Sorghum (Sorghum bicolor (L.) Moench) is an economically important C₄ cereal crop for more than 500 million people around the globe due to its versatile usage as staple food, bioenergy, feed for livestock and industrial products (Maikasuwa and Ala, 2013; Tari et al., 2013; Ciampitti and Prasad, 2020). Sorghum occupies an important role in global food security along with other cereal crops such as wheat, millets, rice and maize (Ciampitti and Prasad, 2020; Maswada et al., 2020). Sorghum is relatively hardy crop as compared to C₃ cereal crops, mainly grown in the semi-arid regions. In C₄ crops, C₄ photosynthetic pathway and related anatomical advantages enable them to tolerate warmer temperatures compared to C₃ crops by maintaining photosynthesis via, concentrating CO₂ and promoting the carboxylase activity of ribulose 1,5 bisphosphate carboxylate/oxygenase (Rubisco) and reducing the oxygenation activity (Sage and Kubien, 2007; Wang et al., 2012). However, anticipated more abrupt changes in weather, high daytime and nighttime temperatures can be important limiting factors for the growth, development and productivity of sorghum (Prasad et al., 2006b; Prasad et al., 2008a; Djanaguiraman et al., 2014).

Temperature and light cycles of light (day) and dark (night) phases of 24 h of a day synchronize circadian rhythms which includes subcellular level (gene expression, calcium signaling and activities of enzymes) and cell and tissue level (gas exchange, seed germination, hypocotyl elongation and behavior of leaves, flowers, and stomata) cycles to adjust with existing environmental conditions. Meanwhile, changes in temperature cycle, either increase or decrease in daytime or nighttime temperature, can adversely affect normal rhythmic response of plants (McClung, 2006; Srivastava et al., 2019). Negative impacts of high daytime and nighttime temperatures on different cereal crops has been well documented including in finger millet (Opole et al., 2018), maize (Sunoj et al., 2016; Lizaso et al., 2018), rice (Prasad et al., 2006a; Lin et al., 2020; Wang et al., 2020), and wheat (Prasad et al., 2008b; Prasad et al., 2015; Impa et al., 2018).

Each crop has cardinal or critical temperatures (minimum temperature, optimum temperature, and maximum temperature) with thresholds below (minimum temperature) or above (maximum temperature) in which no growth or development or physiological process occurs. Temperatures below and above optimum negatively impacts physiological process, traits, growth, development, and yield. Any growth temperature is the mean of daytime maximum and nighttime minimum temperatures. The same mean temperature can be obtained by different daily temperature (either daytime maximum and nighttime minimum temperatures or diurnal temperature range or amplitude). Identification of mean temperature threshold of growing season (different phenological stages) and responses of crop or process to different temperatures provides basis for quantifying impacts of climate change and adaptation options (Luo, 2011). For sorghum, the maximum critical threshold mean temperature is considered as 34°C (Hammer et al., 1993; Maiti, 1996), while, the optimum daytime and nighttime temperatures are 32 and 22°C, respectively (mean temperature of 27°C) (Prasad et al., 2006b). On a global scale prediction by inter-governmental panel for climate change (IPCC), pointed out a rise of 1.5 to 2°C in mean temperature by end of the 21^st century which is expected to be accompanied with more frequent heat waves and warmer nights (IPCC, 2018). These can negatively affect sorghum productivity, thereby a threat to global food security and economic status of farming community and allied industries.

In sorghum, high daytime and nighttime temperatures are reported to cause abortion of flowers, reduced pollen germination, phospholipid saturation of pollen, seed set, grain number and yield, biomass accumulation, photosynthesis, quantum yield of PS II, activities of antioxidant enzymes and increased reactive oxygen species (ROS), leaf night respiration, and thylakoid membrane damage (Prasad et al., 2006b; Prasad and Djanaguiraman, 2011; Djanaguiraman et al., 2014; Prasad et al., 2015; Singh et al., 2015; Sunoj et al., 2017; Djanaguiraman et al., 2018a; Narayanan et al., 2018). Even though, high daytime and nighttime temperatures negatively affect crops, nighttime temperature is more critical for crops grown on larger spatial or temporal scales. High nighttime temperature is negatively affecting at wide range of growth and developmental stages of crops and influencing responses of morphological, physiological, biochemical, and yield traits (Peng et al., 2004; Prasad et al., 2008a; Prasad et al., 2008b; Mohammed and Tarpley, 2009; Welch et al., 2010; Lobell et al., 2012; Djanaguiraman et al., 2013; Narayanan et al., 2015; Djanaguiraman et al., 2018b; Impa et al., 2018; Opole et al., 2018). Furthermore, due to climate change the rate of increase in daytime and nighttime temperatures can differ leading to lower diurnal amplitude (difference between daytime maximum and nighttime minimum temperatures) (Vose et al., 2005; Wang et al., 2017). Lower diurnal amplitude can negatively influence crop growth (Sunoj et al., 2016).

Studies to understand the effect of different diurnal temperature amplitudes with same mean temperature on C₄ cereal crops are limited as compared to response of crops to either high daytime and nighttime temperatures (Myster and Moe, 1995; Sunoj et al., 2016). There are few studies to understand the effect of diurnal temperature amplitudes has been reported in different C₃ crops such as cucumber (Xiong et al., 2011), orange (Bueno et al., 2012), eggplant, sweet pepper, tomato, melon, and watermelon (Inthichack et al., 2013; Inthichack et al., 2014; Matsuda et al., 2014). These studies revealed that diurnal temperature amplitude influence several morphological traits, foliar mineral composition, gas exchange and carbohydrate composition and metabolism. Hence, it is important to understand such responses in key food grain crops such as sorghum.

Important role of photosynthesis during light phase of a day in plant growth is beyond doubt. Response of light and dark phase gas exchange [day-time photosynthesis (CO₂ assimilation) and night-time respiration (CO₂ release), respectively] revealed that diurnal temperature amplitude induced night respiration during dark phase of a day has important role in carbon tradeoff (photosynthesis verses night respiration) which is one of the important factor determines the magnitude of growth along with other leaf traits and light interception (Hammer and Muchow, 1994; Ravi Kumar et al., 2009; Sunoj et al., 2016; Impa et al, 2018). Our study on C₄ cereal crop maize [hybrid maize (DKC 47-27RIB, DEKALB, USA); Sunoj et al., 2016] systematically demonstrated the impact of different diurnal temperature amplitudes (2, 8, and 10°C) with optimum (30°C) and high (35°C) mean temperatures on vegetative growth. The study proved that, narrowing of diurnal temperature amplitude did not affected photosynthesis, but it increased leaf night respiration and reduced growth. At the same time, intensity of impact of narrowing of diurnal temperature amplitude on plant growth was varied among optimum and high mean temperature conditions.

Though, above study on maize provided a new insights on plant response to diurnal temperature amplitude and night respiration along with photosynthesis, it does not account the possibility of crop to crop variation and maintaining same or showing contrasting pattern of responses among different genotypes of same crop. It is also crucial to understand how sorghum, a relative more tolerant crop to abiotic stresses compared to maize, respond to these changes and current study is relevant as the sorghum grown in such regions which are experiencing and expecting higher temperatures in near future. Furthermore, based on available literatures, there are no reports dealing with response of sorghum to diurnal temperature amplitudes with an emphasis on carbon tradeoff and growth. Hence, this study was conducted to understand change in the pattern of response among different genotypes of same crop. To fill this knowledge gap, we performed current study with specific objective to quantify the impact of diurnal temperature amplitudes on growth, carbohydrate composition and carbon tradeoff in two sorghum hybrids. This research will help us to understand the dark phase processes, night respiration, and carbon utilization under temperature stress conditions. The results can be used to modify and strengthen the screening protocol for high temperature tolerance in sorghum. We hypothesize that dark phase (carbohydrate utilization via night respiration) and light phase (carbohydrate synthesis via photosynthesis) response are equally important and differentially influenced by diurnal temperature amplitude, thus affecting carbon trade-off and growth of sorghum genotypes.

Across two indepentend experiments, there were significant (P<0.05) effect of temperature and hybrids when exposed to optimum (27°C; ODNT) and high (35°C; HDNT) daytime and nighttime temperatures with three diurnal temperature amplitudes (2, 10, and 18°C) on several physiological, biochemical and growth traits (Tables 1 and 2). Interaction effect of hybrids and temperature conditions was significant (P<0.05) on nine out of 10 measured traits (Tables 1 and 2). When exposed to different temperature conditions, both hybrids and measured traits were followed the same pattern in both experiments, which confirming the consistency and repeatability in results and response of hybrids. To demonstrate this, data from two independent experiments are independently presented here.

Our current study shows that narrowing of diurnal temperature amplitude (18 to 2°C) caused reduction in total leaf area and above ground total biomass along with an increase in night respiration and SLA and changes in carbohydrate composition [total soluble sugars, reducing sugars, non-reducing sugars and starch] in selected sorghum hybrids (Figures 2–4). However, photosynthesis was not affected with changes in diurnal temperature amplitude ranges (2, 10, and 18°C) tested in this study indicating altered carbon tradeoff (Figure 3). Overall response of physiological, biochemical and growth traits of sorghum hybrids were similar to those observed on maize (Sunoj et al., 2016). Growth traits of sorghum hybrids were severely reduced under HDNT (35°C) followed by ODNT (27°C) with narrow diurnal temperature amplitude of 2°C having lower daytime and higher nighttime temperatures as compared to diurnal temperature amplitudes of 10 and 18°C, except SLA. Simultaneously, opposite trend in growth traits was observed in wider diurnal temperature amplitude of 18°C, where temperature at daytime was high and nighttime was low (Figure 2). The impact of above contrasting daytime and nighttime temperatures of extremes of diurnal temperature amplitudes (2 and 18°C) on growth of sorghum hybrids specifies importance of diurnal temperature amplitude for sustaining optimum plant growth.

Light and dark phases in 24 h of a day are crucial for plant growth due active photosynthesis (CO₂ assimilation) in daytime and respiration (CO₂ release) in nighttime which are two components of foliar gas exchange correspondingly contributing and significant for carbon tradeoff and are inevitable for overall growth, development and yield (O’Leary and Plaxton, 2016; Sunoj et al., 2016). Despite similar rates of photosynthesis, there were significant differences in night respiration and total above ground biomass accumulation in both sorghum hybrids across all the deployed temperature conditions (ODNT and HDNT) in current study (Figures 2 and 3). This trend raises questions about the utilization of assimilated CO₂ in the form of photosynthates and its end products (sucrose and starch) for growth and the influence of respiration. These differential responses in photosynthesis and above ground biomass accumulation point towards changes in night respiration and carbohydrate composition in response to temperature. However, there was also a strong relationship between total leaf area and biomass accumulation (Figure 7). Such an effect may arise from differential effects of temperature treatments on leaf area expansion (Tardieu et al., 1999), as consequent differences in leaf area will affect extent of light interception and hence, plant growth. This confounding effect of plant size needs to be considered when considering treatment effects on growth and biomass.

Correspondingly, Prasad et al., 2008 reported the same kind of response in photosynthetic performance of sorghum hybrid (DK 28E), in which, high day and night temperatures was set to 40/30°C (diurnal temperature amplitude of 10°C with mean temperature of 35°C and one of the day/night temperature condition used in the current study) and such response can be attributed to the ability of C₄ crops to undergo thermal acclimation (Dwyer et al., 2007; Sage and Kubien, 2007; Wang et al., 2012). This was further supported by the stable photosynthesis in C₄ crop maize under high temperature conditions (Sunoj et al., 2016; Rotundo et al., 2019). Additionally, previous studies on diverse C3 and C4 crops also demonstrated zero impact of high temperature on the subsequent day photosynthesis (potato: Lafta and Lorenzen, 1995; tomato: Huckstadt et al., 2013), while it has impact on night respiration, carbohydrate composition and growth (rice: Peraudeau et al., 2015; maize: Sunoj et al., 2016). Contrastingly, in C3 crop quinoa, photosynthesis and night respiration was increased under high temperature condition (Eustis et al., 2020).

Furthermore, decrease in photochemical efficiency of photosystem II (PSII) (Fv/Fm) is considered as indicator of photoinhibition or photodamage of PSII in plants under diverse abiotic stresses (Huang et al., 2016; Sunoj et al., 2016), varied across different temperature conditions (Figure 3). Besides, initial and reversible photoinhibition of PSII protects PSI from photoinhibition by limiting supply of electrons from PSII, which allow to keep the oxidative status of PSI under stress conditions. Hence, Fv/Fm values indicates health of leaves as well as total functionality of photosystems (PSII and PSI) (Joliot and Johnson, 2011; Takagi et al., 2017; Kadota et al., 2019). However, all the Fv/Fm values in both sorghum hybrids and temperature conditions in current study were equal or higher than 0.77 and this was only a slight change from the standard value (0.78) of F_v/F_m representing uninterrupted functioning of PSII (Huang et al., 2016; Huang et al., 2018). The above minor reduction implies that photosystems (PSII and PSI) and linear electron flow (LEF) were slightly affected with narrow diurnal temperature amplitudes, even under HDNT. Nevertheless, the impact of variation in temperatures was not sufficient to tamper the light dependent reaction and thereby light independent (dark) reaction of photosynthesis in sorghum hybrids. It was one of the reasons resulted for and reflected from stable photosynthesis.

In plants, other than energy cost for ion uptake in roots and transport inside root and shoot, respiration [light phase (daytime) and dark phase (nighttime) respiration] has two main functions, which are maintenance and growth (Johnson, 1990; Atkin et al., 2000). Respiration is the controlled oxidation of energy-rich photosynthates or its end-products (sucrose and starch) synthesised from photosynthesis by the collective reactions of glycolysis, the tricarboxylic acid cycle and mitochondrial electron transport chain which produce CO₂, adenosine 5-triphosphate (ATP), and low-molecular-weight molecules required for the biosynthesis of proteins and lipids and various metabolites essential for growth and plants to acclimation to stress (Atkin et al., 2000; O’Leary and Plaxton, 2016). Approximately, 30 to 70% of photosynthetically generated carbohydrates serve as major substrate as compared to the seldom used fatty acids that are utilized for respiration; and rate of respiration varies with plant species and status of growth conditions (optimum or sub or supra optimum) (Mohammed and Tarpley, 2011).

Respiration, especially at dark phase (night respiration), plays a significant role in recovery from injuries or damages caused due to the oxidative stress under optimum and diverse abiotic stress (moderate to severe injuries) conditions, in addition to contribution towards overall growth (Sunoj et al., 2016; Impa et al., 2018). However, depending on the extent of injuries or damage due to any type of stress, more resources are channelled for repair or maintenance respiration rather than contributing to growth, therefore, growth processes of the plants are negatively affected (Loka and Oosterhuis, 2010; Mohammed and Tarpley, 2011; Sunoj et al., 2016).

In current study, higher night respiration and reduction in assimilated total soluble sugars (reducing and non-reducing sugars), starch and biomass accumulation in both sorghum hybrids under narrowing diurnal temperature amplitude of ODNT and HDNT (Figures 2–4) imply higher level of carbohydrate utilization for the maintenance rather than growth. This was further evident from the negative correlations of night respiration with carbohydrate composition (Figure 6) and total biomass and positive correlation among total biomass and carbohydrate composition (Figure 5). Carbohydrate composition consist of glucose, fructose, and maltose which are reducing sugars, sucrose is major transported non-reducing sugar in plants and starch is a polysaccharide stored in carbon sinks of plants and synthesized from sucrose (Loka and Oosterhuis, 2010; Halford et al., 2011). Meanwhile, highest night respiration under HDNT with 2°C diurnal temperature amplitude (Figure 3) specifies that balance between two functions (maintenance and growth) of night respiration differs with the type and intensity of stress (Djanaguiraman et al., 2013; Sperling et al., 2015; Sunoj et al., 2016; Impa et al., 2018). High daytime and nighttime temperature reported to increase the night respiration and reduce ATP, carbohydrate composition and growth in diverse crops as well (cotton: Loka and Oosterhuis, 2010; soybean: Djanaguiraman et al., 2013; maize: Sunoj et al., 2016). In contrast, non-significant relatioship between night respiration and growth was also reported (tomato, soybean and lettuce: Frantz et al., 2004; rice: Peraudeau et al., 2015). At the same time, dark phase in plants are equally important as light phase not only due to contribution of night respiration but also because of its effect on synchronization of circadian clock to manage the redox state of photosystems, response of hormones and induction or control of different mechanisms which are collectively responsible for sustainable growth under optimum and tolerance under stress conditions (McClung, 2006; Sunoj et al., 2016; Srivastava et al., 2019).

Studies in sorghum have also shown a strong influence of temperature on leaf initiation, appearance, and expansion rates, all of which are important for development of plant leaf area and hence, light interception (LI) and biomass accumulation (Hammer and Muchow, 1994; Ravi Kumar et al., 2012). In the current experiment, overall, there was positive correlation of total leaf area with total biomass and carbohydrates (Figure 7). This indicates the contribution of plant size via leaf area to differences in biomass accumulation in addition to night respiration and utilization of carbohydrates (Figures 3–6), rather than from photosynthesis, which was stable across different temperature conditions (Figure 3). Progressive increases in total leaf area and biomass with increasing diurnal temperature amplitudes under both ODNT and HDNT further indicates the important role of leaf area in effecting biomass accumulation (Figure 2).

Some research has shown that leaf area expansion under different temperature conditions is independent of carbohydrate availability (Tardieu et al., 1999; Volkenburgh, 1999), so that SLA is a consequence of the temperature-driven potential leaf area expansion and available assimilates. SLA increases (i.e. more leaf area per unit dry weight) when environmental conditions have greater negative effect on assimilate availability than on leaf expansion rate (Tardieu et al., 1999). The negative correlation of SLA with total leaf area (Figure 7) indicates reduced availability of assimilates for plants that had been restricted in leaf area. The smaller plants found in narrow diurnal temperature amplitudes under both ODNT and HDNT (Figure 2) also had higher night respiration, which is consistent with their increased SLA and the overall positive association of SLA with night respiration (Supplementary Figure S1). Hence, it is likely that a combination of temperature effects on leaf expansion and night time respiration are generating the consequences observed on biomass accumulation. The importance of leaf area expansion on growth under diverse temperature conditions might also explain findings in studies in other crops where a nonsignificant relationship between night respiration and growth was reported (Frantz et al., 2004; Peraudeau et al., 2015).

Although the contribution of night respiration for growth may be confounded by effects on leaf expansion, understanding the crop and genotypic response of night respiration to the variation in day and night temperatures remains important. This is due to the fact that overall contribution of respiration is approximately 50% of the total annual CO₂ input from the terrestrial ecosystem to atmosphere by taking part in plant growth and development (Gifford, 2003; O’Leary and Plaxton, 2016). Furthermore, our results indicate that effects of temperature on night respiration have a significant impact on biomass accumulation. This implies the importance of maintaining an optimum diurnal temperature amplitude that does not cause extreme maximum or minimum temperature is critical for attaining maximum growth and biomass.

The results from the current study clearly demonstrated the importance of diurnal temperature amplitude on growth of sorghum. Relative to optimum mean daytime and nighttime temperature (27°C), high mean daytime and nighttime temperature (35°C) had negative impacts on physiology and growth of sorghum. Carbon tradeoff in sorghum was significantly altered under different diurnal temperatures amplitude with optimum or high mean temperatures. This was evident from stable CO₂ assimilation (light phase photosynthesis), altered utilization (dark phase night respiration) and changes in total leaf area, specific leaf area and biomass accumulation and their relationships. Narrowing of diurnal temperature amplitude resulted increase in night respiration by utilizing the carbohydrate for maintenance rather than growth. These reveals the importance of understanding the dark phase mechanisms along with better understating the above ground biomass, leaf area components, and related light interception under different temperature conditions. Both hybrids monitored in current study to understand inter genotypic differences showed same pattern of response to diurnal temperature amplitudes. However, research with larger number of genotypes are needed to further confirm and understand genotypic response to different diurnal amplitude for the same mean temperature. Results from current study ascertain that impacts of diurnal temperature amplitude on leaf area, biomass accumulation and night respiration should be incorporated while quantifying the impact of temperature on growth and yield of crops. In addition, methods should consider appropriate temperature conditions and temperature amplitude while screening for temperature tolerant/sensitive genotypes. Furthermore, greater importance should be given to breed for genotypes/hybrids to maintain productivity and yield in areas facing or expecting high night temperatures and changing temperature amplitude. However, it is strongly recommended that separate studies should be conducted for different crops irrespective of their photosynthetic pathway (C₃ or C₄) under field conditions to achieve conclusions that are more realistic conditions without restrictions of root growth and natural light conditions. Finally, further research to understand optimum respiration under different temperature conditions will be important to identify crops or genotypes within crops that will be more suitable under changing climatic conditions.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

VS, IC, and PP designed the research. VS and HM conducted the experiment, collected and analyzed the data and contributed to statistical analysis. VS and HM drafted the manuscript. PP and IC suggested critical comments and corrections while preparing manuscript. All authors contributed to the article and approved the submitted version.

Financial support from Center for Sorghum Improvement at Kansas State University (KSU) and United States Agency for International Development (USAID) through Sustainable Agricultural and Natural Resource Management (SANREM) Collaborative Research Support Program for conducting the research. Preparation and publication of the manuscript was supported through Feed the Future Innovation Lab for Collaborative Research on Sustainable Intensification (Grant no. AID-OAA-L-14-00006).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.