High Temperature Stress in Plants

In this article we will discuss about High Temperature Stress in Plants. After reading this article you will learn about: 1. Subject-Matter of High Temperature Stress 2. Effects of High Temperature Stress.

Subject-Matter of High Temperature Stress:

All the living organisms, either plants or animals, are adapted to grow within a narrow range of temperature limits. Temperatures within the range influence metabolism by its effect on chemical reactions, which are catalysed by enzymes.

A slight increase in temperature even for a short duration, may affect the physiological and biochemical processes of plants. At extremely high temperature beyond the limit of survival life is destroyed by losing control on chemical reactions, proteins undergo denaturation along with physical changes detrimental to the organism.

Effects of High Temperature Stress:

i. Cellular Organisms:

High temperature stress adversely affects every physiological activity or metabolic process in plants. Land plants have to tolerate an environment of changing temperature, and such changes may diurnal or seasonal. The upper temperature limits at which the different organisms may grow and survive have been found to vary considerably.

A simple organism like heterotrophic bacteria can grow at 110°C, while photosynthetic bacteria are not found above 73°C and eukaryotes are able to live above 60°C. Although higher plants cannot withstand a temperature above 50°C for a long time, there are some flowering plants like cactus plants which can survive for quite some time at 55°C (day) and 45°C (night) temperatures.

ii. Enzymes:

K_m for an enzyme represents the concentration of substrate required to half-saturate the enzyme. The value of K_m indicates enzyme-substrate affinity. A higher K_m reflects lower affinity of the enzyme for substrate. Conversely, a lower K_m reflects higher affinity of the enzyme for substrate.

The temperature has large effects on enzyme-substrate affinities as shown by K_m variations. This is because the specific shape of the active site is stabilized by temperature-sensitive interactions within enzyme protein, which in turn influence the attraction between enzyme and substrate.

Catalytic efficiency of an enzyme rises with increase of V_max but falls with increase of Km and the ratio V_max/K_m is a measure of catalytic activity. At high temperatures within physiological limits, V_max will rise and Km will decrease leading to increased activity. But at still higher temperatures, proteins will be denatured with loss in catalytic activity.

iii. Photosynthesis:

Photosynthesis represents an integration of photochemical as well as biochemical processes.

Thus, temperature will have a direct impact on photosynthesis through its effects on temperature- sensitive biochemical and physiological processes. It has been observed that exposure of leaves to elevated temperature (35 – 50°C) in the biologically relevant range, CO₂ assimilation, O₂ evolution and photophosphorylation are generally inhibited.

Chloroplast activities associated with thylakoids appear to be more heat-sensitive than those of the stroma, and PS II is the most heat-labile component of thylakoid membranes. Inactivation of PS II by high temperature is associated with denaturation of PS II polypeptides and the inhibition of the oxygen-evolving complex as a result of release of Mn²⁺ ions.

It is to be noted that in contrast to PS II components, which are less stable to high temperature, PS I components are more thermos-table. Chloroplast biogenesis has been shown to be affected when plants are grown at high temperature. Chlorophyll content also declines and the leaf senescence process begins.

Photophosphorylation is one process that is inhibited by high temperature owing to thermal uncoupling. This will result in a decrease in the supply of ATP necessary for carbon assimilation.

In addition, damage to PS II at the level of O₂ evolving complex or the destabilization of the light- harvesting chlorophyll binding complex (LHCP) associated with PS II will lead to the reduced supply of NADPH concerned in CO₂ fixation as well as will result in decreased efficiency of the thioredoxin system responsible for the light activation of key enzymes of the Calvin cycle including Rubisco.

It has also been suggested that the decline in Rubisco activation may be due to an inhibition of Rubiscoactivase.

iv. Ultra-Structural :

The ultra-structures of cells undergo considerable changes when exposed to high temperature stress. It is generally observed that the cellular activities are always separated from outside environment by suitable barriers represented by membranes which very often lead to the formation of cellular compartments.

Heat stress has also been reported to cause prominent ultra-structural changes in nucleus, mitochondria, endoplasmic reticulum and plastids. In secretory cells of barley aleurone, amylase translation process is arrested by dissociation of lamellar structure.

During heat stress, fatty acids comprising the membrane lipids shift from long-chain unsaturated to short-chain saturated ones. It is possibly an adaptive change by which membrane with decreased level of fluidity is formed through a conversion of unsaturated to saturated state that will be able to maintain membrane integrity at high temperature.

v. Metabolism (Thermogenesis):

Several species belonging to family Araceae show a special kind of respiration that is cyanide resist ant and thermo-genic. These species have been found to heat their tissues much above the surrounding temperature. In this case, heat is produced by cyanide-resistant respiration in mitochondria of thermo-genic tissues of the inflorescence of such aroids.

The phosphorylation site of cyanide- resistant respiration is either one (in case of NAD-linked substrate) or zero (in case of succinate) in contrast to three present in normal, cyanide-sensitive respiratory chain. Since energy conservation is less, the tissue produces much waste heat as the energy that is not conserved as ATP.

(HSPs):

The response to high temperature or heat shock is one of the very well-known environmental responses at the molecular level. When seedlings are shifted to temperatures five or more degrees above optimal growing temperature, synthesis of most normal proteins and mRNAs is repressed, and transcription and translation of a small set of “heat-shock proteins” (HSPs) is initiated. This was first discovered in Drosophila.

Several classes of HSPs have been described in eukaryotes including plants. They are designated by their approximate molecular weight in kDa as HSP 110, HSP 90, HSP 70, HSP 60 and low molecular weight (LMW) HSPs (15 – 20 kDa). In addition, ubiquitin is also referred to as an HSP because its synthesis increases during heat stress.

Ubiquitin is a highly conserved protein composed of 76 amino acids and conjugation of ubiquitin to various acceptor proteins in eukaryotic cells regulates a number of cellular processes. Ubiquitin has protease activity and it helps in removal of denatured or non-functional proteins. Thus, it prevents the accumulation of such proteins to toxic levels and recycles them as peptides or amino acids.

Heat-shock proteins have potential role in plant protection from high temperature stress. It is possible that HSPs contribute to an organism’s ability to survive at high temperature. The principal role of HSPs involves stabilization of proteins in a particular state of folding.

Through this mechanism, HSP 90, HSP 70 and HSP 60 facilitate many processes like protein folding, transport of proteins across membranes, assembly of oligomeric proteins, and modulation of receptor activities.

All these functions require alteration or maintenance of specific polypeptide conformations. Based on these activities, HSP 90, HSP 70 and HSP 60 have been termed “molecular chaperones”or “polypeptide chain binding proteins”.

One HSP 60 has been identified in chloroplasts as the Rubisco subunit binding protein. It is involved in the assembly of Rubisco holoenzyme. Which is composed of 16 subunits of 2 types, i.e., 8 chloroplast-encoded large subunits and 8 nucleus-encoded small subunits.