposing the seed to specific conditions, environmental factors, such as temperature, trigger physiological and biochemical processes causing the seed to germinate. Although temperature positively correlates with accelerated germination for multiple species (Barpete et al., 2015; Buriro et al., 2011), once the species-optimum temperature is reached, every increase from that point decreases germination rate rapidly (Lamichhane et al., 2019; Tyagi & Tripathi, 1983).
7. BIOCHEMICAL PROCESSES UNDER HIGH TEMPERATURE
High temperature affects several biochemical processes in plants. High day temperature impacts photosynthesis (light-dependent reactions and carbon assimilation) and photorespiration (Ahammed, Xu, Liu, & Chen, 2018; Ainsworth & Ort, 2010; Bianchetti et al., 2020; Crafts-Brandner & Salvucci, 2002; Gupta et al., 2013; Kume, Akitsu, & Nasahara, 2019; Lipova, Krchnak, Komenda, & Ilik, 2010; Nagar et al., 2015; Sharma, Andersen, Ottosen, & Rosenqvist, 2015), while respiration is mainly affected by high night temperature (Dusenge et al., 2019)(Figure 5) .
Rubisco activase (Rca) regulates the proportion of catalytically active Rubisco, the central photosynthetic enzyme (Bracher, Whitney, Hartl, & Hayer-Hartl, 2017; Portis Jr, 2003) (Figure 5) . Noteworthy, a functional Rca gene encodes two or more isoforms based on alternative splicing of pre-mRNA in a temperature-dependent manner. One of the isoforms is more thermotolerant in crops (Crafts-Brandner & Salvucci, 2002; Scafaro et al., 2018; Scafaro, Bautsoens, den Boer, Van Rie, & Galle, 2019; Yamori, Masumoto, Fukayama, & Makino, 2012; Yin et al., 2014).
Notably, efficient photosynthesis depends on how the Rubisco enzyme (carboxylase or oxygenase) discriminates between CO2 and O2 as the substrate (Figure 5) . Photorespiration, a major productivity limitation for C3 crops, is exacerbated as temperature increases. A decreased [CO2]/[O2] ratio due to the reduction of stomatal conductance caused by warm temperature, results in the decreased Rubisco specificity for CO2 relative to O2 (Walker, VanLoocke, Bernacchi, & Ort, 2016). Compared with Rubisco, the rate of regeneration of the CO2 acceptor ribulose 1,5-bisphosphate (RuBP) is more sensitive to higher temperature (e.g. less ATP supply caused by the impaired electron transport) (Sage, 2002). In maize, however, Rca and Rubisco activation are the major limitation to net photosynthesis (Crafts-Brandner & Salvucci, 2002).
In addition, mean night-time temperatures are rising at a faster rate than those during the daytime (Davy, Esau, Chernokulsky, Outten, & Zilitinkevich, 2017; Sadok & Jagadish, 2020), which speeds up respiration and results in crop yield and quality reduction (Coast, Šebela, Quiñones, & Jagadish, 2020; Dusenge et al., 2019; Impa et al., 2019; Impa et al., 2020; Schaarschmidt et al., 2020).
Acclimation of the above photosynthetic reactions to heat stress is significant when the temperature is elevated gradually, which, however, is less likely during sudden heat waves or intense heat stress (Posch et al., 2019; Way & Yamori, 2014). These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures (Kaur, Sinha, & Bhunia, 2019; Ruiz-Vera, Siebers, Drag, Ort, & Bernacchi, 2015).