Heat Stress and its Impact on Plant Function: An Update

An astonishing increase in temperature is posing several harmful impacts on crop plants. Heat stress is an abiotic environmental phenomenon that causes limits, inhibits plant growth, metabolism, and productivity worldwide, resulting in losses in production yields. Heat stress is caused by human activities and global warming,s such as greenhouse gases, carbon dioxide, methane, nitrous oxide, and water vapour. There are many pieces of evidence to support that heat stress reduces the crop plants yield worldwide, and the effects of heat stress are challenging to meet nutritional security and global food security for human beings. Heat stress has negative impacts on each developmental stage, including from germination to harvesting. Prevalent approaches for heat adaption is inadequate management that is unable either to increase the crop productivity or sustain ld. Several responses to dissect the relevant knowledge about heat stress mechanism involving morphological phenomena, physiological phenomena, reproductive replies, and molecular responses such as heat shock proteins act as mRNA synthesis, mRNA control (effects of genes during heat stress), the translation process, heat response element. There are such phenomena involving disseminating the knowledge concerning heat stress. In this review, we summarise the effect of heat stress on plant mechanisms, including morphological, biochemical and molecular responses.


Introduction
Heat stress is a primary concern for agriculture production due to its changes in an irreversible way for crop plants (Hall, 2000). There are many causes for the increase in heat stress, namely greenhouses gaseous, chlorofluorocarbon, human activities and CO2 assimilation. Numerous models have been put forth to predict climate change's impact on crop yields (Zhou et al., 2017). Keeping everything constant in the simulation models has been seen by a study that increases in temperature, which reduces the global yield of maize (7.4%), wheat (6.0%), soybean (3.1%) and rice (3.2%) (Zhou et al., 2017). Forecasts predict the frequency and duration of extreme heat events to rise by 50 % in 2050 and 90 % by 2100, resulting in significant yield losses (Handmer et al., 2012).
Crops adapted to the warmer regions show better tolerance to high temperatures, as evident in cucumber, cowpea, and cotton (Driedonks et al., 2016). While in the excellent season, crops such as lentils and wheat show decreased germination at soil temperatures above 24-26 0 C (Hall, 2000). Rising temperature resulting in heat stress is causing an alteration in the mechanism of crop plants, including morphological, physiological and molecular (Porter, 2005). Heat stress leads to the death of cells within minutes or could destroy the whole plant (Schoffl et al., 1999). Out of all the factors, the timing and duration of heat stress have the worst effect on plant growth (Dufault et al., 2009). Heat stress damage is severe when it coincides with the critical crop development stage, particularly the reproductive stage (Rieu et al., 2017). The reproductive stage is highly vulnerable to damage and thus causing significant yield penalties (Telfer et al., 2013).
Heat stress is causing direct or indirect effects on plants, including protein, lipid denaturation, mitochondria death, membrane degradation resulting in plant death. Eventually, these direct and indirect effects lead to cells' starvation, ion flux reduction, toxic compounds production, etc. (Howarth, 2005). Heat stress ultimately results in molecular, transcriptional, phenological and physiological changes for plant growth and survival. Different crops plants can tolerate a specific threshold temperature without significant damage to their development and mechanism. This review summarizes the effect of heat stress on plants mechanisms and different changes induced in the plants.

Responses of plants during heat stress:
Heat stress is a significant problem for plants that limits and restricts plants' growth, development, metabolism, and productivity. Therefore, alternation of these responses may positively and negatively impact both (Figure 1). These effects may be on plants, including morphological, physiological, hormonal response and biochemical responses.

Morphologiccal responses
In a tropical climate, heat stress is the limiting factor affecting crops growth, plant development stage, and crop yield. Heat stress causes significant limitations in all crops, including wheat, rice, maize, pearl millet, sorghum, barley, brachypodium, arabidopsis, pea and tomato. Heat stress has an irreversible effect on pre and post-harvest losses, including sunburn on leaves, scorching and twigs on leaves, stem, shoot, root growth inhibition, fruit discolouration, and reduced yield (Vollenweider and Günthardt-Goerg, 2005). In addition, heat stress reduces the total biomass, indirectly influencing the characters associated with biomass production. However, morphological appearance can be seen in every plant stage involving the physiological and reproductive stages. For instance, heat stress would reduce seed germination capacity, loss of vigour, reduce seedlings, and ultimately destroy the plants. These responses may differ from the phonological stage to other stages. Another example, heat stress reduced coleoptile at 40 0 C and ceased at 45 0 C in maize (Weaich et al., 1996).
Moreover, it has been seen that one of the effects on shoot growth of plants is effected in which the first internode of shoot growth and some other internodes is inhibited by heat stress. For instance, sugarcane exhibited smaller internodes, high tillering capacity, and ultimately reduced total biomass production (Ashraf . In addition, cell membrane thermostability is a good indicator in which electrolytic leakage from leaf disks are exposed during heat stress. Moreover, chlorophyll fluorescence and membrane leakage is a sensitive method for quantifying the responses of cotton during high temperatures. Hence cell thermostability is a good measure for heat stress. In addition, canopy temperature measures the heat stress in plants. Canopy temperature is calculated using the physiological inframeter technique in which several genotypes are detected (Singh et al., 2007). In addition, photosynthesis phenomena are more sensitive and have adverse effects on plants, including C3 and C4 plants (Young et al., 2004). during heat stress. Photosynthetic rate mainly depends on CO2 concentration in the leaf intracellular, especially in C3 plants. When photosynthesis occurred in thylakoid lamella and carbon assimilation in the stroma in the chloroplast, it is the primary site where photosynthesis is reduced (Wise et al., 2004). Photosynthetic is affected by the activity of the supply of carbohydrates in the developing walls.
Moreover, photosynthesis and chlorophyll content is decreased during heat stress. In addition, reduction of chlorophyll a :b the ratio was more pronounced than developed leaves under high temperatures (Karim et al., 1997). These effects on plants show that chlorophyll and photosynthetic activity is reduced with the production of active oxygen species. Heat stress poses several negative phenomena on plants involving reduced leaf area and water potential that negatively impact photosynthesis of plants. In which photosynthesis transitions from noncyclic photophosphorylation to cyclic photophosphorylation and changes in photosynthetic activity occur by the destruction of electron transport, several proteins, and damages of photosynthetic pigment. In the vegetative stage, high temperature decrease the physiological mechanism in the leaf in that CO2 damage the component of photosynthesis II in thylakoid membrane of chloroplast and resulted in membrane damage (Hall, 2004). The changing of photosynthesis rate along with transpiration rate is a primary indicator of heat stress. During heat stress, significant alternations in chloroplast, thylakoid stability disturbance, swelling in grana in which carbon metabolism activity in the stroma taking place in thylakoid is a disturbance.
Moreover, heat stress has a negative effect on photoinhibition of photosystem I and photosystem II ( PSII transfers an electron to PSI; then PSI is damaged immediately because PSI cannot consume soon by electron sinks. Moreover, photosynthesis is limited by photoinhibition of the two photosystems, i.e., PSI and PSII. These two photosystems, either PSI or PSII, limits the photosynthetic CO2 assimilation. The CO2 oxide assimilation rate per unit area in C4 plants is higher than that of C3 plants, and more assimilation power is required (Atkinson et al., 2016). Ting et al. reported that PSII was significantly inactivated after Heat temperature in maize. Identical results are found in previous research in C4 plants sweet sorghum (Yan et al., 2013).
In addition, respiration is vital for photosynthetic, if respiration is inhibited, then suppression of photosynthesis and aggravates of photoinhibition is occurred (Dinakaret al., 2010; Gardeström and Igamberdiev, 2016; Nunes-Nesi et al., 2011). Limitation of respiratory electron transfer inhibits photorespiration and hence aggravates photoinhibition (Zhang et al., 2017). The efficiency of oxidative phosphorylation is occurred by disturbance of mitochondrial membrane structure. Moreover, the nuclear genome encodes many chloroplast proteins when nuclear envelopes resulting in inhibition of the photoprotection mechanism, hence repair of photoinhibition and structural damage is damaged and delayed by aggravating photosynthetic mechanism (Tana et al., 2019).
Moreover, the respiration rate seemingly increases when the temperature is above 40-50 0 C. This high-temperature affect and damage to respiratory mechanism. Heat stress involves increases in respiratory carbon losses, reduction in ATP production, and increases in ROS production in which kinetics of rubisco and CO2 solubility are adversely affected. The rubisco enzymes catalyze photosynthesis and photorespiration, the rate of rubisco enzyme mainly depends on oxygenase and carboxylase enzymes (Laing et al., 1974). The mesophyll concentration of CO2 limits carboxylase activity. Photosynthetic activity is reduced by the CO2 losses that led to photorespiration.

Hormonal response
Several plants hormones are responsible for maintaining the plants during heat stress. Such plants hormones such as abscisic acid (ABA), salicylic acid (SA) and ethylene are increased, and some hormones such as auxin, cytokinin and gibberellin acid are decreased under heat stress (Larkindale and Huang, 2005). ABA is an abiotic stress hormone that is responsible for combating heat stress. Moreover, it mediates the biosynthetic pathway in which it maintains the heat stress by closure the stomata by osmotic pressure. Furthermore, Abscisic acid save plants under heat stress to alter the expression of several numerous genes. Moreover, it is also related to reactive oxygen species (ROS) generation in guard cells through rubidium hydroxide (Rboh) regulation Furthermore, salicyclic acid (SA) is involved in heat shock responses (HSRs) too. SA prevents oxidative damage to membranes through detoxification of superoxide radicals. Salicylic acid provides strong thermotolerance in plants by association with heat shocks protein (HSP) genes, antioxidant phenomena that improved fertility and increased yield (Larkindale and Knight, 2008). Some other hormones just opposite to ABA, such as gibberellins and cytokinins are involved with heat tolerance. The concentration of these hormones is being declined by roots and shoots growth and dry accumulation under heat stress. In addition, it has been proved that the amount of endogenous auxin is reduced under high temperatures, mainly in anthers (Teale et al., 2006).

Reproductive response
As we know, plants can give high performance in a suitable condition, but under heat stress, the plants survival rate is meager; if they do not change the abnormal environment, they cannot survive hopefully in that environment. Therefore, plants alter the mechanism activity to increase the frequency of survival rate under heat stress conditions and give the expected yield. During the reproductive stage, there are various activities occurred under heat stress (Table 1). There is many reproductively activity such as inflorescence development, sporogenesis, gametogenesis, anthesis, pollination and fertilization etc. (whole reproductive activity during heat stress). As a next developmental step, floral organs originate in the spikelets formed on the central 5 axis of inflorescence or its branches. In 1991, the research found that the ABC flower development   However, the most widely used interpretation is the latter one.  It is reported that the reproductive organ is less sensitive than that of the male reproductive 82 organ. However, it is reported in many studies that female reproductive is sensitive with heat stress 83 and differentiate across different crops. Heat stress has an adverse impact on eggs, syndergids,