Scientists Discover Non-Stomatal Control of Water Loss in Critical Crops

Scientists have discovered that some plants can survive stressful, dry conditions by controlling water loss through their leaves without using their usual mechanism: tiny pores called stomata.

Non-stomatal control of transpiration in maize, sorghum, and proso millet, all C4 crops critical to global food security, gives these plants an advantage in maintaining a beneficial microclimate for photosynthesis in their leaves.

This allows plants to absorb carbon dioxide as part of the photosynthesis and growth process, despite higher temperatures and increased atmospheric demand for water, without increasing water expenditure.

They published their findings on September 16 in PNASResearchers from the University of Birmingham, the Australian National University in Canberra and James Cook University in Cairns are challenging the traditional understanding of plant transpiration and photosynthesis under stressful, dry growing conditions, which is that stomata alone control water loss from leaves.

Co-author Dr Diego Márquez, from the University of Birmingham, commented: “This has revolutionised our understanding of plant-water relationships by showing that non-stomatal control of transpiration limits water loss without compromising carbon gain, challenging what is generally accepted as an inevitable trade-off.

“Our results have important implications for plant adaptation to climate change and for how crops can be grown in arid environments. Understanding this mechanism could open new avenues for improving water use efficiency in C4 crops, which are vital for global food security.”

The study confirms that C4 plants maintain reduced relative humidities in the substomatal cavity, up to 80% under vapor pressure deficit (VPD) stress, thereby reducing water losses and highlighting the critical role of non-stomatal control in water use efficiency.

This mechanism helps plants maintain photosynthesis by reducing water loss without significantly decreasing intercellular CO.₂ photosynthesis levels. This is essential to maintain growth and ensure crop prosperity.

The results also suggest that non-stomatal control mechanisms may have evolved before the divergence of the C3 and C4 photosynthetic pathways, indicating a common evolutionary trait.

“Our research redefines the understanding of water use efficiency in C4 plants and reveals that this alternative mechanism helps plants continue to grow and capture carbon dioxide even when atmospheric water demand is high, challenging traditional assumptions about how these plants survive droughts,” added Dr. Márquez.

Photosynthesis is how plants use light and carbon dioxide to make sugars needed for growth, using an enzyme called Rubisco. Plants use the carbon dioxide that enters through the open stomata to make sugar, while the open stomata also let water vapor out.

While C3 plants rely solely on CO₂ By diffusing through their stomata for carbon gain, C4 plants possess specialized leaf structures and enzymes that concentrate carbon dioxide around Rubisco, thereby improving their photosynthetic performance and water use efficiency.

However, this advantage comes with a trade-off, as these plants are vulnerable to a substantial reduction in photosynthesis when the stomata close. Therefore, the non-stomatal mechanism is essential to ensure their success in controlling water loss while allowing the stomata to remain open.