In this article we will discuss about Flooding Stress. After reading this article you will learn about: 1. Detrimental Effects of Flooding Stress on Metabolism 2. Adaptive Responses to Flooding Stress.
Detrimental Effects of Flooding Stress on Metabolism:
i. Effects on Respiration:
A reduction in aerobic respiration in roots is the initial effect of anaerobic soil conditions. Since conservation of energy in the form of ATP is much less in anaerobic respiration, the amount of ATP, the total energy profile of root cells and the ATP/ADP ratio are reduced under flooded condition.
Another detrimental effect of flooding is the accumulation of ethanol to toxic levels during anaerobic conditions. This is because ethanol is usually produced as an end product of anaerobic respiration. In order to produce enough ATP for cell survival, the rate of anaerobic respiration must be kept at a high level so that an excessive amount of ethanol may be produced in cytosol and stored in vacuole.
ii. Effects on Photosynthesis:
Under flooded condition, plant roots remain submerged while shoots are generally above the water level. Although the shoots are not directly influenced by anaerobic conditions, but they respond to the metabolic conditions of roots.
One of the immediate effects of root zone flooding is stomatal closure of leaves. As a consequence of increasing stomatal resistance, photosynthesis decreases quickly following flooding. Besides stomatal inhibition of photosynthesis, direct inhibition of photosynthetic enzymes may also occur by H2S, a product of submerged soil.
Because of the relative inefficiency of anaerobic respiration as opposed to aerobic respiration, roots of plants under flooded condition demand a utilization of a large amount of carbohydrate. As a result root tissues rapidly become depleted in carbohydrates and this situation has been very often described as “carbohydrate starvation” during flooding.
Furthermore, the roots experience an increased carbohydrate starvation because translocation of carbohydrates from leaves (source) to roots (sink) is hampered during flooding conditions.
The accumulation of inhibitory plant hormones in leaves may also cause non-stomatal inhibition of photosynthesis. Translocation of cytokinins from roots to leaves suffers a decline during flooded condition, while translocation of ABA and ethylene increases.
It is envisaged that hypoxia in roots induced by flooding, leads to an increase in ethylene biosynthesis and its translocation. This is because the internal tissue like the stele of the roots is totally devoid of oxygen (anoxic) as compared to the peripheral cortical tissues with less oxygen (hypoxia).
It is known that the last enzyme in the ethylene biosynthetic pathway (ACC oxidase) requires O2 and stops in anoxic condition, while another key enzyme, ACC synthase activity increases in centre of roots (anoxic), ACC diffuses to outer cells (more hypoxic), where ACC oxidase can complete ethylene synthesis.
Ethylene also induces aerenchyma formation and through such gas passages ethylene transport from root to leaves may increase.
iii. Effects on Water and Nutrient Relations:
Flooding with salt water creates an osmotic stress. On the other hand, flooding with fresh water may decrease root permeability to water and root hydraulic conductance. As a consequence, wilting symptom may result from flooded condition.
Flooding induces the formation of adventitious roots which are, however, more permeable to water and hence are able to enhance water transport to shoot. Total water flow in plants will be less because adventitious roots have inferior hydraulic conductance.
Anaerobic respiration is unable to provide adequate ATP for active nutrient uptake. Thus, nutrient accumulation in flooded situation is very much limited. Transpiration is reduced by flooding and as a result the rate of delivery of nutrients, dissolved in xylem fluid, to the shoots is reduced. A rapid occurrence of chlorosis may happen in sensitive plants after flooding because of reduced nutrient uptake and iron toxicity.
Adaptive Responses to Flooding Stress:
i. Development of Aerenchyma:
Flooding induces the formation of gas spaces called aerenchyma in different plant parts like roots, rhizome, stem, petioles and leaves. It is now believed that aerenchyma tissue helps in proper growth of roots and their survival under reduced O2 regime.
This can be achieved by promoting the conductance of O2 and other gases during their movement through the plant body and also by enhancing the O2 storage capacity. Plant hormone ethylene may be involved in aerenchyma development.
ii. Reorientation of Leaves and Stems:
It has been suggested that petiole epinasty in response to flooding may be an adaptive response enabling plants to withstand stress. Such epinasty response is caused by ethylene production in the shoots of plants, which is induced by flooding. In roots growing under anaerobic condition, ACC, the immediate precursor of ethylene, cannot be converted to ethylene.
It can be transported via the transpiration stream to the shoot remaining in aerobic condition and consequently can be converted to ethylene in the shoot.
iii. Adventitious Root Formation and Hypertrophy:
During flooding, the old roots are replaced by adventitious roots, which act as a survival mechanism. Resistance to flooding is found to be correlated with adventitious root formation. Water and nutrient uptake in flood-resistant plants are facilitated by adventitious roots. Auxin and ethylene may be involved in the formation of adventitious roots, which is induced by flooding.
The ability of flooding to induce hypertrophy at the base of the stem has been observed in a number of plants.
Accelerated lateral cell expansion with increased intercellular space and cell lysis may lead to the induction of hypertrophy. It is generally accepted that hypertrophy increases the porosity at the base of the stem to promote aeration for adventitious root formation and thus acts as a survival mechanism in plants during partial submergence.
iv. Fast Shoot Elongation Under Water:
Deep water rice plants show a unique physiological adaptation with an enhancement of inter-nodal length. As a result, the leaves are kept above water level, which permits the movement of air to the submerged plant parts. Ethylene has been shown to be responsible for internode elongation of deep-water rice.
v. Biochemical Changes Induced by Flooding:
Two theories have been proposed to explain flooding tolerance. According to Crawford’s metabolic theory for flooding tolerance reported in 1971, it has been stated that flooding tolerance depends on reduction in ethanol production.
This can be achieved by low alcohol dehydrogenase (ADH) activity, which reduces the accumulation of ethanol having toxic effects. Flooding tolerance has been related with the ability to shift glycolytic intermediates to alternate end products like malate, lactate and other organic acids.
According to Davies-Robert’s pH stat hypothesis first reported in a total regulation of pH (pH stat) is necessary to prevent the possible change in acidification of cytoplasm. The nature of pH existing in cytoplasm controls the relative rates of synthesis of either lactate at high pH favouring lactate dehydrogenase (LHD).
When the cytoplasmic pH decreases , LHD is reduced while pyruvate decarboxylase is increased resulting ethanol synthesis.