Plants drink in unknown ways on gypsum hills

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Across the world, you can find gypsum drylands. Here, access to water is critical but also challenging. Burrowing into the depths of the soil can allow plants to reach the water table, but what about shallow-rooted plants. In a study published in Annals of Botany, Laura de la Puente and colleagues find that water held in gypsum crystals is an important water source for these plants.

Gypsum soils pose a difficulty for plants living in them. They are often found in drylands, meaning that there is not a lot of water arriving. The soils also retain little water, so the rain that arrives might not be around for long. However, examining previous research, de la Puente and colleagues found that in some cases, water availability in the summer was higher in gypsum soils than nearby non-gypsum soils. Previous research has shown that Helianthemum squamatum can extract water from gypsum crystals. So, could other shallow-rooted plants in gypsiferous soils also have this ability?

Plants living in gypsum soils either have traits for stress tolerance or are gypsophiles, plants that specialise in gypsum soils. The botanists decided that these gypsum specialists were the most likely plants to use crystallisation water.

The team set out to examine the distribution of water sources among plant species in a gypsum community. The scientists could tell where the water was coming from by measuring the isotopes in the elements that made the water. Some isotopes of hydrogen and oxygen are heavier than usual. In upper soil layers, water with these heavier atoms is less likely to escape through evaporation, so water in the topsoil is slightly heavier than typical water. Evaporation isn’t happening in the deeper layers, so the water here has a high proportion of lighter water molecules.

It’s this difference in the water’s weight that de la Puente and colleagues used to track the water sources.

“We characterized the variation in the isotopic composition of water along the soil profile and evaluated the effect of species rooting depth and affinity for gypsum soils on their water use both in spring and summer. We also analysed how plants interacted with the soil beneath them. Considering plant water-uptake patterns, we hypothesised that… shallow-rooted, gypsum-exclusive species will preferentially use crystallization water from gypsum in summer, whereas shallow-rooted, non-exclusive species will be restricted to the (scarce) free water available in the topsoil. Conversely, deep-rooted species, regardless of gypsum affinity, will rely mainly on the use of deep soil water and/or groundwater during summer drought,” write de la Puente and colleagues.

The team examined the plants on a gypsum hill in Zaragoza, in northeastern Spain. The rain in this part of Spain falls mainly in the spring and autumn. In the summer, it is dry, so the plants have to contend with drought. The soil is almost two-thirds gypsum.

The scientists sampled plants for isotope analysis in the spring and summer to have a comparison.

“On each sampling date, we harvested the main stems (including the root crown) of five individuals of each species. We selected vigorous, medium-sized individuals located at least 5 m away from each other.”

The team then used the water in the xylem to indicate where the plants were getting their water from.

“We identified gypsum crystallization water as a crucial component of the water balance in gypsum drylands,” write de la Puente and colleagues. “Water held in the crystalline structure of gypsum was the most important water source for almost all shallow-rooted species and a highly relevant water source for deep-rooted species during summer drought. Our results demonstrate that gypsum crystallization water is widely used by plants, irrespective of their affinity for gypsum soils. Contrary to our predictions, both gypsum endemic and non-endemic species… with shallow roots used gypsum crystallization water as the preferential water source during summer.”

The botanists admit this is a puzzle because it’s unclear how the plants are accessing the crystallisation water. They note that recent research shows how some microorganisms can dissolve gypsum rock by secreting organic acids. Are plants exuding something from their roots to break into water held in the crystals? Or are they encouraging microbes to do the work for them?

While the mechanism is unknown, it certainly seems that the plants are getting their water from gypsum. de la Puente and colleagues point out that this makes gypsum crystallization water a potentially unaccounted for water source in studies of drylands, which are found on all continents.