**1. Introduction**

The rising temperature is an intrinsic component of global climate change that controls the carbon fluxes in all the crops. High temperature affects the major plant physiological processes, such as photosynthesis and respiration; therefore, it becomes important to estimate the plant carbon dioxide (CO2) balance that finally decides the crop productivity [1–3].Through these two pathways, the terrestrial ecosystems exchange about 120 Gt of carbon per year with the atmosphere [4]. A rough estimate states that half of the CO2 assimilated annually through photosynthesis is released back to the atmosphere by plant respiration [5–7], and merely 15–25% of the fixed carbon finally translates into yield [8,9]. The projected elevation in temperature beyond 2.0 ◦C by the end of the decade [10] may increase the magnitude of carbon loss exponentially in the physiological temperature range of 0 to 38 ◦C [11], which will further exacerbate in a species-and environment-dependent manner at higher temperatures between 48 and 60 ◦C [12–15].

**Citation:** Sharma, N.; Thakur, M.; Suryakumar, P.; Mukherjee, P.; Raza, A.; Prakash, C.S.; Anand, A. 'Breathing Out' under Heat Stress—Respiratory Control of Crop Yield under High Temperature. *Agronomy* **2022**, *12*, 806. https:// doi.org/10.3390/agronomy12040806

Academic Editor: Daniel Mullan

Received: 16 February 2022 Accepted: 24 March 2022 Published: 27 March 2022

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The carbon lost through the 'breathing out' processes in plants can occur via two mechanisms, namely photorespiration and dark/mitochondrial respiration. These processes release CO2, but dark respiration occurs regardless of light in the plant cells [16,17]. Biochemically, dark respiration is an enzymatically regulated, multistep, amphibolic process that produces ATP by the oxidation of glucose formed during photosynthesis. Glucose is initially broken into pyruvate during glycolysis, which is oxidized to form acetyl-CoA, releasing a molecule of CO2. The acetyl-CoA then enters the tricarboxylic acid (TCA) cycle, where it is oxidized to CO2 and also produces reductants (nicotinamide adenine dinucleotide: NADH; dihydroflavine-adenine dinucleotide: FADH2) that pass through the mitochondrial electron transport chain (ETC). The oxidation of the reductants produces a proton gradient across the inner membrane of the mitochondria that drives the synthesis of ATP. High temperatures impact dark respiration in plants with an exponential increase [18], which can become detrimental due to irreversible damage to the enzymatic machinery [15]. Climate change prediction models have speculated a 3–20% decline in the yield of major crops like wheat, rice, maize, and soybean with every 1 ◦C increase in the global mean temperatures [19,20], which makes it pertinent to relate this loss to the waste of carbon due to respiration. The contribution of dark respiration in limiting the productivity of crops under elevated temperatures has not been extensively reviewed, in comparison to photorespiration. Therefore, our present review discusses the heat-induced alterations in dark respiration in plants and proposes strategies to reduce the carbon loss under the inevitable reality of a changing climate.

#### **2. Respiratory Carbon Loss-A Constraint to Crop Yield**

Respiration, rather than photosynthesis, may be the primary contributor to yield losses in a high temperature climate [11]. Low respiration rates are generally correlated with high crop yields [21,22]. Walker et al. [23] reported that photorespiration decreased soybean and wheat yields by 36% and 20%, respectively, in the United States. In another study, a 10–12% and 17–35% decrease in the yields of wheat and rice, respectively, was reported due to high temperatures [24]. The yield loss in wheat and rice due to high night temperature (HNT) is mainly ascribed to higher dark respiration, which increases the consumption of photoassimilates, thereby resulting in the reduction of non-structural carbohydrates (NSCs) in stem tissues [25,26]. Glaubitz et al. [27] reported that increasing night temperature from 25 ◦C to 35 ◦C resulted in increased leaf respiratory carbon losses in grapevines, as reflected by the decrease in NSCs of 0.025 and 0.041 mg g<sup>−</sup>1dry weight, respectively. Such losses are consistent with metabolite profiling studies in wheat and rice, which revealed an increase in TCA cycle intermediates in leaves exposed to HNT, supporting increased respiration in the photosynthesizing tissue [25,28]. Xu et al. [29] suggest that increased dark respiration restrains source availability under the combined stress of high day and night temperatures, leading to a considerably more severe yield penalty due to carbon loss.
