**3.3 Temperature stress**

*High temperature/heat stress*: Heat stress often increases the evapotranspiration loss of water and thereby, creates drought like situation. This situation is very critical when soil temperature increases along with drought stress. Elevation in temperature causes denaturation of protein, inactivation of essential enzyme activities (*viz*., sucrose phosphate synthase, adenosine diphosphate-glucose pyro-phosphorylase, invertase etc.) reductions of starch and sucrose synthesis [17], generation of ROS and death of the cell [18]. Further, light dependent reactions in stroma of thylakoid and carbon metabolism are hampered under elevated temperature. Impairment of electron supply to PS II under heat stress leads to restriction of the activity of PS II. Seed germination and seedling stand establishment are greatly affected by this stress. At high temperature, respiration exceeds photosynthesis and high loss of carbon through photo-respiration is seen (respiration rate increases two times with each 10°C increase of tissue temperature). Root number, length and biomass are greatly affected under high temperature rise, resulting in decreased uptakes of nutrients and water from soil. Further, drying or scorching of aerial part of the plants, reduction of plant growth and development through depleting chlorophyll as well as hampering photosynthetic activity and cell division are witnessed under heat stress. It is noted that sudden increase in temperature is comparatively more harmful as compared to gradual temperature rise as it disrupts biochemical, physiological and molecular activities of the plant. C3 plants suffer comparatively more under heat stress over C4 plants due to variations in energy supply and carbon metabolisms [19].

*Low temperature/cold stress*: Cold stress causes chilling (dysfunctioning of physiological properties) and freezing (dehydrating the cell) injuries to the plant. Low temperature stress hampers membrane integrity resulting in electrolyte leakage/ plasmolysis, alteration of cell metabolic activities and reduction of protoplasmic streaming [20]. It alters nucleic acid and protein synthesis and down-regulates PS II [21]. Wilting, bleaching through pigment photo-oxidation, leaf necrosis, browning, cell death etc. are the common examples of impact of cold stress on plant.
