1. Introduction
Plants face a wide range of temperatures during their life cycle, both on a daily and seasonal level, and need to continuously adapt (Gourdji, Sibley, & Lobell, 2013; Hatfield et al., 2011; Lobell & Gourdji, 2012; Ray, Gerber, MacDonald, & West, 2015) (Figure 1A-B ). In addition to aboveground organs, root systems are also exposed to a soil temperature range dependent on radiation absorption, reflection and permeation, with most variation in the topsoil (Farias et al., 2018; H. Lu et al., 2020; Ren et al., 2017) (Figure 1C ). Furthermore, due to global warming (Figure 1D ), crops are exposed to greater variation in environmental conditions, and this has an impact on their performance (Asseng et al., 2014; Lesk, Rowhani, & Ramankutty, 2016; Liu et al., 2016; Lobell & Gourdji, 2012; Lobell, Schlenker, & Costa-Roberts, 2011; Schauberger et al., 2017; Tack, Barkley, & Nalley, 2015). The yield of staple crops (e.g. wheat, maize, rice, and soybean) already significantly dropped due to increased temperature (Lobell et al., 2011; Lobell, Sibley, & Ivan Ortiz-Monasterio, 2012; Zhao et al., 2017), and this impacts the future demands of the increasing world’s population (World Resources Institute, 2018). Furthermore, it has been estimated that for each degree Celsius (°C) increase, crop production will reduce by 6% (wheat) or 10-12% (rice), thus impacting global food security further (Asseng et al., 2014; Nelson et al., 2010). In this context, nearly all the warmest years in the last 136 years have occurred since 2000 (Figure 1D) and temperature is predicted to increase even further in the coming decades, with up to 4.8°C by 2100 and with a likely increase of at least 1.5°C (Global Climate Change, 2020; The Intergovernmental Panel on Climate Change (IPCC), 2007).
Several physiological and developmental responses to increased temperature have been described. Heat stress is defined as the increase in temperature above a critical threshold for a period of time sufficient to cause irreversible damage to plant growth and development, even death, which is frequently occurring during a hot season. Moderately high temperature causes morphological or photosynthetic changes that together are likely to contribute to adaptive growth acclimation to otherwise detrimental high ambient temperature conditions (Lippmann, Babben, Menger, Delker, & Quint, 2019; Quint et al., 2016; Vu, Xu, Gevaert, & De Smet, 2019). Unlike warm temperature acclimation during vegetative development, (early) reproductive development is more vulnerable to moderately warm temperature, which directly causes grain yield reduction (Draeger et al., 2020; Hedhly, Hormaza, & Herrero, 2009). Physiological effects of high temperature on crops include protein denaturation, aggregation and degradation, degradation of chlorophyll, increased fluidity of membrane lipids, increased membrane permeability, disruption of cell organelle function, inhibition of protein synthesis, reduced rate of net photosynthesis, and cell death (Cossani & Reynolds, 2012; Los & Murata, 2004; Nagar, Singh, Arora, Dhakar, & Ramakrishnan, 2015). Furthermore, warm temperature is likely to shorten the time of growth and development of many crop species, and further affects grain number (when heat stress occurs before anthesis, at meiosis), seed size and early seed setting, accelerates senescence in photosynthetic organs and induces chlorophyll loss, thereby limiting seed setting (Asseng et al., 2014; Hatfield et al., 2011; Wang, Dinler, Vignjevic, Jacobsen, & Wollenweber, 2015; Zhao et al., 2016).
In this review, we assess various thermal responses of crops, focusing on knowledge gained from both monocots (e.g. wheat, barley, maize, rice) and dicots (e.g. soybean or tomato), at different developmental stages. These responses include architecture, photosynthesis during vegetative development, early reproductive phase (e.g. floral transition, and inflorescence, pollen and pistil development) and late generative stage (seed and fruit setting and development). Additionally, crosstalk between high temperature and biotic stresses is assessed. We discuss some molecular mechanisms underlying the physiological and developmental changes caused by moderate or critical high temperature, from which biotechnological and breeding strategies can benefit (Chen et al., 2020; Shen et al., 2019; South, Cavanagh, Liu, & Ort, 2019; Whitney, Birch, Kelso, Beck, & Kapralov, 2015).