Summary Understanding plant thermal tolerance is fundamental to predicting impacts of extreme temperature events that are increasing in frequency and intensity across the globe. Extremes, not averages, drive species evolution, determine survival and increase crop performance. To better prioritize agricultural and natural systems research, it is crucial to evaluate how researchers are assessing the capacity of plants to tolerate extreme events. We conducted a systematic review to determine how plant thermal tolerance research is distributed across wild and domesticated plants, growth forms and biomes, and to identify crucial knowledge gaps. Our review shows that most thermal tolerance research examines cold tolerance of cultivated species; c. 5% of articles consider both heat and cold tolerance. Plants of extreme environments are understudied, and techniques widely applied in cultivated systems are largely unused in natural systems. Lastly, we find that lack of standardized methods and metrics compromises the potential for mechanistic insight. Our review provides an entry point for those new to the methods used in plant thermal tolerance research and bridges often disparate ecological and agricultural perspectives for the more experienced. We present a considered agenda of thermal tolerance research priorities to stimulate efficient, reliable and repeatable research across the spectrum of plant thermal tolerance.

The high temperature responses of photosynthesis and respiration in wheat are an underexamined, yet potential avenue to improving heat tolerance and avoiding yield losses in a warming climate.

Greater availability of leaf dark respiration (Rdark) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of Rdark data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non‐destructive and high‐throughput method of estimating Rdark from leaf hyperspectral reflectance data that was derived from leaf Rdark measured by a destructive high‐throughput oxygen consumption technique. We generated a large dataset of leaf Rdark for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for Rdark. Leaf Rdark (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7‐ to 15‐fold among individual plants, whereas traits known to scale with Rdark, leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf Rdark, N, and LMA with r2 values of 0.50–0.63, 0.91, and 0.75, respectively, and relative bias of 17–18% for Rdark and 7–12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf Rdark is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of Rdark are discussed.

Mitochondrial respiration (R) is central to plant physiology and responds dynamically to daily short‐term temperature changes. In the longer‐term, changes in energy demand and membrane fluidity can decrease leaf R at a common temperature and increase the temperature at which leaf R peaks (Tmax). However, leaf R functionality is more susceptible to short‐term heatwaves. Catalysis increases with rising leaf temperature, driving faster metabolism and leaf R demand, despite declines in photosynthesis restricting assimilate supply and growth. Proteins denature as temperatures increase further, adding to maintenance costs. Excessive heat also inactivates respiratory enzymes, with a concomitant limitation on the capacity of the R system. These competing push‐and‐pull factors are responsible for the diminishing acceleration in leaf R rate as temperature rises. Under extreme heat, membranes become overly fluid, and enzymes such as the cytochrome c oxidase are impaired. Such changes can lead to over‐reduction of the energy system culminating in reactive oxygen species production. This ultimately leads to the total breakdown of leaf R, setting the limit of leaf survival. Understanding the heat stress responses of leaf R is imperative, given the continued rise in frequency and intensity of heatwaves and the importance of R for plant fitness and survival.

Climate change and future warming will significantly affect crop yield. The capacity of crops to dynamically adjust physiological processes (i.e., acclimate) to warming might improve overall performance. Understanding and quantifying the degree of acclimation in field crops could ensure better parameterization of crop and Earth System models and predictions of crop performance. We hypothesized that for field‐grown wheat, when measured at a common temperature (25°C), crops grown under warmer conditions would exhibit acclimation, leading to enhanced crop performance and yield. Acclimation was defined as (a) decreased rates of net photosynthesis at 25°C (A25) coupled with lower maximum carboxylation capacity (Vcmax25), (b) reduced leaf dark respiration at 25°C (both in terms of O2 consumption Rdark_O225 and CO2 efflux Rdark_CO225) and (c) lower Rdark_CO225 to Vcmax25 ratio. Field experiments were conducted over two seasons with 20 wheat genotypes, sown at three different planting dates, to test these hypotheses. Leaf‐level CO2‐based traits (A25, Rdark_CO225 and Vcmax25) did not show the classic acclimation responses that we hypothesized; by contrast, the hypothesized changes in Rdark_O2 were observed. These findings have implications for predictive crop models that assume similar temperature response among these physiological processes and for predictions of crop performance in a future warmer world.

Wheat photosynthetic heat tolerance can be characterized using minimal chlorophyll fluorescence to quantify the critical temperature (Tcrit) above which incipient damage to the photosynthetic machinery occurs. We investigated intraspecies variation and plasticity of wheat Tcrit under elevated temperature in field and controlled-environment experiments, and assessed whether intraspecies variation mirrored interspecific patterns of global heat tolerance. In the field, wheat Tcrit varied diurnally—declining from noon through to sunrise—and increased with phenological development. Under controlled conditions, heat stress (36 °C) drove a rapid (within 2 h) rise in Tcrit that peaked after 3–4 d. The peak in Tcrit indicated an upper limit to PSII heat tolerance. A global dataset [comprising 183 Triticum and wild wheat (Aegilops) species] generated from the current study and a systematic literature review showed that wheat leaf Tcrit varied by up to 20 °C (roughly two-thirds of reported global plant interspecies variation). However, unlike global patterns of interspecies Tcrit variation that have been linked to latitude of genotype origin, intraspecific variation in wheat Tcrit was unrelated to that. Overall, the observed genotypic variation and plasticity of wheat Tcrit suggest that this trait could be useful in high-throughput phenotyping of wheat photosynthetic heat tolerance.

Plant water use theory has largely been developed within a plantperformance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models.

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