Extreme Temperature Responses, Oxidative Stress and Antioxidant Defense in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 179

between 20 and 35°C, and the temperature of 10°C is cited as the minimum critical value below
which rice does not germinate [101]. There are many reports on positive correlation between
germination at LT and root development at an early stage; and between the germination and
seedling establishment [102]. Angadi et al. [103] observed that the number of days to 50%
germination in B. napus was only 3 d at 8°C which was nearly 13 d at 2°C. This LT effect was
more pronounced in B. rapa , because at 2°C, emergence was less than 50%, even after 20 d of
sowing [103]. Buriro et al. [104] reported that the increase in temperature significantly
enhanced germination and related traits in wheat cultivars. All the wheat varieties germinated
well (80-97%) sown at 10-30°C. The maximum seed germination, vigor index occurred at
20-30°C and these temperature regimes were identified as optimum for wheat seed germina‐
tion. The delay in germination percentage and the reduced germination percentages were
observed in Gossypium hirsutum at LT below 20°C [105]. In T. aestivum , the germination is
drastically hampered at temperature below 8-10°C [106].
3.2. Growth and morphology
Low temperature stress inhibits various metabolic reactions thus preventing the expression of
full genetic potential of plants which is expressed by different phenotypic symptoms [107].
Some of the common LT injury in plants are reduced seedling growth, seedling discoloration,
leaf yellowing, leaf whitening, white specks, white bands, withering after transplanting, a re‐
duced rate of tillering, stunting and so on [102]. According to Angadi et al. [103], temperatures
below 10°C result in slower and reduced growth and premature stem elongation in B. napus , B.
rapa and Raphanus sativus . It is well reported that plants at their seedling stage are very much
sensitive to cold stress. At early stage of plant growth, and various phenotypic symptoms in re‐
sponse to chilling stress are surface lesions, chlorosis, necrosis, desiccation, tissue break down
and water soaked appearance of tissues, reduced leaf expansion, wilting [108, 109]. Nahar et al.
[99] observed varieties of cold injury symptom is rice including stunted growth, yellowing of
leaves, abnormal number of tiller, malformed grain, abnormal colors in grain.
Generally, exposure to cold temperature affects crop growth and development in two ways
concurrently. First, developmental events in the shoot apex are affected which directly
determine the differentiation of the panicle and hence potential yield and spikelet fertility
resulting in fewer grains. Second, photosynthesis is impaired which reduces growth and
results in indirect yield loss because there is less carbohydrate available for grain production
[110]. The cellular structures and components are also damaged due to the cold which have
been studied for a long time. The development process and ontogeny of the organelles may
be disrupted by chilling stress [111]. Nahar et al. [98, 99] observed cold stress induced
morphological symptoms like stunted plant, bushy plants, early maturity, yellowing of leaves.
Modifications of cellular components include swelling of mitochondria, plastids and thylakoid
lamellae, vesiculation of thylakoid, accumulation of lipid droplets and eventually the disor‐
ganization of the entire plastid [112], reduction in ribosomal numbers, dilation of endoplasmic
reticulum, vesiculation of cytoplasmic membranes, condensation of nuclear chromatin,
invagination of plasmalemma and vacuolation of membranous vesicles [111, 113] were noticed