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Tuesday, 23 February 2016

Keeping your head above water: plants coping with waterlogging

By Helen Roberts

Flooding on the Somerset Levels.
Photo credit: Nigel Mykura [CC BY-SA 2.0],
via Wikimedia Commons
Britain has had its fair share of flooding over the last couple of years. In 2014, the Somerset Levels was under water for weeks and 2015 saw some truly devastating flooding occurring in the northwest of England. Flooding can have detrimental effects on our own lives, but also on plant communities.

Waterlogging of plants can cause chlorosis (loss of the normal green colour) of the leaves, root rot and eventually death. It’s a common problem that many gardeners face every day and there are different techniques to cope with this ever persistent problem on our shores. Precautions are even taken at the University of Bristol Botanic Garden during this wet weather.

"As far as the garden borders go, we're very careful about never walking on them when there's been heavy rain," explained Andy Winfield, horticultural technician at the Botanic Garden. "If we have to get on a border for any reason, we use a board and then fork over where it was to prevent compaction and a pan forming. When a pan forms, then water is more likely to sit on the surface and create problems."

How does waterlogging affect soils and plants?

The profile of a soil will greatly affect its interaction with water. Soils are composed of solid material with spaces filled with water, gases, roots and other living organisms - these attributes impact water retention and drainage. For example, clay soils have small pore spaces and so retain more water compared with sandy loams.

Subsoils can also influence soil structure and its interaction with water. Waterlogged soils are not only affected by the amount of water coming into the system, but by the soil’s ability to disperse and absorb that water.

When soils are waterlogged, the air spaces between the particles are filled with water and the movement of gases within the soils is inhibited preventing the roots from respiring properly. Gases such as ethylene and carbon dioxide begin to accumulate, which leads to further negative impacts on root growth. Anaerobic processes begin to changes the soil biochemistry, which leads to plant death through the build up of toxins within soils.

What is happening to plants at a cellular level when faced with anoxic or hypoxic conditions? 

When plants are waterlogged, they are not getting enough oxygen via the roots for cellular respiration and energy production. Because the plants cannot obtain oxygen via the roots, plants turn on their own energy reserves. This is much like when we use our muscles during strenuous exercise and we can't get sufficient oxygen to the hard working cells - the cells undergo anaerobic respiration, which produces lactic acid. Plants can also undergo anaerobic respiration, but it is not sustainable and eventually, the plant dies as the demand for energy exceeds the supply.

Until recently little was known about how some plants cope with the stress of waterlogging. However, researchers from the Max Planck Institute of Molecular Plant Physiology, with colleagues from Italy and the Netherlands, have discovered a protein that triggers the activation of stress response genes when oxygen levels drop due to waterlogging. This protein is attached to the cell membrane under normal aerobic conditions, but when levels drop it detaches from the membrane and relocates to the nucleus where it switches on the stress genes. When oxygen levels return to normal, the protein degrades and the stress response genes switch off again.

How some plants have evolved to cope with anoxic and hypoxic conditions

When out walking as a child on Exmoor, I would often pick the stems of the soft rush, Juncus effusus, and peel back the green outer coating to reveal the soft, husky white pith inside. I was amazed when an adult told me this material was once used for making rush lights. The pith would be extracted from the rush leaves and combined with fat or grease to provide a source of artificial light. This pithy material is interesting though in this context as it contains a tissue called aerenchyma, which is usually found in the roots and stems of many hydrophytes (plants adapted for living in water). The tissue has large interconnected intercellular gas spaces that help to oxygenate the roots and increase buoyancy.

Other plants adapted to soggy conditions will produce fine surface roots called adventitious roots. These roots scavenge oxygen from the surface where there is a thin aerobic layer. Many of the Melaleuca species, mostly from Australia, use this way of coping with water hypoxia.

Some plants are adapted to rise above it all; they elongate their shoots to get above the water, as is the case with some floodplain Rumex species (docks and sorrels). Nymphaea species (the water lilies) - which you can see in the Botanic Garden glasshouses -  have a hugely elongated petiole, often more that two metres long, to keep their leaves and flowers at the water surface.

Arial roots (pneumatophores) of the grey mangrove
(Avicennia marina var resinifera) from South Australia.
Photo Credit: Peripitus (Own work) [GFDL, CC-BY-SA-3.0 )
or CC BY-SA 2.5-2.0-1.0 ], via Wikimedia Commons
Large tree species have also adapted their roots to cope with swamp-like conditions. These strange looking roots are known as pneumatophores - woody extensions that grow vertically upwards from the underground root system to reach above water and capture that much needed oxygen. The bald cypress, Taxodium distichum, which is found in the southern USA in lowland river floodplains and swamps, forms these roots that look like knees sticking up out of the water. The actual surface of the root is pockmarked with many lenticels, which are small stomata-like pores found in the bark that allow gaseous exchange. Other swamp and mangrove species have variations of these root adaptations to cope with low oxygen levels including pencil and cone roots (which belong to the pneumatophore group) and other types of aerial roots like knee, stilt, peg and plank roots. These roots differ in both their morphology and function, but are ultimately adapted to cope with waterlogging and often saline conditions.

The importance of wetlands as carbon sinks

Waterlogged lands are not all doom and gloom, in fact, bogginess is vitally important in terms of the Earth's climate. Peatlands fall into that category. They act as important carbon sinks and currently cover about four per cent of the Earth's land surface. Drainage of these areas of peatlands and wetlands for agricultural use leads to increases in greenhouse gas emissions. Researchers are actively trying to understand the effects of climate change on peatlands globally and there have been pushes to effectively  conserve and manage these precious ecosystems.


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