Extreme Temperature Responses, Oxidative Stress and Antioxidant Defense in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 182

113]. Low temperature also disrupts the systems including electron transport, carbon cycle
metabolism and g s . Among the photosynthetic apparatus PSII is the primary target of damage
under LT stress. Moreover, LT reduces the activities of stromal and carbon assimilation
enzymes like Calvin cycle enzyme, ATP synthase, and restricts RuBisCO regeneration and
limits the photophosphorylation [125]. Another impact of LT exposure is the decline of carbon
export from leaves which results in the accumulation of soluble carbohydrates [126]. Yorda‐
nova and Popova [127] stated that exposure of wheat plants to a LT (3°C) for 48 h and 72 h
resulted in decreased levels of Chl, CO 2 assimilation and transpiration rates. Photosynthesis
is strongly reduced below 18ºC [128], while temperatures around 4ºC dramatically depress
photosynthetic performance [129]. The decline of photosynthetic capacity in LT is related to a
decrease in the quantum efficiency of PSII and the activities of PS I, the ATP synthase and the
stromal enzymes of the carbon reduction cycle [125]. Partelli et al. [130] showed that coffee
plant resulted in 30% reduction in Chl a , 27% reduction in Chl b , 29% reduction of total Chl
when the day/night temperature decreasing from 25/20° to 13/8°C. For Car, 86% reduction of
α-carotene, 57% reduction in β-carotene, 68% reduction in α/β-carotene ratio, 32% reduction
in lutein, but 21% increase in zeaxanthin. In O. sativa , the total Chl content was reduced by
50% due to exposure to LT (15/10 ºC) for 2 weeks [131]. In a recent study, Reda and Mandoura
[48] reported that even at LT stress of 3°C the enzyme chlorophylase is still activated and led
to a decline in Chl in T. aestivum plant.
3.6. Water and nutrients movements and uptakes
Cell membrane plays major roles in water and nutrient movements within and outwards the
cell. Intra- and extracellular water and nutrient movement are inhibited due to LT due to
memebrane damage under LT. There can be of two types of abnormalities during LT stress.
Cold damages the membrane that makes the membrane permeable to undesired nutrients and
ions and causes ion leakage; another is cell membrane and cell wall can be ruptured by the
cold which is also responsible for disrupting cellular homeostasis by destroying both intra and
extracellular nutrient and water movements [132, 133]. Severe dehydration may also occur due
to freezing of cell constituents, solutes and water [134]. Available literature states that when
temperatures drop below 0°C, the ice formation generally begins in the intracellular spaces
because the intracellular fluid has a higher freezing point as compared to the other suborga‐
nales of cell [134, 135]. Low and freezing temperatures also lead to cellular dehydration, reduce
water and nutrient uptake and conduction by the roots in some plants, thus causing osmotic
stress [107]. Yadav [134] stated that dehydration during cold occurs mainly due to reduction
in water uptake by roots and a hindrance to closure of stomata. The success or failure of a
seedling in the field is strongly related to the development of its root system under cold stress
[136]. In root of cucumber it was found that chilling caused injury to the cortical cells and
further long time exposure increased the density of cytoplasm and damage the endoplasmic
reticulum [137]. Chilling-sensitive plants exposed to LT usually show water-stress symptoms
due to decreased root hydraulic conductance; and decreased leaf water and turgor potentials
[96]. Freezing-induced increase in water viscosity is partly accounted for an initial decrease in
root hydraulic conductance [138]. During cold stress another phenomenon is common with
the imbalanced water movement. The metabolic functions are altered those include production