2.2 Elevated temperature stress
It is estimated that human activities have led to an approximate increase of 0.8°C to 1.3°C in global temperatures since pre-industrial levels, with a likely increase to 1.0°C to 2.0°C by mid-century given current rates of increase (IPCC, 2021). In addition to increases in mean temperature in most land and ocean regions, regional climates are also likely to see an increase in extreme heat events (IPCC, 2021). This is predicted to lead to a widespread and significant negative impact on crop yields if global warming exceeds 1.5 °C above pre-industrial levels (Battisti & Naylor, 2009; Hatfield & Prueger, 2015; Perkins et al., 2012; IPCC, 2021; Zhu et al., 2021). It is estimated that 3-12% of global crop yields will decline for every 1°C of warming for major global crops (soybean (Glycine max ), rice (Oryza sativa ), wheat (Triticum aestivum ), and maize) (Zhao et al., 2017; Zhu et al., 2021). Temperature (along with daylength) also signal significant physiological transitions in plants, which is a major determinant of yield in grain crops (Ruiz-Vera et al., 2015; Zhu et al., 2018) and proper developmental timing in perennial cropping systems (Leisner, 2020).
Due to this importance, much work has been done to understand the physiological response of crop plants to elevated temperatures (for review see Hatfield & Prueger, 2015; Moore et al., 2021; Zhu et al., 2021). This work has illustrated key impacts of elevated temperature on photosynthesis (Moore et al., 2021), growth, development, and other biochemical and physiological processes (Hatfield & Prueger, 2015; Zhu et al., 2021). From these reviews we enumerate a few key impacts of elevated temperature on photosynthesis (Moore et al., 2021). First, C3 crop plants are sensitive to elevated temperature impacts on photosynthetic enzymes involved in carbon assimilation. This is due to a decline in specificity of the key carboxylation enzyme Rubisco (Ribulose-1,5- bisphosphate carboxylase/oxygenase), deactivating the enzyme under supra-optimal temperatures (Moore et al., 2021). Second, regulation of Rubisco activity by Rubisco activase is another possible area of improvement of plant photosynthetic responses to high temperatures, as manipulation of the thermostability of Rubisco activase at higher temperatures has been shown to increase photosynthetic thermotolerance in Arabidopsis and rice (Kurek et al., 2007; Kumar et al., 2009; Shivhare & Mueller-Cajar, 2017; Scafaro et al., 2016; Scafaro et al., 2019; Wang et al., 2010).
Third, plant photosynthetic responses to heat stress can be modulated through changes in stomatal density and size which in turn, affect rates of stomatal conductance, which is a key control point for gas exchange between the leaf interior and the atmosphere (Moore et al., 2021). Elevated temperature also determines the air vapor pressure deficit, plant transpiration rate, and plant water status, all of which affect stomatal behavior and photosynthetic capacity (Moore et al., 2021). Work to improve stomatal anatomy and metabolism is underway to improve stomatal resilience to heat stress (Moore et al., 2021).
Fourth, elevated temperature can negatively impact photosynthetic capacity through alterations in source-sink relationships (Moore et al., 2021). Changes in the translocation of the products of photosynthesis (carbohydrates) determines source-sink relationships, and changes in this relationship can also affect the timing of vegetative and reproductive development, and ultimately affect yield. Previous work has found that structural changes in the phloem, along with changes in activity and gene expression of key enzymes involved in sucrose transport and metabolism effect source-sink relationship in plants exposed to heat stress (Moore et al., 2021), and are future targets for developing heat-resistant cultivars of plants. Finally, increased temperature has also been shown to cause denaturation of proteins and inhibition of protein synthesis, degradation of chlorophyll, changes in membrane fluidity and permeability, and alterations in respiration and cell death (Zhu et al., 2021), all of which directly affect plant photosynthesis, growth, development, and productivity.
The ultimate impact of elevated temperature on plant growth and development is also dictated by the timing of temperature stress during a plant’s life cycle (Hatfield & Prueger, 2015). Generally, vegetative development has a higher temperature optimum than reproductive development, but a range of acceptable maximum and minimum temperatures for growth and temperature extremes exist (Hatfield & Prueger, 2015; Zhu et al., 2021). Elevated temperatures during vegetative growth leads to accelerated development in non-perennial crops, which can decrease yield potential by reducing vegetative growth and decreasing the duration of reproductive growth (Hatfield & Prueger, 2015). Additionally, elevated temperature can significantly negatively affect reproductive structures, including impacts on pollen viability, fertilization, grain/fruit formation (CCSP, 2008; Hatfield et al., 2011), and chronic exposure to elevated temperatures during pollination can lead to decreased grain/fruit set and yield (Hatfield & Prueger, 2015).
Previous work suggests crop plants that exhibit variation in flowering times during the day may be more resilient to future elevated temperatures, as flowering at cooler times of the day would be beneficial (Caviness & Fagala, 1973; Sha et al., 2011; Sheehy et al., 2005; Wiebbecke et al., 2012). Additionally, the length of anthesis has a strong correlation with crop sensitivity to temperature extremes, as exhibited in the range of anthesis times in maize, rice, sorghum, soybean, peanuts (Arachis hypogaea ), and cotton (Gossypium hirsutum ), with longer anthesis times potentially leading to more resilience to extreme heat events (Hatfield & Prueger, 2015). Taken together, these impacts on plant growth and development may cause declines in yield in annual crop plants but are dependent on CO2 emission scenarios and crops evaluated (Hatfield et al., 2011; Lobell et al., 2011; Schlenker & Roberts, 2009). Further work is needed to understand the complex interaction of elevated CO2 and temperature, crop genetics, biotic stresses, and adaptive management strategies on yield loss estimates (Hatfield & Prueger, 2015). Furthermore, precise evaluations of maximum andminimum temperature, atmospheric water vapor demand and duration of heat stress in both annual and perennial plants is needed to gain a more complete understanding of temperature impacts on plant productivity (Hatfield & Prueger, 2015; Leisner, 2020).