2. Oxygen deficiency
Oxygen deficiency in plant also know as hypoxia and anarobiosis
Total absence of oxygen is called anoxia
• Hypoxia:
• Hypoxia has been define as an expressions of inadequate oxygen
availability.
3.
4. 1. INTRODUCTION
• Hypoxia and/or anoxia conditions around the root system are
a major issue for plant growth and development, not only in
natural ecosystems but also in agriculture, horticulture or
silviculture.
• In horticulture and agriculture, under field or greenhouse
growth conditions, hypoxia is frequently observed as a result
of either poor drainage and/or root aeration.
5. 2. SYMPTOMS OF HYPOXIA/ ANOXIA
• The effect of oxygen deprivation on plant growth and development
has long been recognised.
• There are numerous historic examples of dramatic flood events
destroying annual crop production and reducing harvest yield.
• Floods, caused by a seasonal rise in the water table (spring or fall),
will have marked effects on plant growth and development.
• When these events occur in the spring there will often reduce seed
germination and seedling establishment.
6. • In general, root hypoxia will only be detected after the plant has
been experiencing the stress for some time and, only once the aerial
part of the plant shows symptoms of stress such as leaf wilting,
bleaching, and senescence.
• As a result,of hypoxia sensitive plants exposed to hypoxia often wilt
in a similar way as plants exposed to drought.
7.
8. • The most commonly observed early symptoms on the aerial part of a
plant include: leaf curling (epinasty) and stem twisting,leaf chlorosis
and wilting, marginal browning of the leaf and shedding/defoliation,
as well as fruit drop .
• Other signs of stress include reductions in specific leaf area and
weight, a decline in shoot growth and sprouting, dieback of the stem
and limb and eventually death of the plant.
9. 3. MORPHOLOGICAL ADAPTATIONS
TO HYPOXIA/ANOXIA
• Aerenchyma development:
• One of the most important response of wetland and some non-
wetland species to waterlogging is the development, in the root
cortex, of gaseous spaces that could help provide O2 to active cells
in the roots .
• The development of gas spaces in the root cortex is one of the most
important adaptive response to soil flooding in many flood tolerant
species.
10.
11. • The proportion of aerenchyma generally distinguishes wetland from
non-wetland plants .
• Ethylene dependent cell death and lysis occurs behind the root apex
in the region of cell expansion and forms continuous gas-filled
channels which help oxygen diffusion from shoot to roots .
12. • Aerenchymaare also observed in the leaf sheath following soil
flooding, forming an interconnecting system of shoot-root
ventilation.
• Their development increases root porosity which itself can be
initiated as a result of osmotic dependant changes in cell shape.
• Nuclear changes (clumping of chromatin, fragmentation,
disruption of the nuclear membrane), are the earliest events
observed following flooding.
13. • These nuclear changes are followed by plasma membrane
becoming crenulated, tonoplast disintegration, organelle swelling
and disruption, loss of cytoplasmic contents and collapse of the cell .
• Development of aerenchyma is often associated with the
enlargement of cortical cells and aerenchyma formation correlates
with increased cellulase and/or xyloglucan endotransglycosylase
(XET) abundance.
14. • These transglycosylase enzymes play a role in cell wall metabolism
during cell expansion.
• Wetland species often have roots with lignified and suberized
secondary walls that develop within a few centimetres of the root
tip, thus limiting the radial diffusion and loss of O2 to the
surrounding environment.
15. Hypertrophied lenticels :
• The presence of hypertrophied lenticels is a common anatomical change
observed in many woody species during flooding.
• Hypertrophic growth appears as swelling of tissues at the stem base
and is believed to result from radial cell division and expansion.
16. • It is associated with auxin and ethylene production and the
development of large root cortical intercellular air spaces in flood-
tolerant species .
• The development of hypertrophied lenticels at the stem base is
believed to facilitate the downward diffusion of O2 as well as
potential venting structures to release phytotoxic compounds
produced as by-products of anaerobic metabolism (ethanol, CO2).
17. • Interestingly, the tendency for stomatal conductance to return
towards control levels after a transient decrease, has generally been
associated with the development of hypertrophied lenticels, lateral
roots and flood Tolerance.
18. • Adventitious roots :
• Another important morphological adaptation to flooding is the
growth of adventitious roots, which functionally replace basal roots.
• The formation of these specialised roots takes place when the
original root system becomes incapable of supplying the shoot with
the required water and minerals, because the energy required for
active transport is insufficient.
19. • The formation of adventitious roots takes place at the base of the
stem of flooded plants and their growth is horizontal rather than
vertical like normal roots.
• The ability to produce adventitious roots is commonly associated
with enhanced tolerance to flooding and their development is
mediated by ethylene, as evidenced by experiments using ethephon.
20. • SIGNALLING AND HORMONAL CONTROL OF PLANT RESPONSES TO
HYPOXIA/ANOXIA :
• Flooding is characterised by changes in the levels of several plant
growth regulators.
• The most studied, ethylene, is induced by a variety of biotic and
abiotic stresses.
21. • The enzyme ACC (1-aminocyclopropane 1-carboxylate) synthase
catalyses the rate limiting step in ethylene biosynthesis, the
conversion of S-adenosyl-L-methionine to 1-aminocyclopropane 1-
carboxylate .
• In higher plants the immediate precursor of ethylene is ACC.
• In addition, ethylene is required for aerenchyma and adventitious
root formation in maize .
22. • Ethylene production rate in leaves of flooded plants closely
correlates with the induction of ACC synthase and the accumulation
of ACC in the Roots.
• In tomato plants exposed to flooding, ACC is rapidly synthesised in
the roots, and transported to the shoot within 6 to 12 h.
23. • In addition to the convincing example of root-to-shoot ethylene
signalling events during flooding, there is increasing interest in the
possible role played by abscisic acid (ABA), gibberelin (GA) and
auxin .
• In tomato, foliar ABA concentrations are increased transiently
following soil flooding and exogenous ABA application increased
anoxia tolerance in maize .
24. • The ethylene increase following flooding causes an increase in GA
concentration and sensitivity to that hormone.
• Other signalling molecules such as salicylic acid (SA) may also be
involved in the responses and adaptation to hypoxia.
• SA has been implicated with ABA in the regulation of stomatal
opening during drought stress.
25. • SA stimulating root growth and adventitious root formation
although, to date no direct evidence is available on a potential role
for SA during hypoxia.
26. • CONCLUSION :
• In general, survival to flooding includes one or more of the following
responses: control of energy metabolism, availability of abundant
energy sources, provision of essential gene products and synthesis of
macromolecules, and protection against post-anoxic injury.
• This understanding is crucial to identify and select plants capable of
withstanding increased environmental pressure.
27. • In addition, the natural selective pressures are increased in
frequency and duration with climate change and species may be
unable to evolve in time to survive.
• It is therefore crucial that plant strategies are analysed and
characterised to help better select species and individuals capable of
tolerating these stresses.
