When drought strikes, the limit on a plant's water supply lies not in the plant but in the soil, according to a study in Science by Andrea Carminati, professor of soil physics at ETH Zurich, and Tim Brodribb, professor of plant physiology at the University of Tasmania. Plants must pull water against the capillary forces that hold it in soil pores, and those forces strengthen dramatically as the pores dry out.
Combining soil physics with plant physiology in model calculations of water potential, the team found that once soil water potential falls below about minus 1.5 megapascals, plants can no longer extract water fast enough to meet their needs, regardless of their own suction capacity. Even sequoias, which lift water more than 130 metres into their crowns, are bound by the same physics at the interface between root and soil.
The finding helps explain why decades of breeding programmes for drought-resistant crops have disappointed: many sought to enhance plants' internal water-extracting traits, including solute storage in cells, when the bottleneck was in the ground all along. The authors argue that drought strategies should start from soil conditions, for instance by timing irrigation to keep soils above the critical threshold, rather than from the plant alone.