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).