Other than morphology and anatomy, physiology of the crop plants is found to be extremely affected by high temperature. Development of tolerance towards continuously elevating temperature is a complex process driven by the factors like the surrounding environment and the genetic potential of the plants under stress (Zrobek, 2012). Respiration and photosynthesis, the two most important physiological processes are susceptible to the harm caused by high temperature. Temperature has a direct impact on the respiratory processes leading to a rapid increase in them but after certain threshold there is an extreme drop in the respiration cycle. Unlike respiration, photosynthesis is relatively less susceptible to high temperature, but the fall in the photosynthetic pathway is much similar to that seen in the respiratory one. It has been reported that with every 10°C rise in the surrounding temperature, a bifold increase is seen in the pace of the enzyme catalyzed reactions (Zrobek, 2012). Since eveiy process involved in the life cycle of crop plants works efficiently within its desired range of temperature, above or below which these processes do not work efficiently and may cease, leading to a long lasting injury to the cell structure and function and in many cases such conditions mark the onset of the end of plant life.
One of the most significant components of plant physiology which should be considered during HT- stress is the water content in the plants and this character is found to be quite fluctuating under changing temperatures (Mazorra et ah, 2002). The continuous long time exposure of sugarcane plants and tomato cultivars to heat stress have a huge impact on the hydraulic conductance of roots (Morales et al, 2003)and brings about remarkable changes in the leaf water potential and its constituents, inspite of the favorable humidity in the surrounding air and the presence of water rich soil (Wahid and Close, 2007). When studied under field conditions, it has been observed that the HT- stress is often linked to minimal soil water availability (Simoes-Araujo et al., 2003). In plants like lotus (Lotus creticus), an amenable increase in the night temperature is often associated with preeminent deceleration in the leaf water potential (Anon et al, 2004). Generally, increased day temperature accelerates the rate of transpiration and generates water deficit plants having altered physiological processes (Tsukaguchi et al, 2003). Substantially, the day time HT- stress is predicted to stimulate the water deficit in plants at a much faster rate than the HT- stress during night (Wahid, 2007).
Photosynthetic machinery is an integral part of the plant system and it often falls prey to the high temperature stress. Being highly perceptive to the increased temperature (Crafts-Brandner and Salvucci, 2002), the photosynthetic efficiency of a plant is frequently considered to be temperature dependent. In green plants, the immediate target sites for heat stress injury are the thylakoid lamellae (site for photochemical reactions) and the stroma (site for carbon metabolism) (Wise et al, 2004). Plastids (chloroplasts), the prime location for the photosynthetic machineiy undergo extensive modifications under HT- stress such as: disruption of the thylakoid structure, inability of grana to form stacks and Major alterations occur in chloroplasts like altered structural organization of thylakoids, inflammation of grana and its inability to form stacks (Rodriguez et al, 2005; Ashraf and Hafeez,
2004). The antagonistic behaviour of high temperature towards photosynthesis is accelerated by secondary factors like HT- induced reduction in the leaf water potential and leaf area along with the early leaf senescence as well as the exhaustion of the carbohydrate resources (Greer and Weedon, 2012; Young et al, 2005; Djanaguiraman et al, 2009). Since the elevated temperature modifies and ceases the activity of the enzymes involved in carbon metabolism (especially mbisco) (Haldimann et al., 2004) along with oxygen involving enzymes in the PS II, which hinders the electron transport and effects the pace of the RuBP synthesis leading to the conclusion that among the photosynthetic efficiency of C3 and the C4 plants, photosynthesis in C3 plants is more sensitive to temperature (Salvucci and Crafts- Brandner, 2004; Yang et al., 2006). Closing of stomata to prevent water loss, decrease in the amount and activity of certain proteins and enzymes like soluble proteins, mbisco binding proteins (RBP), sucrose phosphate synthase (Chaitanya et al, 2001), ADP-glucose pyrophosphorylase, and invertase (Vu et al, 2001) as well as reduction in the rate of biochemical reactions are certain other factors that affect the photosynthesis of theplants under heat stress (Siunesh et al, 2008; Rodriguez et al, 2005; Djanaguiraman et al., 2009; Ashraf and Hafeez, 2004; Nakamoto and Hiyama, 1999). Apart from curtailing the number of photosynthetic pigments (Marchand et al, 2005) increase in temperature influences the thylakoid structure and the working of the thennolabile photosystem II (PSII) (Morales et al, 2003; Bukhov et al, 1999; Camejo et al, 2005; Mcdonald and Paulsen, 1997). While studying the effect of HT- stress (38/28 °C) on the cultivated soybean crop, 18% reduction was seen in the total chlorophyll content, 7% and 3% decline in chlorophyll a and chlorophyll a/b ratio respectively. The sucrose content dr opped to 9 % while as an exceptional increase of 47% and 36% was observed in the reducing sugar content and in the quantity of leaf soluble sugars present (Tan et al., 2011). The two varieties of rice Shuanggui 1 and T219 showed a reduced rate of photosynthesis by 16% and 15% respectively, when exposed to heat (Hurkman et al, 2009). Keeping in consideration the increasing CO, content in the atmosphere, the resulting high temperature may have harmful effects on the photosynthetic machinery, yield and productivity of crop plants (Wahid, 2007). Concisely, the photo synthetic efficiency of plants has been found positively correlated to high temperature (Schuster and Monson, 2002). Hence, development of the strategies to prevent the alarming effects of heat stress on photosynthesis of crops is the need of the hour.