Critical crops' alternative way to succeed in heat and drought

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

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

Publishing their findings in PNAS, researchers from the University of Birmingham, Australian National University, Canberra, and James Cook University, Cairns, challenge traditional understanding of plant transpiration and photosynthesis under stressful and dry growing conditions -- namely that stomata alone control leaf water loss.

Co-author Dr Diego Márquez, from the University of Birmingham, commented: "This revolutionised our understanding of plant-water relations by showing that nonstomatal control of transpiration limits water loss without compromising carbon gain -- challenging what is typically accepted as an unavoidable trade-off.

"Our findings have significant implications for plant adaptation to climate change and how crops might 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, down to 80% under vapour pressure deficit (VPD) stress, reducing water loss and highlighting a critical role of nonstomatal control in water-use efficiency.

This mechanism helps plants sustain photosynthesis by reducing water loss without significantly lowering intercellular CO2 levels for photosynthesis. This is crucial for maintaining growth and ensuring that the crops thrive.

The findings also suggest that nonstomatal control mechanisms may have evolved before the divergence of C3 and C4 photosynthetic pathways, indicating a shared evolutionary trait.

"Our research reframes 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 for growth, using an enzyme called Rubisco. Plants use the carbon dioxide that enters through open stomata to produce sugar, whilst open stomata also let water vapour out.

While C3 plants rely only on CO2 diffusion through their stomata for carbon gain, C4 plants possess specialised leaf structures and enzymes that concentrate carbon dioxide around Rubisco, enhancing their photosynthetic performance and water-use efficiency. However, this benefit comes with a trade-off, as these plants are vulnerable to substantial photosynthesis reduction when the stomata close. Therefore, the nonstomatal mechanism is crucial in ensuring their success in controlling water loss while allowing stomata to remain open.