Rice, a staple for over half the global population, is highly vulnerable to heat stress. When exposed to temperatures above 35°C during the flowering stage, rice can experience a 10% yield reduction. Physiologically, heat stress leads to spikelet sterility, chalky grains, and reduced grain size. In extreme cases, temperatures exceeding 40°C can result in complete sterility, decimating yields. This article explores rice’s optimal growth conditions, stages vulnerable to temperature shifts, and its physiological and molecular reactions to heat stress.
Table of Contents
Rice Growth Stages
Rice thrives best in warm climates with temperatures between 20-35°C.
Optimal temperatures are crucial throughout these stages, with any significant deviations, especially during the reproductive phase, potentially affecting yield and grain quality.
Stages of Rice Growth Affected by Heat Stress
Vegetative Stage: In the early growth stages, rice prefers a temperature range of 30-35°C for germination. Outside this range, germination is hampered, affecting plant height, tiller number, and overall dry weight.
Reproductive Stage: Rice is most vulnerable during its heading and flowering stages. High temperatures can cause spikelet sterility and reduce the grain-filling period, leading to fewer grains and a lower yield.
Grain Filling Stage: Rice prefers cooler nights during grain filling. However, high temperatures can lead to abnormal grain shapes and colors, reducing grain weight and overall yield.
Physiological Effects of Heat Stress on Rice
Membrane Damage: The plant cell’s biomembrane, a sensitive component, can suffer structural and functional impairments under high temperatures. This can lead to increased fluidity and leakage of ions, compromising the cell’s integrity.
Reactive Oxygen Species Accumulation: Elevated temperatures cause a surge in reactive oxygen species (ROS) inside the cell. This can lead to cell death, grain chalkiness, and even seedling death. Excessive ROS can further damage biomembranes, impairing proteins and nucleic acids.
Photosynthesis Disruption: Photosynthesis, vital for plant growth, is highly sensitive to heat. Elevated temperatures can reduce chlorophyll content, leading to decreased photosynthetic rates. The most affected component is Photosystem II, which can be severely impacted by oxidative stress.
Carbohydrate Metabolism Disturbance: Heat stress can disrupt carbohydrate metabolism, affecting the sugar content in anthers. This impairs pollen development, leading to grain chalkiness and reduced starch accumulation.
Phytohormone Imbalance: Heat stress can cause an imbalance in phytohormones, affecting spikelet number, pollen fertility, and kernel weight. This includes reduced levels of cytokinin, gibberellin, and indole-3-acetic acid.
Molecular Mechanisms in Response to Heat Stress
Heat Stress Sensing:
Cell Wall Changes: The cell wall is a protective barrier surrounding plant cells. When plants are exposed to high temperatures, this wall undergoes structural modifications. These changes can influence various cellular processes.
Apoplastic Ca2+ Release: The space outside the cell membrane but within the cell wall is known as the apoplast. Calcium ions (Ca2+) present in this region can be released in response to certain stimuli, like heat stress.
Cytosolic Ca2+ Content: As a result of the release from the apoplast, there’s an increase in the calcium ion content within the cell’s cytoplasm (cytosolic Ca2+). This rise in calcium levels acts as a signal for the cell to respond to the heat stress.
Elevated ROS: Reactive Oxygen Species (ROS) are molecules that can cause damage to cells if their levels rise too much. An early response to heat stress is the increased production of ROS.
Heat-induced Signal Cascades:
Cytosolic Ca2+ as a Messenger: The increased cytosolic Ca2+ acts as a messenger molecule. It helps in rapidly transmitting the external heat signals inside the cell, initiating a cascade of responses.
Role of ROS in Thermotolerance: While ROS can be harmful, they also play a crucial role in helping plants tolerate heat. They are involved in various signaling pathways that enhance the plant’s ability to withstand high temperatures.
Transcriptional Regulatory Network:
Heat Stress Transcription Factors (HSFs): These are proteins that bind to specific DNA sequences and regulate the expression of certain genes. In response to heat stress, HSFs activate genes that produce ROS scavenging enzymes (which reduce ROS levels), metabolic enzymes, and Heat Shock Proteins (HSPs).
Other Transcription Factors: Apart from HSFs, other transcription factors also play a role in regulating plant responses to heat, ensuring the plant can adapt and survive.
Protein Homeostasis:
Disruption Due to Heat: High temperatures can disrupt the normal structure and function of proteins within the cell. This leads to the accumulation of proteins that are unfolded or not correctly shaped.
Role of HSPs: Heat Shock Proteins (HSPs) come to the rescue in such situations. They help stabilize these misfolded proteins and assist in refolding them back to their functional forms, ensuring the cell remains healthy and functional.