Iron Toxicities Caused by Water Logging

Australia is the driest inhabited continent, with rainfall distribution varying yearly.
Rainfall distribution examples:
- 2021: Higher than average rainfall projected during the cropping season (winter) in the wheat belt of Western Australia (60-65% chance). However, the weather is hard to predict.
- More recently: Drier winter projected, with a less than 50% chance of average rainfall.
Water logging occurs in depressions in the landscape, saturating the soil and potentially leading to surface water (flooding).
Water logging may last a short time (hours or a day), but the stress is severe, significantly depressing grain yield (50-80% or complete loss).
The lecture focuses on iron toxicities due to water logging, contrasting with typical discussions of aerenchyma formation in roots.

Water logging drastically changes soil properties, increasing the concentration of specific ions, causing toxicity to plants.
Comparison of iron and manganese concentrations in soil:
- Normal soil: typical range of iron and manganese concentrations.
- Waterlogged soil: significant, multi-fold increase in iron and manganese concentrations.
Plants must cope with toxic concentrations of these nutrients under waterlogged conditions.
High concentrations of manganese and iron in plant shoots can reach toxic levels. Local site-specific example:
- Manganese: High concentrations in wheat shoots, even without water logging, due to parent material composition naturally high in manganese. If there is water logging on top of it then that's even worse.
- Iron: Low concentrations in shoots under non-waterlogged conditions, but very high concentrations under waterlogged conditions across all genotypes tested.

Iron:
- Influenced by redox state (reduction and oxidation).
- $Fe^{2+}$ (reduced form) is toxic to plants; high concentrations under reducing conditions.
- $Fe^{3+}$ (oxidized form) is less problematic as plants have poor uptake capacity.
Manganese:
- Redox element - accepts/releases electrons based on soil conditions.
- $Mn^{4+}$ is poorly water-soluble and precipitates.
- Under reducing conditions, converts to $Mn^{2+}$ (form plants can uptake).
- Excessive concentrations lead to toxicity.
- The forms of manganese and iron are interchangeable depending on electron availability and redox state of soils.

In well-aerated soils, the concentration of various ionic forms of iron are influenced by pH.
Negative logarithm relationships: Axes represent negative logarithms of hydrogen and iron concentrations.
$Fe^{2+}$ : concentrations are very low, regardless of pH.
$Fe^{3+}$ : concentrations are higher under acidic conditions.
Iron hydroxides: non-charged forms are insoluble and do not change concentration with pH.
Charged iron hydroxides: higher concentrations under low pH levels.
Total iron concentration: high under acidic conditions, declines as pH goes down, then increases again.
Plant iron requirements: plant species require between $10^{-4}$ and $10^{-6}$ M concentration, which would be between 10 micromolar and 100 micromolar.
Availability: soil chemistry may not provide sufficient iron for plant uptake; plants evolved strategies (strategy 1 and strategy 2) to enhance iron uptake.
Under reduced conditions, the relationship between pH and iron changes significantly.

Under oxidized conditions, manganese concentration increases with decreasing pH.
Under reduced conditions, manganese concentration is much higher.
Example: At pH 6, manganese concentration increases from close to $10^{-12}$ molar under oxidized conditions to about $10^{-3}$ M (one millimolar) under reduced conditions, causing toxicity.
This demonstrates the soil chemistry behind manganese and iron toxicity in waterlogged (reduced) conditions.

Engineering solutions, such as water removal, are expensive and may negatively impact plant growth during drier periods.
Changing plants: Screening genotypes for tolerance to manganese and iron toxicity caused by waterlogging, using nutrient solutions for preliminary screening, then testing under realistic soil conditions.
Iron toxicity: root growth is more affected than shoot growth.
Differential genotypic tolerance: some genotypes (e.g., BH1146) have poor tolerance; others (e.g., Ceteros) have higher tolerance.

High concentrations of manganese used to create toxicity conditions.
Differential genotypic tolerance: foreign germplasm with good and poor tolerance; Australian genotypes with high tolerance.
ET8 (Egret) is highly tolerant to manganese toxicity.
Modern wheat genotypes: Krikov more tolerant than Trident, though Trident is more widely grown due to other agronomic properties.
Realistic conditions: different soils with different soil properties show that there's different soil plant interactions.

Aluminum, manganese, and iron: sensitive genotype always performed less well under waterlogged situations.
Severe waterlogging stress: significant growth reduction, root growth particularly inhibited.
Aluminum-tolerant varieties showing better root results.
Aluminum, manganese, and iron: tolerant one always produced a little bit better and bigger root growth than the intolerant one.
Intolerant genotypes: greenness indicates that the waterlogging stress slowed down the growth of these intolerant genotypes, meaning if there is terminal drought that is bad news.

Iron: intolerance is associated with iron accumulation; tolerance involves iron exclusion or avoidance.
Manganese: tolerance is associated with manganese accumulation, internal tolerance to bound extra manganese to organic acid anions.

Recurrent parent: good agronomic properties but lacking a specific trait.
Donor: has the desired trait (e.g., aluminum tolerance) but may have undesirable traits.
Process: cross recurrent parent with donor, select progeny with desired traits, backcross into recurrent parent to eliminate undesirable genes from the donor. Want to eliminate the undesired traits and keep the desired trait.
Examples:
- ES8 and ES18 are eighth-generation backcrosses into EGRIT to incorporate aluminum tolerance.
- Striking difference between donor and recurrent parent in aluminum tolerance.
- Segregation for aluminum tolerance in the second-generation backcross.
- After five backcrosses, there is a range of root growth, with some progeny potentially better than the donor parent.

Water logging is a severe stress, even for short periods, affecting grain yield.
Major problems are iron and manganese toxicities due to anaerobic (reducing) conditions.
Engineering solutions are expensive and may not be appropriate.
Improving genotypic tolerance to iron and manganese toxicity is a viable option, especially given concerns about aluminum toxicity in acidic soils.

Traits for tolerating iron and manganese toxicity not commonly incorporated into current genotypes.
Cost of investment was not justified by frequency and extent of stress and aluminum testing is always completed.
All wheat, barley, and canola breeding in Western Australia includes aluminum tolerance testing.
Eastern Australia does not have to do this as often.
Rice grows in flooded conditions, but not suitable for Western Australia.
There are species that grow with their feet in water.