FLOODING STRESS IN PLANT GROWTH AND DEVELOPMENT DETERMINAL EFECTS ADAPTIVE RESPONSE

1. FLOODING
STRESS IN PLANT
GROWTH AND
DEVELOPMENT
Submitted by MOHAMMED ANFAS K T
anfasnellikuth@gmail.com
https://www.linkedin.com/in/mohd-anfas-5409431a0

2. 1.
Detrimental Effects
of Flooding Stress
on Metabolism

3. i. Effects on Respiration
3
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.

4. ii. Effects on
Photosynthesis
4
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.

5. 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.

6. 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.

8. iii. Effects on Water and
Nutrient Relations
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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.

9. 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.

10. 2.
Adaptive Responses
to Flooding Stress

11. 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.

12. 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.

13. 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.

14. 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

15. 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.

16. 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.

17. THANK
YOU
17
Submitted by MOHAMMED ANFAS K T
anfasnellikuth@gmail.com
https://www.linkedin.com/in/mohd-anfas-5409431a0