Depending upon the extremity and duration, and also depending upon the plant types and other environmental factors in the surroundings, plants show dynamic responses to HT, but identification and confirmation of the traits that confer tolerance to HT still remain elusive [ 23 , 24 ]. Plant scientists involved in research on HT stress are endeavoring to discover the plant responses that lead to heat tolerance and they are also trying to investigate how plants can be managed in HT environments.
Recent widely studied molecular approaches have included omics techniques and the development of transgenic plants through manipulation of target genes [ 25 — 27 ]. Investigation of these underlying molecular processes may provide ways to develop stress tolerant varieties and to grow agriculturally important crop plants under HT. In this chapter, we focus on these new strategies and we review the recent research into the physiological and biochemical events and the molecular responses seen in plants in response to HT stress.
We also review the roles of exogenous protectants, the underlying mechanisms for transduction of HT stress signals, and transgenic approaches currently being taken to promote HT stress tolerance in plants. Plant responses to HT vary with the degree of temperature, duration and plant type. At extreme HT, cellular damage or cell death may occur within minutes, which may lead to a catastrophic collapse of cellular organization [ 28 ].
Heat stress affects all aspects of plant processes like germination, growth, development, reproduction and yield [ 5 , 29 — 31 ]. Heat stress differentially affects the stability of various proteins, membranes, RNA species and cytoskeleton structures, and alters the efficiency of enzymatic reactions in the cell for which the major physiological processes obstacle and creates metabolic imbalance [ 32 — 35 ]. Some common effects of heat stress have been summarized in Table 1. Among the growth stages of plant the germination is affected first of all.
Heat stress exerts negative impacts on various crops during seed germination though the ranges of temperatures vary largely on crop species [ 49 , 50 ]. Reduced germination percentage, plant emergence, abnormal seedlings, poor seedling vigor, reduced radicle and plumule growth of geminated seedlings are major impacts caused by heat stress documented in various cultivated plant species [ 50 — 52 ].
Inhibition of seed germination is also well documented in HT which often occurs through induction of ABA [ 53 ]. Plant height, number of tillers and total biomass were reduced in rice cultivar in response to HT [ 55 ]. High temperature causes loss of cell water content for which the cell size and ultimately the growth is reduced [ 24 , 56 ]. Reduction in net assimilation rate NAR is also another reason for reduced relative growth rate RGR under HT which was confirmed in maize and millet [ 57 ] and sugarcane [ 58 ]. The morphological symptoms of heat stress include scorching and sunburns of leaves and twigs, branches and stems, leaf senescence and abscission, shoot and root growth inhibition, fruit discoloration and damage [ 24 ].
Damage to leaf-tip and margins, and rolling and drying of leaves, necrosis, was observed in sugarcane due to HT stress [ 59 ]. In common bean Phaseolus vulgaris morphophysiological characteristics such as phenology, partitioning, plant-water relations, and shoot growth and extension are seriously hampered by heat stress [ 60 ]. Reduced number of tillers with promoted shoot elongation was observed in wheat plant under heat stress [ 50 ].
High temperatures may alter the total phenological duration by reducing the life period.
At extreme heat stress plants can show programmed cell death in specific cells or tissues may occur within minutes or even seconds due to denaturation or aggregation of proteins, on the other hand moderately HTs for extended period cause gradual death; both types of injuries or death can lead to the shedding of leaves, abortion of flower and fruit, or even death of the entire plant [ 14 , 24 ].
Photosynthesis is one of the most heat sensitive physiological processes in plants [ 65 ]. High temperature has a greater influence on the photosynthetic capacity of plants especially of C 3 plants than C 4 plants [ 66 ]. In chloroplast, carbon metabolism of the stroma and photochemical reactions in thylakoid lamellae are considered as the primary sites of injury at HTs [ 67 , 68 ].
Thylakoid membrane is highly susceptible to HT. Major alterations occur in chloroplasts like altered structural organization of thylakoids, loss of grana stacking and swelling of grana under heat stress [ 24 , 56 ]. Heat shock reduces the amount of photosynthetic pigments [ 68 ]. The ability of plant to sustain leaf gas exchange and CO 2 assimilation rates under heat stress is directly correlated with heat tolerance [ 66 , 70 ].
Molecular mechanisms underlying stress response and adaptation
Heat markedly affects the leaf water status, leaf stomatal conductance gs and intercellular CO 2 concentration [ 71 ]. Closure of stomata under HT is another reason for impaired photosynthesis that affects the intercellular CO 2 [ 56 ].
All these events significantly decreased the photosynthesis compared with OT in sorghum [ 40 ]. Some other reasons believed to hamper photosynthesis under heat stress are reduction of soluble proteins, Rubisco binding proteins RBP , large-subunits LS , and small-subunits SS of Rubisco in darkness, and increases of those in light [ 73 ]. High temperature also greatly affects starch and sucrose synthesis, by reduced activity of sucrose phosphate synthase, ADP-glucose pyrophosphorylase, and invertase [ 24 , 74 ].
Heat imposes negative impacts on leaf of plant like reduced leaf water potential, reduced leaf area and pre-mature leaf senescence which have negative impacts on total photosynthesis performance of plant [ 71 , 75 ]. Under prolonged heat stress depletion of carbohydrate reserves and plant starvation are also observed [ 74 ]. Although all plant tissues are susceptible to heat stress at almost all the gowth and developmental stages, the reproductive tissues are the most sensitive, and a few degrees elevation in temperature during flowering time can result in the loss of entire grain crop cycles [ 30 ].
During reproduction, a short period of heat stress can cause significant decrease in floral buds and flowers abortion although great variations in sensitivity within and among plant species and variety exists [ 76 ]. Even heat spell at reproductive developmental stages plant may produces no flowers or flowers may not produce fruit or seed [ 77 , 78 ].
The reasons for increasing sterility under abiotic stress conditions including the HT are impaired meiosis in both male and female organs, impaired pollen germination and pollen tube growth, reduced ovule viability, anomaly in stigmatic and style positions, reduced number of pollen grains retained by the stigma, disturbed fertilization processes, obstacle in growth of the endosperm, proembryo and unfertilized embryo [ 79 ].
High temperature often causes excessive ethylene Eth production and leads to male sterility of rice pollens. The Eth is hypothesized to inhibit the key enzymes in sugar—starch metabolism which weaken sink strength and restrict grain filling and ultimately produce sterile grain. Edreira and Otegui [ 47 ] observed that heat stress at flowering periods, more specifically at pre-silking and silking stages resulted higher yield reduction relative to the heat stress at grain filling stage of maize.
High temperature stress resulted in abscission and abortion of flowers, young pods and developing seeds, resulting in lower seed numbers in soybean [ 82 ].
High temperatures at flowering are known to decrease pollen viability in soybean [ 44 ]. Elevated temperatures are raising apprehension regarding crop productivity and food security [ 62 ]. Its affect is so terrible that even a small 1. Higher temperatures affect the grain yield mostly through affecting phenological development processes. Heat induced yield reduction was documented in many cultivated crops including cereals e. The sensitive crop varieties are more severely affected by heat stress relative to tolerant varieties. The higher yield reduction was also observed in heat-sensitive rice cultivar Shuanggui 1 Heat stress modifies the early dough and maturity stage shorten the kernel desiccation period and cause grain yield loss in wheat [ 48 ].
Molecular mechanism for relaxation of and protection from light stress 
Heat also reduces the single kernel weight and it is the major contributor to the yield loss [ 87 ]. In canola Brassica spp.
The cause of this yield decline was due to heat induced infertile pods, reduced seed weight and seeds per pod [ 88 ]. Loss of productivity in heat stress is chiefly related to decreased assimilatory capacity [ 89 ] which is due to reduced photosynthesis by altered membrane stability [ 22 ] and enhanced maintenance respiration costs [ 90 ], reduction in radiation use efficiency RUE, biomass production per unit of light intercepted by the canopy.
These occurrences were documented in wheat [ 91 ] and maize [ 92 ]. Elevated temperature affects the performance and crop quality characteristics. Grain quality characteristics in barley significantly changed under heat stress. In barley grain several proteinogenic amino acids concentrations and maltose content increased, where the concentrations of total non-structural carbohydrates, starch, fructose and raffinose, lipids and aluminum were reduced [ 93 ].
Damages in pod quality parameters such as fibre content and break down of the Ca pectate were found in okra Abelmoschus esculentus at HT stress [ 46 ]. Different metabolic pathways are depended upon enzymes which are sensitive to various degrees of HTs. It is suggested that because of thermal damage to photosystems under such HTs less absorbtion of photon occurs [ 96 ]. Singlet oxygen is formed during photoinhibition, and PS II electron transfer reactions in chloroplasts [ 99 , ].
Physiological, Biochemical, and Molecular Mechanisms of Heat Stress Tolerance in Plants
Sites of production of reactive oxygen species in plants [ 5 ]. Various physiological damages occur in plants upon exposures to varying levels of heat stress [ 96 ]. Hydroxyl radicals can potentially react with all biomolecules, like pigments, proteins, lipids and DNA, and almost with all constituents of cells [ 97 , 98 ].
Singlet oxygen can directly oxidize protein, polyunsaturated fatty acids and DNA [ 99 , ]. Thermal stress can induce oxidative stress through peroxidation of membrane lipids and disruption of cell membrane stability by protein denaturation [ 24 , ]. Functional decrease in photosynthetic light reaction even under moderate HTs was documented to induce oxidative stress through ROS production caused by increased electron leakage from the thylakoid membrane [ 96 , ].
The HT increased leaf temperature which reduced the antioxidant enzyme activities that increased malondialdehyde MDA content in leaves of rice plant [ 37 ]. Populations of perennial ryegrass Lolium perenne L. High temperature stress provoked membrane lipid peroxidation and aggravated membrane injury was also observed in cotton, sorghum and soybean [ 40 , 44 , 75 ].
Moreover, the ROS produced by HT stress are involved in proteolysis of protein or degradation of polymeric protein into simple soluble forms those are the cause of premature leaf senescence in cotton [ 75 ]. In such extreme cases, ROS accumulation in cells can trigger programmed cell death [ ].
Although the ROS have tremendous destructive effects on plant metabolic processes they have also hypothesized to have signaling behaviors to trigger the heat shock responses towards the development of heat tolerance in plant which are inexplicable and should be divulged [ 94 ]. Living organisms can be classified into three groups, subject to the preferred temperature of growth Figure 3.
There is a great variation among the plant species in terms of their response and tolerance to HT. On the basis of thermotolerance, Larcher [ ] classified all the plant species into three groups Figure 3. Survival in hot, dry environments can be achieved in a variety of ways, by combinations of adaptations [ ].
Plant adaptation to heat stress includes avoidance and tolerance mechanisms which employ a number of strategies Figure 4. Different adaptation mechanisms of plants to high temperature. A: Avoidance, T: Tolerance. Under HT conditions, plants exhibit various mechanisms for surviving which include long-term evolutionary phenological and morphological adaptations and short-term avoidance or acclimation mechanisms such as changing leaf orientation, transpirational cooling, or alteration of membrane lipid compositions.
Closure of stomata and reduced water loss, increased stomatal and trichomatous densities, and larger xylem vessels are common heat induced features in plant [ 58 ]. In many crop plants, early maturation is closely correlated with smaller yield losses under HT, which may be attributed to the engagement of an escape mechanism [ 24 , ]. Plants growing in a hot climate avoid heat stress by reducing the absorption of solar radiation. This ability is supported by the presence of small hairs tomentose that form a thick coat on the surface of the leaf as well as cuticles, protective waxy covering.
In such plants, leaf blades often turn away from light and orient themselves parallel to sun rays paraheliotropism. Solar radiation may also be reduced by rolling leaf blades.
Related Stress Responses of Photosynthetic Organisms. Molecular Mechanisms and Molecular Regulations
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