Reducing a root’s oxygen supply makes it more difficult to examine nutrient stresses

Often the perennial C4 grass Urochloa humidicola is planted on infertile acidic and waterlogging-prone soils of tropical America. But what effect does that waterlogging have on its growth? Juan de la Cruz Jiménez and colleagues carried out some experiments to find out.

Urochloa genotypes
Urochloa genotypes. Photo: Juan De La Cruz Jimenez Serna.

You may think plants breathe through their leaves, and need moist soil, so why worry if the ground gets waterlogged? Juan Jiménez explained that it’s not that simple: “All of the cells in the plant body (not only leaves) need oxygen to respire (like people breath) to produce energy for maintenance and growth. In flooded soils, oxygen is too low to support root respiration, so the internal movement of oxygen from shoot (above water tissues) to roots is essential for plant survival. This is like a snorkel when people go swimming.”

To test the importance of aeration, Jiménez and colleagues set up comparison between aerated and stagnant growth conditions. The team grew plants in a mix of nutrient conditions, with some low in nutrients and some high. They then aerated some of the plants and not others. What they found was that under low nutrient conditions, plant growth in stagnant solution was equal to that in aerated solution. However, when there were a lot of nutrients available for the plants, it was the aerated plants that performed best.

The results weren’t quite what the team expected, but Juan Jiménez found out why. “We expected that the combination of both stresses (low oxygen and low nutrition) would have a higher deleterious impact on plant growth than only one stress. However, it turned out that under low nutrition, plant growth is slow and the proportion of air spaces in the roots increased, so those traits help the plants to tolerate better such conditions of combined stress as compared with the aerated ‘control’ plants.”

Growing plants hydroponically allows a lot of control for experimentation, but it’s not without its problems. Juan Jiménez said: “Initially we had long discussions on which nutrient solutions we would use in this study and how these would simulate the conditions of soils dedicated to tropical grasses production. We evaluated a large range of concentrations in the growing solution with the aim to reflect the variable nutrient soil conditions where these plants grow. Imposing precise treatments and maintaining the plants in this soilless culture in nutrient solutions takes a lot of time. The measurements of the root radial oxygen loss made use of specialised equipment designed by a famous wetland plant researcher from Hull University in the UK, Professor William Armstrong. Our lab is one of the lucky few in the world to have this equipment and expertise for its use.”

Now that the team know low-oxygen conditions can mask problems plants have with waterlogging, they can refine their experiments. The plan now is to examine the plants further to see what are the best strains for coping with waterlogging. Juan Jiménez said: “Our next research plans to use the stagnant agar method and appropriate nutrient levels for screening of root phenotypes of different species and accessions to identify genetic variation in tolerance for future breeding of more robust forages for flood-prone areas.”

The results matter to a wide range of people. In academia the paper obviously matters to people studying flooding stress in roots and the ecological consequences of floods resulting from climate change. Yet there are uses for the research outside the lab too. Jiménez notes finding a better performing grass will help tropical forage breeders and livestock farmers in tropical regions.

Further reading

Jiménez, J. de la C., Kotula, L., Veneklaas, E. J., & Colmer, T. D. (2019). Root-zone hypoxia reduces growth of the tropical forage grass Urochloa humidicola in high-nutrient but not low-nutrient conditions. Annals of Botany. https://doi.org/10.1093/aob/mcz071