Whole plant response to abiotic stress: water stress, salinity and toxic soils Flashcards
Abiotic factors that cause stress
- Water availability
- Temperature
- Light intensity and quality
- Mineral levels
- Wind
- Soil salinity and pH
^None act in isolation, learn more at:
https://heatherkellyblog.wordpress.com/2015/02/08/plants-get-stressed-too/
A lot of the crop plants we grow originate from the middle east and are not suited to the environments that we grow them in so we must manage abiotic factors carefully for good yields
Water flows through a plant down a concentration gradient
More at: https://heatherkellyblog.wordpress.com/2015/02/14/more-thirsty-plants/
Water status of plants: osmosis
*Water enters and leaves cells by osmosis, moving from area of high water potential to low
*Facilitated by aquaporins
*Carries on until equilibrium is reached
*Rigid cell walls mean plant cells cannot take up water indefinitely
*Water enters vacuole, swells and stretches cell wall until pressure potential is balanced by inward pressure from wall
*Pressure potential prevents further water uptake - cell is fully turgid - dynamic equilibrium.
*Ultimately this physical pressure drives cell growth…. Plant cells can only grow if turgid
Cell walls vary in rigidity
^ http://legacy.hopkinsville.kctcs.edu/instructors/Jason-Arnold/VLI/VLI/VLI818/m2cellfunctionandenergetics/m2cellfunctionandenergetics3.html
Water status of plants: role of aquaporins
hydrolic conductivity – how easily water is transported through cells
Achieved through aquaporins, determine how plant responds to water stress
May be gated or chemically controlled e.g. by pH or Ca+ presence
Effect of water stress: deserts and ‘mini deserts’
desert conditions e.g. rainfall of 10 cm per year in Ladak region of the Himalayas creates a desert-like environment
‘mini desert’ conditions e.g. Scottish stonewort shows the same features of desert plants as water is not readily available on the stones it grows on – despite high rainfall in the region.
Effects of water stress
- water deficit leads to cellular dehydration and hydraulic
secondary effects include:
*reduced cell/leaf expansion
*reduced cellular activity
*stomatal closure - cavitation (air bubbles in xylem)
*photosynthetic inhibition
*leaf abscission
etc. (see diagram for more) - salinity leads to water potential reduction, cellular dehydration and ion cytotoxicity
^ which has the same secondary effects as water deficit - flooding and soil compaction leads to hypoxia and anoxia
secondary effects:
*reduced respiration - fermentative metabolism
*Inadequate ATP production
*production of toxins by anaerobic microbes
*ROS production
*stomatal closure
-
Effects of water stress: soil (edaphic) factors
soil factors determine how much water the plant can actually take up (see diagram)
- When soil is fully saturated there are no air spaces at all and soil is waterlogged resulting in hypoxia and anoxia in plants
- Field capacity is just below full saturation. This is ideal, soil is close to saturated, potential close to 0 but there are air spaces present.
- When the soil is too dry the permanent wilting point is reached – water remaining is strongly adhered to soil particles and cannot be accessed by plants
^ This is why breaking up soil and adding organic materials increases water availability to plant
Additionally: Low air humidity can draw moisture from soil without it reaching the plant
^ http://www.nature.com/scitable/content/water-content-and-water-potential-at-saturation-59719594
Permanent changes can occur when a plant is dehydrated
Even if sufficient water becomes available water deficit damage can be too great for the plant to recover e.g. root hair contraction reduces connection, cavitation in xylem etc.
Plant may survive but will not grow.
Effects of flooding
What impact does waterlogging/anaerobic conditions in paddy fields have on rice plants?
-Rotting due to anaerobic microbial activity
- Roots unable to respire
- May disrupt ion balance
Coping with water deficit and drought – survival strategies
Avoidance
*Dormancy – drought-deciduous shrubs or trees/desert ephemerals
*Water tapping – many legumes
*Water storage – cacti and other succulents
*Desiccation – ‘resurrection’ plants
Tolerance
*Xerophytes - grasses such as marram
Coping with water deficit and drought: tolerance
tiny leaves to reduce water loss
e.g. cactus spines
or Ephedra gerardiana scale-like leaves on stem
trichomes
leaf hairs prevent water loss by reducing evaporation and diffusing excess sunlight/ uv radiation damage
e.g. Nepeta floccosa leaves covered in tiny hairs appear silver
carbon allocation patterns
e.g. E. saligna (coastal) and E. dives (dry interior) allocate carbon differently. E. dives is more tolerant to low water - able to allocate more carbon to root formation whereas E. saligna requires higher water levels. However E. dives is unable to utilise higher levels of water (growth is capped at a lower rate)
Coping with water stress and drought - delayed senescence
In crop plants it is useful to delay senesence to allow increased yield e.g. stay green maize variety
Coping with water drought and deficit: stomata
Most plants open their stomata in the morning, close their stomata in the middle of the day, and open them again in the afternoon. In heat/water stress plants may only open stomata in the morning and close them for the rest of the day. In CAM metabolism type plants stomata are open only at night and closed during the day to retain water.
Coping with flooding
Programmed cell death in the stem can create tunnels to allow air to reach the roots even in water-logged conditions
as seen in lotus stems and some maize varieties
Coping with physiological drought: high salinity
different plant types have different salt tolerance levels
Suaeda corniculata grows in the shrinking lakes in the Himalayas in Ladakh, it is covered in salt crystals that it extrudes through its leaves
Salicornia europaea (samphire) has to fight the oceans salt concentration gradient to take up water – the salty soil has a negative water potential
