**3. Heat-Induced Changes in the Proportion of Maintenance Respiration**

Dark respiration (Rd) is typically partitioned into two functional components, i.e., growth respiration (Rg) and maintenance respiration (Rm), which are impacted upon by environmental stresses [9,30,31]. Figure 1 illustrates the differences in these components under elevated temperatures. Growth respiration is a dominant component of respiration in younger tissues, while the latter contributes majorly to the older tissues [32]. Growth respiration is defined as the amount of photoassimilates respired to provide energy for the synthesis of additional biomass [33]. It also provides a carbon skeleton and reductants to facilitate nutrient uptake/assimilation followed by biosynthesis of cellular components to drive the growth of tissues. Thus, the relationship between the growth rates of a species and temperature is actually a measure of the rate of the growth respiration component [34]. A recent analysis of 101 evergreen species growing in different biomes (boreal to tropical) showed that respiration increased with an increase in growth temperatures in accordance with previous studies [35,36]. Leaf form accounted for the response ratio of Rg to warming,

as species with needle-like leaves had a significantly higher response (25 ± 9%) than broad-leaved ones [36].

**Figure 1.** Growth respiration and maintenance respiration under elevated environmental temperature. HSPs: heat shock proteins; NSCs: non-structural carbohydrates; Rg: growth respiration; Rm: maintenance respiration; Rt: total respiration.

On the other hand, maintenance respiration comprises the respiratory processes that help in supporting the already established biomass of the plant [33]. It depends upon the amount and composition of the biomass, as both these factors undergo change depending on the environment and developmental stage of the plant. Although the role of both the components is integral to the life cycle of the plants, their estimation can only be done by employing physiological models [32,37–39]. The higher temperature responsiveness of Rm over Rg in mature tissues was concluded from various studies, e.g., Marigolds when exposed to a 10 ◦C increase in temperature resulted in a 43% to 55% increase in the proportion of maintenance respiration to total respiration (Rt) [40]. Additionally, a significant reduction in ATP content and total biomass was observed in rice plants subjected to 10 ◦C higher temperature at the reproductive stage than the ambient temperature (28 ◦C), thereby suggesting that energy produced by respiration under high temperature conditions was mainly attributed to maintenance respiration rather than growth respiration [32]. Mathematically, maintenance respiration is expressed as the product of maintenance respiration coefficient and plant size. The Q10 value (proportional increase in rate of respiration with a 10 ◦C rise in temperature) of the maintenance respiration coefficient varies between 1.35 and 3.0 depending upon the species, developmental stage, and environmental conditions as shown in the data compiled from various studies (Table 1). The sensitivity of Q10 to temperature indicates that the response of respiration to temperature cannot be represented by one value.


**Table 1.** Q10 values for maintenance respiration coefficient in various crops.
