**4. Substrate Availability for Respiration under High Temperature**

The considerable variation observed in the Q10–temperature relationship is influenced by the supply of the respiratory substrate and the respiration capacity [4,46]. Environmental variables that affect the biosynthesis of the substrates [18,46] or increase the metabolism of energy consuming processes like turnover of proteins and maintenance of ion gradients [47], make Q10 values highly dynamic in response to temperature. Additional energy costs are incurred by mechanisms imparting heat tolerance in the crops, e.g., upregulation of the antioxidant defense system to counteract the upsurge in the level of reactive oxygen species (ROS), synthesis of osmoprotectants, and accretion of heat shock proteins (HSPs). The need for respiratory substrate in the plants is mainly met from the non-structural carbohydrates [25–27,48] and the protein turnover [11,32]. Studies on the effect of elevated night temperatures have shown that the high rate of nighttime respiration exerted pressure on the supply of NSCs, which subsequently reduced the biomass and yield of rice [25,26]. The concentration of sugars has been positively correlated with the rate of dark respiration in *Pinus* [49], *Quercus rubra* [50], and *Spinacia oleracea* [51]. The light control of carbohydrate synthesis affected the rate of dark respiration in *Geum urbanum* plants grown under 75% shade as it declined due to limited photosynthate supply, but Q10 declined only when the leaves experienced near darkness for long periods. It was concluded that intense shade for a prolonged period would cause a reduction in both respiration and Q10 due to adenylate restriction on respiration in addition to the substrate availability [4].

#### **5. Regulation of Respiratory Flux at High Temperature**

Adenylates (in particular the ratio of ATP to ADP and the concentration of ADP per se), are likely the most important in regulating respiratory flux at warm temperatures [52]. Adenylate control would indicate that the respiratory capacity at warmer temperatures exceeded the level required for cell processes to proceed [18], which in turn would lead to elevated ATP:ADP ratios or low ADP concentration, causing downregulation of respiration [53]. The increased leakiness of membranes at high temperatures could further contribute to substrate limitation because concentration gradients of TCA cycle intermediates are more difficult to maintain when mitochondrial membranes are excessively fluid [18].
