Thirsty Plants: How African grasses shield themselves from drought

Africa is famous for its wide-open spaces that are home to charismatic wildlife, such as giraffes, elephants, and rhinos. For ecologists, these wide-open spaces are an opportunity to study systems that have limited industrial human impact compared to the areas many of us live and work in. Many ecologists are interested in how large animals influence the landscape as well as how organisms survive in harsh environments. For me and 8 other ecologists from Kansas State University and Oklahoma State University, the opportunity to study how grasses survive drought inspired our recent trip to Botswana.

After a 16-hour flight from Atlanta to Johannesburg, South Africa, our team crammed into a 9-passenger van to drive the 4 hours to Botswana where we studied the mechanisms grasses use in order to survive drought. Drought is a force that shapes ecosystems over time, and organisms have evolved a variety of mechanisms to deal with these stressors. In the coming years, precipitation is projected to become more unpredictable, and many areas of land are expected to become drier. Studying drought tolerance in plants is important because it provides information on how plants will tolerate environmental change.

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Rhinoceroses and zebras grazing at Khama Rhino Sanctuary, Botswana.

Our goal was to explore the question: what role do sponge-like sheaths that surround the root (known as rhizosheaths) play in drought tolerance? Rhizosheaths form naturally from roots, fungi, and soil stuck together with a natural adhesive to encapsulate the roots of grasses and enhance their ability to tolerate drought. Rhizosheaths can be thought of as a sponge surrounding the root. When it rains, the rhizosheath quickly absorbs and stores the water. When it is dry, the rhizosheath becomes hard and seals off the root, creating a shield that protects the root from the dry environment and reduces water loss. Rhizosheaths are found in 75% of African grasses and in other arid environments around the world. Yet, there is little research addressing how rhizosheaths are made, how they work, and how climate influences their formation.

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A closer look at a rhizosheath. The root is surrounded by the rhizosheath. The small portion of bare root where the rhizosheath has fallen off can be used as a size comparison to visualize the thickness of the rhizosheath

The general function of rhizosheaths as well as their microbial and fungal compositions are not well understood. In order to address this, we sampled rhizosheaths and the surrounding soil from two areas in Botswana: the wetter eastern side of the country and the drier south western side of the country. Collecting samples from both a wet and dry area allowed us to assess how the composition and function of rhizosheaths vary when exposed to different amounts of precipitation. We found that rhizosheaths hold more water than the surrounding soil. This indicates that rhizosheaths aid the grass in the uptake of water after rainfall and the retention of water during dry periods. Further research is being done by members of the group on how the microbial composition of rhizosheaths differs from the surrounding soil. Assessing the microbial and structural composition could be a key factor in understanding how rhizosheaths are formed.

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Tins full of rhizosheaths. These tins were weighed before and after drying them in order to quantify water content of the rhizosheaths.

Understanding rhizosheaths and other mechanisms plants use to survive drought is becoming increasingly important as rainfall and climate becomes more variable. The ability of grasses to cope with drought has implications for the human population, as we are all reliant on grasslands in some way whether it be food, fiber, livestock, or livelihood.

The opportunity to travel and experience a new ecosystem is one of the many benefits of being an ecologist. Along the way we enjoyed the culture and wildlife of southern Africa. Trips like these build community. There are few better ways to get to know a group of people than to squeeze into a van with each other for two weeks. I have always dreamed of visiting Africa and coupling that dream with intriguing research and a light-hearted group made the experience even more fulfilling. Experiences in the field are a balance of work and play – fun, exciting, and driven to find the answer to tough ecological questions such as how plants tolerate drought and what we can learn from them in order to be prepared for a drier future.

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A group of giraffes on their way to a watering hole.

 

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This post was written by Emily Wedel. Emily is a Masters student studying the expansion of woody species into the tallgrass prairie.

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