Heat Stress and Its Management in Agriculture is very important. Heat stress is an abiotic stress and defined as the rise in temperature beyond a threshold level for a period of time enough to cause irreversible damage to plant growth and development.
This ultimately leads to low yield of the crop. Generally, a transient elevation in temperature 10-15oC above ambient is considered as heat stress.
Due to global warming and greenhouse gases, heat stress is becoming a major limiting factor for better crop production. There exist a lot of variability in plant responses to heat stress.
According to the sensitivity to heat stress plants are categorized into sensitive, facultative tolerant, and tolerant to heat stress. Sensitive crops would be more prone to heat stress than other crops. So, heat stress management is necessary for better production.
Heat-Stress Threshold
A threshold temperature refers to a value of daily mean temperature at which a detectable reduction in the growth of plants begins. Threshold temperature is different for different species. In some plant species, the higher threshold temperature may be lower or higher than 35°C.
High temperature sensitivity is more important and heat stress is a big issue in tropical and sub-tropical climates. In those areas, heat stress becomes a major limiting factor for low yield of crops.
In addition, heat stress makes the plant susceptible to more pests attack and other environmental problems. So, there is a need to understand the response and mechanism of plants for heat stress management.
Plant’s Response to Heat Stress
Direct Injuries
Heat stress causes delayed germination and loss of seed vigor. Reduced plant emergence leads to patchy crop establishment which is a restraining factor towards high yield.
Due to heat stress plants show wilting, scorchy leaves appearance, sunburn of twigs or branches, Leaf senescence, abscission and stunted growth. At the anatomical level, there are high transpirational losses, closure of stomata and reduction in cell size.
At the sub-cellular level, heat stress disrupts the organization of the thylakoid membrane, loss of thylakoid stacking or its swelling leads to low chlorophyll and photosynthesis. Due to the high temperature mitochondrial cristae also affect.
Indirect Injuries
Indirect injuries are inactivation of enzymes of chloroplast and mitochondria, denaturation of protein, inhibition of protein synthesis and loss of membranes integrity.
Membrane fluidity increases due to damage of structural organization of the membrane. At the molecular level, oxidative stress also occurs as a secondary stress in response to heat stress cause major damage to cell functions. Singlet oxygen (1O2), superoxide radical (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (OH–) are reactive oxygen species. In all these, hydroxyl radical is more damaging than other ROS.
Growth Stage More Prone to Heat Stress
Heat stress causes damage to plants at each stage but the reproductive stage is more sensitive to heat stress. At the germination stage of seed, heat stress affects the plant population of crop.
If heat stress occurs at the anthesis stage, pollens viability affects. This results in less fertilization and less number of grains. If heat stress occurs at post anthesis or grain filling stage, due to less dry matter partitioning from leaves to grains, the grains size remains short which also affects 1000 grains weight or economic yield of crop.
Mechanism of Heat Tolerance in Plants
Plants have evolved various mechanisms against heat stress. They include short term avoidance, acclimation or long term adaptations to thrive in stressed environmental conditions.
In short term response plants change their leaves orientation, transpiration for cooling effect and change in the composition of cell membrane lipids. Plant show this response for initial heat stress signals in which cell osmotic balance disturbs or fluidity of cell membrane change.
In response to this signal, plants try to reestablish proper osmoregulation and repair cell membrane structure.
Heat Shock Proteins
When plants sense the heat stress, there are gene activation in cells to make some proteins, Heat Shock Proteins (HSP’s) which protect the cell from damage. Plants also produce late embryogenesis proteins (LEA), osmoprotectants such as glycine betaine, proline and some polyols which provide adaptation against heat stress.
Osmoprotectants
Accumulation of osmoprotectants (soluble solutes) in the cell helps in osmotic adjustment by maintaining water balance and membrane fluidity. Osmoprotactants also act as buffers for cellular redox potential.
This mechanism of plants against heat stress is known as thermotolerance. This may be the inherent ability of plants or acquired by regular exposure of plant to heat stress.
Improvement in Heat Tolerance Mechanism
When high temperature occurs year after year, for heat stress management plants adapt themselves. Plants adapt changing their leaves size into smaller, rolled leaves and having small hairs or thick waxy cuticle layer on leaves surface.
So, that direct exposure of leaves with heat would be minimum. Plants have a mechanism to make their own protection against heat stress by producing different kinds of proteins or sugars. There is a need to improve the efficiency of plants to make these protecting agents.
Genetic Approach
For this genetic engineering is being used to add genes responsible for making more heat shock proteins or osmolytes. Overexpression of these genes will produce more osmoprotectants, which is a key mechanism of plants against abiotic stresses. These transgenic plants have more capabilities to survive under heat stress.
Conclusion
Future global climate change, with a predicted rise in temperature from 1.5-5.8°C by the year 2100 is creating heat stress. Its a major threat to agricultural production in Pakistan and worldwide. So to increase yield and to combat high temperature or heat stress in upcoming years due to global warming, there is a need to make heat-tolerant verities of crops by genetic improvement.
This article is jointly written by Sadia Sajjad1, Dr. Muhammad Tahir1, Nabgha Imran1. Department of Agronomy, University of Agriculture Faisalabad.