2. Optimum temperature varies according to CROP species and developmental stage
Different plant species respond differently to the wide range of temperatures they are exposed to throughout their life cycle. Each species, shows a minimum, maximum or optimum temperature that regulates growth and that is specific for a particular developmental stage (Hatfield et al., 2011). It is obviously not surprising that the average optimal temperature for growth and developmental progression tends to be lower for the crops originating from temperate regions than for the species from subtropical or even tropical zones (Figure 2A ).
Warm season crops (e.g. maize, rice, tomato and soybean) generally have a higher optimum temperature from emergence to the vegetative growth and development than those crops from temperate zones (e.g. wheat and barley), which are in the range of 25-32°C and 15-25°C, respectively (Ali Tahir, Nakata, Yamaguchi, Nakano, & Mukhtar Ali, 2015; Alsajri et al., 2019; Cannell, 1969; Chavan, Duursma, Tausz, & Ghannoum, 2019; Ford et al., 2016; Friend, 1965; Garmash, 2005; Hakim, Hossain, Teixeira da Silva, Zvolinsky, & Khan, 2012; Havko et al., 2020; Hemming, Walford, Fieg, Dennis, & Trevaskis, 2012; Jumrani & Bhatia, 2018; Krishnan, Ramakrishnan, Reddy, & Reddy, 2011; Liu et al., 2020; T. Lu et al., 2017; Lyu et al., 2020; Sanchez, Rasmussen, & Porter, 2014; Singh, Reddy, Reddy, & Gao, 2014; Tsai, Weng, Chen, Lin, & Tsai, 2019; Yoshida, 1973) (Figure 2B and Table 1 ).
For (cereal) crops with a winter habit, low temperature (referred to as vernalization) is, next to photoperiod, the most important environmental cue to stimulate the transition into the reproductive stage (Dixon et al., 2019; Kiss et al., 2017). Crops with a spring habit only depend on photoperiod (Gol, Tome, & von Korff, 2017). After the transition to reproductive development, the plant is more vulnerable to elevated temperature than the vegetative phase and high temperature causes yield (seed and fruit set) reduction (Figure 2B and Table 1 ) (Draeger et al., 2020; Hedhly et al., 2009). The optimum temperature range for floral development in wheat and barley under inductive photoperiod conditions is 15-20°C (Dixon et al., 2019; Draeger et al., 2020; Draeger & Moore, 2017; Ejaz & von Korff, 2017; Ford et al., 2016; Hemming et al., 2012; Oshino et al., 2007; Oshino et al., 2011), while for (sub)tropical crops this range is relatively higher (24-30°C) from the transition (double-ridge initiation or flower initiation) to pre-anthesis (Table 1 ) (Ayenan et al., 2019; Begcy et al., 2019; Dielen, Lecouvet, Dupont, & Kinet, 2001; Lyu et al., 2020; Martínez-Eixarch & Ellis, 2015; Sanchez et al., 2014; Suwa et al., 2010; Thomas & Raper, 1978; Xu, Wolters-Arts, Mariani, Huber, & Rieu, 2017). During flowering, a temperature higher than 30-33°C increases the risk of spikelet sterility in conventional rice (Bheemanahalli et al., 2017; Jagadish, Craufurd, & Wheeler, 2007; Shi et al., 2018; Zhang et al., 2018). Male fertility is reduced in maize and soybean as the temperature rises above 28-33°C and 26-30°C, respectively (Djanaguiraman, Prasad, Boyle, & Schapaugh, 2011; Sanchez et al., 2014; Thomas & Raper, 1978; Wang, Tao, et al., 2019; Wang et al., 2020; Wiebbecke, Graham, Cianzio, & Palmer, 2012). Wheat and barley have an optimum temperature range during the flowering of around 18-21°C (Alghabari, Lukac, Jones, & Gooding, 2014; Ejaz & von Korff, 2017; Porter & Gawith, 1999; Saini, Sedgley, & Aspinall, 1983, 1984; Sakata, Takahashi, Nishiyama, & Higashitani, 2000). The optimal temperature during the generative phases of the subtropical crop tomato, encompassing most developmental stages from reproductive to fruit development, is at a relatively intermediate range (21-25°C) (Ayenan et al., 2019; Dielen et al., 2001; Goldberg, Beals, & Sanders, 1993; Paupière et al., 2017; Peet, Willits, & Gardner, 1997; Xu et al., 2017) (Table 1 ). However, the optimal temperature range for post-anthesis development, such as early seed and fruit-setting phases, in many crops is sometimes slightly lower compared to other developmental stages (Sato, Peet, & Thomas, 2002; Sehgal et al., 2018) (Figure 2B and Table 1 ). For example, the optimal range for post-anthesis development for maize is 24-29°C (Commuri & Jones, 2001; Sanchez et al., 2014) and 13-18°C for wheat and barley (Abdelrahman, Ishii, El-Sayed, & Tran, 2020; Cochrane, Paterson, & Gould, 2000; Howard et al., 2012; Kino, Pellny, Mitchell, Gonzalez-Uriarte, & Tosi, 2020; Koga et al., 2016; Wardlaw, 2002).
Soil temperature, which is several degrees lower than the air temperature (Shen, McLaughlin, Zhang, Xu, & Liang, 2018), plays an essential role in the underground root growth and development, and affects the uptake and transport of water and nutrients (Koevoets, Venema, Elzenga, & Testerink, 2016). Depending on the climate, the soil shows distinct temperature regimes. In temperate latitudes, a large majority of soil types are grouped and classified accordingly to their temperature as being mesic (mean annual temperatures: >8 °C & <15 °C), thermic (>15 & <22 °C) and hyperthermic soil (>22 °C) (USDA - Natural Resources Conservation Service, 2020). This definition solely considers mean annual temperatures, but seasonal differences may exceed ± 5°C (e.g. summer and winter). In parallel, tropical latitudes are grouped in similar categories but receive the “iso” prefix (e.g isomesic (>8°C and <15°C), isothermic (mean annual temperatures: >15°C and <22°C) or isohyperthermic (mean annual temperatures: >22 °C) and therefore, experience a narrower range of variation in between seasons (< ± 5°C). Plants develop adaptative traits to overcome limitations imposed by extreme soil temperature in their habitat (Garrett, Huynh, & North, 2010; Iversen et al., 2014; Martre, North, Bobich, & Nobel, 2002). Despite the fact that some polar plants are capable of maintaining growth under extremely low temperatures in cold soils (1-3°C), optimum root growth occurs at 12-20 °C (K. Bell & Bliss, 1978). Commonly, temperature fluctuates for the topsoil, and tends to gradually stabilize with depth (Figure 1C ) (Aydin, Sisman, Gültekin, & Dehghan B, 2015; Chakrabarti, Singh, Kumar, Harit, & Misra, 2013; Pramanik et al., 2018). For instance, in the zone of the temperate crop wheat, the mean soil surface temperature fluctuates between 13-17°C (Chakrabarti et al., 2013). A similar top soil temperature fluctuation is observed in the zone of the tropical crop maize, but with higher absolute temperatures compared to the temperate zone (Pramanik et al., 2018; Yin et al., 2016).
Several key biochemical processes also have optimal temperatures: photosynthesis mainly depends on day-time temperature, while night-time temperature affects respiration (Dusenge, Duarte, & Way, 2019). The maximal rate of net photosynthesis is, depending on the variety, 30°C-35°C/23°C (day/night) in rice (Bahuguna, Solis, Shi, & Jagadish, 2017; Weng & Chen, 1987). In maize, relative photosynthesis reaches a maximum at 38-40°C/22-26°C, but sharply drops when the air temperature rises above 40°C (Rotundo, Tang, & Messina, 2019; Wang et al., 2020). In tomato and soybean, the maximal photosynthetic rate is at the range of 25-30°C/15°C and 28-35°C/18-22°C, respectively (Camejo et al., 2005; Djanaguiraman et al., 2011; T. Lu et al., 2017; Wiebbecke et al., 2012). In contrast, in wheat and barley, photosynthetic activity is already negatively affected when the ambient temperature is above 25°C/15°C (day/night) (Djanaguiraman, Boyle, Welti, Jagadish, & Prasad, 2018; Impa et al., 2019; Jedmowski & Bruggemann, 2015; Oukarroum, El Madidi, & Strasser, 2016; Posch et al., 2019) (Figure 2B and Table 1 ).