Download Vodní režim rostlin

Plant water regime
• Water stress
–
–
–
–
Development of water stress
Adaptation to drought
Signals and signalling pathways
Basic processes affected by water stress
Water stress
• Water stress is induced when transpiration rate (E) is higher than
absorption rate (A)
• High E – low air humidity, high temperature, high irradiance, strong
wind
• Low A – low soil moisture, high concentration of salts, low soil
temperature
• Gradient of water potential between substrate and shoot is prerequisite
for water transport
• Transport of water from storage equilibrates small differences between
E and A
• Transient and permanent water deficit
• Rate of development
• Recovery after rehydration
Adaptations of plants to water stress
• 1) drought avoidance – whole growth cycle in a wet season, leaf fall
under drought
• 2) drought tolerance (resistance)
• a) drought tolerance at low cell water potential – survive with
minimum metabolism: seeds, pollen grains, resurrection plants
• b) drought tolerance at high cell water potential – regulation of water
loss (stomata, cuticle, trichomes, leaf movements, leaf shape, leaf area,
C3CAM), regulation of absorption (amount and morphology of
roots, osmotic adjustment), efficient water transport with low water
potential at cavitation, water storages (stems, trunks, fruits), low
damage under mild water stress, low sensitivity of basic metabolic
processes, production of protection compounds (e.g. carotenoids,
osmolytes, stress proteins, antioxidants)
Water stress signalling
• Signal of water stress can be decreased cell water content, decreased
water potential and its components osmotic and pressure potentials,
increased concentration of solutes, decreased cell volume, change in
membrane tension, changes in structure of macromolecules due to
changes in their hydration, changes in interaction between cell wall
and plasmalemma
• Water stress receptors are not sufficiently known, probably different
for different signals
• For root-shoot communication hydraulic and chemical signals are used
• Direct and indirect effects of water stress
Water stress affects almost all processes in plants
• Inhibition of elongation growth, cell division, changes in cell wall
synthesis
• Inhibition of shoot growth and stimulation of root growth and thus
increase of root/shoot ratio
• Acceleration of ageing
• Production of stress proteins
• Accumulation of osmotically active compounds (proline,
glycinebetaine, sugars, sugar alcohols)
• ROS production and development of antioxidative systems
• Inhibition of photosynthesis, transport of assimilates, respiration
• Changes in enzyme activities (decrease in activity of Rubisco, PEPC,
nitratereductase, but increase in activity of hydrolases or
dehydrogenases)
• Changes in biosynthesis and catabolism of phytohormones, especially
ABA
• Changes in absorption and transport of ions
Multiple effects of water stress
Different water stress sensitivity of different processes
Signalling pathway leading to stress induced changes in gene expression
(Xiong et al. 2002)
Changes in gene expression induced by drought, salinity or cold (Seki et al. 2002)
Role of ABA in plant response to stress (Nakashima and Yamaguchi-Shinozaki 2006)
Water stress and protein synthesis
1) Inhibition of synthesis of some proteins
2) Stimulation of synthesis of other proteins
3) Synthesis of specific stress proteins
• A) proteins taking part in signal transduction and gene expression, e.g.
transcription factors (MYC, MYB), protein kinases (MAPK), enzymes
of phospholipid metabolism (phospholipase C, D)
• B) proteins participating in stress tolerance, e.g. membrane proteins,
proteins of water and ion channels, protection factors (chaperones,
LEA proteins), syntases of osmoprotectants, stress proteins localized
in chloroplasts, specific inhibitors of proteolytic activity, antioxidants,
antioxidative enzymes, proteins taking part in recovery after stress
Oono et al. 2003
Osmotic and elastic adjustment
• Osmotic and elastic adjustment (growth, drought, salinity, low
temperature, etc.), induction by decrease in soil water potential, air
humidity, etc.
• Osmotic adjustment – ion uptake, production and accumulation of
osmotically active substances such as sugars (glucose, trehalose,
saccharose), sugar alcohols (mannitol, sorbitol, glycerol), polyamines,
amino acids (proline), betaines (glycinebetaine)
• Membrane protection, source of C or N, defence against reactive
oxygen species (ROS)
• Dehydrines – ripening of seeds or pollen grains, in plant vegetative
parts during stresses, induced also by abscisic acid (ABA)
• Elastic adjustment – expansin, endoglucanase, transglycosylase,
peroxidase
• At different plant species are different adaptations, amount of
osmoticum is not always in correlation with water stress tolerance
Ogawa and Yamauchi 2006
Synthesis and degration of proline
L-Glu - L-glutamate, GSA - glutamate semialdehyde, P5C - -pyrroline-5-carboxylate, P5CS - P5C
syntetase, P5CR - P5C reductase, ProDH - proline dehydrogenase, L-Pro - proline
Water stress induced accumulation of proline and glycine betaine as affected by
calcium
Fig. 1. Effect of water stress, 1 mM calcium chloride (Ca) and 0.5 mM calcium channel blocker verapamil (VP) on proline and
glycine beatine content in shoots and roots at 1st and 7th day of stress in C306 (C) and HD2329 (H) wheat genotypes. 15-d-old
plants were subjected to PEG-6000 of -1.0 MPa for 7 d and the observations were recorded during stress period. Means  SE of
three different samples are represented by vertical bars. (Nayyar 2003)
Synthesis and accumulation of other osmoprotectants
• Glycine betaine synthesis in chloroplasts by oxidation of choline in
two steps
• Accumulation of mono- and di-saccharides by inhibition of starch
synthesis from new photosynthates, degradation of starch, inhibiton of
respiration
• Sugars serve not only as osmotica but also in signalling pathways, or
gene expression regulation
Stress and reactive oxygen species (ROS)
• ROS: singlet oxygen 1O2, superoxide radical O2.-, hydroxyl radical
(OH.) and H2O2
• ROS can be formed during photosynthesis (under stress is insufficient
regeneration of NADP+ by Calvin cycle, O2 is electron acceptor and in
so called Mehler reaction 1O2 a O2.- are formed), during respiration,
photorespiration or degradation of lipids
• Oxidative stress – lipid peroxidation, membrane damage, changes in
nucleic acids, oxidative stress in chloroplasts can accelerate ageing
• Under stress increased activity of different defence mechanisms is of
vital importance
• Carotenoids, xanthophylls
• Non-enzymatic antioxidants (ascorbic acid, glutathione, tocopherols)
• Antioxidative enzymes (superoxide dismutase, ascorbate peroxidase,
peroxidase, catalase, glutathione reductase)
Water stress induced production of ABA, ROS, and antioxidants (Lei et al. 2006)
Antioxidative enzymes
Fig. 3. Activity staining for SOD (A), POX (B) and
PPO (C) in roots of two sesame cultivars at
different levels of drought: Darab 14 subjected
to 100, 75, 50 and 25 % FC (1 to 4), Yekta
subjected to 100, 75, 50 and 25 % FC (5 to 8).