2.2. Physiological and Molecular Responses
Most of the physiological functions in plants are temperature dependent and any deviation over the optimum temperature hampers the growth and development of plants (
Table 2). HS reduces photosynthetic efficiency which eventually affects the plant growth and biomass production
[22,56][22][56]. The reduction in photosynthesis rate under HS is correlated with the increase in the non-photorespiratory processes and reduction of soluble proteins, Rubisco and Rubisco binding proteins
[72,73][72][73]. These sequential changes results in reduced leaf area, less effective photosynthetic machinery, early leaf senescence, overproduction of reactive oxygen species (ROS), disruption of thylakoid membrane, alteration of enzyme action, and denaturation of heat shock proteins (HSPs), which ultimately reduces wheat productivity
[6,55,74][6][55][74]. The reduction in total chlorophyll content with accelerated leaf senescence diminishes the photosynthetic capacity of the plant
[61]. Furthermore, respiration and photorespiration rate increase with temperature above the threshold level, resulting in the reduction of availability and transport of photoassimilates from leaves to the grain. This hampers the growth and grain filling process
[75,76][75][76].
Water status in the plant is crucial especially under HS as the temperature of plant tissue (canopy temperature) can be optimized by water uptake and transpiration. Thus, leaf relative water content (LRWC), leaf water potential (LWP), rate of transpiration, and stomatal conductance are strongly influenced by canopy temperature (
Table 2)
[59]. Higher vapor pressure deficits under HS lead to a higher evapotranspiration rate which ultimately decreases LWRC and LWP (
Table 2). However, limited information is available about the effect of HS in relation to the nutrient status of the crop
[77]. The uptake and translocation of nitrogen (N) in wheat declined under HS due to a reduction in nitrate reductase activity (NRA) in plants
[62]. Phosphorus (P), potassium (K), sulphur (S), and sodium (Na) helps to maintain the redox state of the cell and hence protects the cell membranes, improving the antioxidant defense system and osmotic potential, which ultimately enhances the photosynthetic rate. These nutrients reduce the activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases and retain the photosynthetic electron transport activity, which helps to reduce ROS thus maintain the redox state of the cell under HS. However, reduced root growth under HS reduces uptake, assimilation, and translocation of most of the nutrients
[78].
HS triggers the production of ROS, such as super-oxides (O
2−), hydroxyl radicals (OH
−), and hydrogen peroxide (H
2O
2), resulting in severe oxidative damage to the plant
[79]. HS can also damage the plasma, mitochondrial, and chloroplast membranes due to lipid peroxidation and concomitant H
2O
2 production
[80]. To the eliminate the accumulated ROS and maintain metabolic activities and productivity
[81], the plant has a well-organized cellular stress-response system (defense systems) composed mainly of transcription factors (TFs) and HSPs
[82]. Plants possess signal transduction molecules (Calcium-dependent protein kinases (CDPKs), mitogen-activated protein kinase (MAPKs), sugar (as signalling molecule), and phytohormones) that act as an activator of the stress-responsive gene during HS
[68,83][68][83]. For example, dehydration-responsive element-binding protein (DREB) is a signal transduction molecule that activates the stress-responsive genes through stress signaling induced expression in wheat. These signal transduction molecules together with TFs activate stress-responsive genes
[84,85][84][85]. For example,
TaHSFA6f gene has been reported to function in sensing the HS and overexpression of this gene imparted heat tolerance in wheat
[86].
2.3. Heat Stress and Sensitive Stages of Wheat
The impact of HS is more severe during reproductive stages in comparison to the vegetative phase
[11,12,52][11][12][52]. HS hinders pollen tube growth, resulting in reduced germination of pollen grains on stigma and impacts embryo formation
[12]. Translocation of assimilates and the rate and duration of grain filling are also directly affected by elevated temperature
[87]. Exposure to high temperature stress during the division of pollen mother cells can reduce grain setting
[11,12,88][11][12][88]. High temperature (3038 °C) during the reproductive phase can lead to the reduction of the total biomass of wheat up to 44%
[48]. HS during the early grain filling stage declines the synthesis of starch in the wheat grain but increases the concentration of total soluble sugars (
Table 2)
[89]. Although the protein content increases under HS, the functionality of proteins significantly decreases, adversely affecting its end-use
[90]. Even after a high percentage of grain protein, total protein yield reduces under HS due to a drastic reduction in the grain yield
[91]. However, an increased level of protein content has been reported to reduce the sedimentation index, which affects the bread-making quality of wheat flour
[92,93][92][93]. The heat-inducible accumulation of gliadins under HS also deteriorates the flour quality of wheat
[94].
Temperature is the most important factor influencing the growth and development of wheat (
Table 1). The growth and development start with seed germination, which requires optimum temperature and moisture. HS is an important key abiotic stress that severely affects the seedling establishment of wheat
[95,96][95][96]. The germination index and germination potential of two wheat cultivars, namely DBA Aurora and L6, were significantly reduced under HS conditions
[97]. The reduction in the germination percentage and seedling establishment under heat stress conditions might be due to a reduction in the antioxidant system, production of ROS, increased lipid peroxidation, and differential expression of miRNAs
[16]. Recent studies on seedling establishment have revealed that the expression of miRNAs and epigenetic changes in wheat seedlings might be key factors that help to optimize the seed vigor for superior breeding of germplasm and varieties under heat stress
[98]. The seedling with high vigor is projected to be more effective in radiation and resource use efficiency. High seedling vigor has no transgenerational effect on the vegetative and reproductive phase
[99]. During the vegetative growth of wheat, the tissues exposed to HS leak electrolytes, damaging leaf members, and decreasing photosynthesis
[100,101][100][101]. HS severely impacts on photosystem II and thylakoid membranes, which are most sensitive cell structures
[102].
HS stress is a severe threat to wheat production when it occurs during reproductive and grain-filling phases (
Table 2)
[11,12,13,14,55][11][12][13][14][55]. Wheat crops experience HS in different phenological stages. However, the impact of HS during the reproductive phase is relatively more detrimental as compared to the vegetative phase which ultimately affects the grain quality and yield (
Table 2). HS during reproductive stages of crop development decreases grain yield up to 30%
[11,19,103][11][19][103]. The optimum temperature for anthesis and grain filling stage was suggested to be in the range of 12–24 °C, and 8–6 days before anthesis stages are identified to be the most sensitive stages to HS
[11,19][11][19]. Likewise, results of 30 wheat crop models from different locations around the globe where mean temperatures in the growing season ranged from 15 to 32 °C with artificial heating indicated that grain yield of wheat started to decline at a majority of the wheat-growing locations
[104]. The simulated median temperature impact on declining wheat yield ranged between 1% and 28% for an increase in temperature of 2 °C, and this value rose to between 6% and 55% for a temperature of 4 °C
[105].
Moreover, the reduction in the number of spikelets per panicle, hundred-grain weight, and seed setting rate was reported in wheat under HS
[2,12,19,106][2][12][19][106] (
Table 2). Pollen tube development after pollination is extremely sensitive to heat stress, leading to abnormal ovary development, ultimately resulting in lower yield as well as quality of grains
[104,107,108][104][107][108]. Harvest index (HI) and its correlation with heat stress is most significant in wheat among all cereals. HI plummets when daily mean temperatures cross 16 °C and a significant loss of yield occurs when it crosses 30 °C
[12]. The impact is more severe when there is higher night temperature, resulting in reduced grain yield, attributed primarily to the reduced spike number
[14,109,110][14][109][110]. HS induced through high night temperatures, especially during the grain filling stage, can reduce grain yield, HI, 1000-grain weight, and grain weight per spike
[111]. These studies provide key traits for wheat breeders to enhance high night temperature tolerance in wheat.