Open or Shut: How Trees Respond to Drought at the Leaf Level
Trees pull water into their roots, where some of it moves up the trunk against the pull of gravity. This upward movement, which is described by the cohesion-tension theory, is possible because of the chemical nature of water. Water molecules are attracted to each other (cohesion), so just before a water molecule evaporates from the leaf’s surface, it pulls (tension) another to the surface, and so on.
Although the surface of a leaf may look smooth, it is lined with tiny openings called stomata. When stomata are open, water vapor and other gases, such as oxygen, are released into the atmosphere through them.
A number of factors can affect the exchange of gases between a leaf and the atmosphere. U.S. Forest Service scientist Chelcy Miniat and her colleagues – including lead author Kimberly Novick, a researcher at Indiana University – recently modeled several of the factors that limit gas exchange. The scientists used tree sapflow data from a U.S. Forest Service monitoring network called Remote Assessment of Forest Ecosystem Stress (RAFES). Their study was published in the journal Plant, Cell and Environment.
Plants close stomata in response to their environment; for example, most plants close their stomata at night. Under drought, plants may also close their stomata to limit the amount of water that evaporates from their leaves. However, this strategy introduces new dilemmas. Because plants must exchange gases through their stomata, closing them prevents plants from taking up carbon dioxide (CO2). Without CO2, plants cannot make carbohydrates, and plants can only obtain this critical molecule when stomata are open. For plants, balancing the need to conserve water, especially in times of drought, with the need to take up carbon dioxide to support growth is a perpetual conundrum.
In general, plants have two options for reconciling the need to take up CO2 with the thirst for water. Some plants limit water loss by closing their stomata when conditions are unfavorable. For example, when the humidity is low, water is more likely to evaporate quickly from the leaf surface, and plants often close or partially close their stomata to maintain a stable water balance in the leaf.
These species are called isohydric, and tend to do poorly during droughts, because without the gas exchange that open stomata allow, they cannot produce carbohydrates for survival. Instead, they rely on stored carbohydrates, and during long droughts these can be depleted and lead to tree mortality. Anisohydric plants keep their stomata open, even when facing water loss, but face significant risks should they run low on water; air bubbles can be pulled into their tissues, a potentially fatal situation.
Plants face several limitations when exchanging gases. In addition to the problem of losing water while obtaining carbon dioxide, water shortages could affect chemical processes – especially enzyme function within leaves. “We developed a new modeling framework linking leaf water loss and carbon gain,” says Miniat. “Our model shows where trees fall on the isohydric to anisohydric spectrum.”
Miniat and her colleagues compared several tree species from southwest Georgia to Arkansas, including white oak and loblolly pine. White oak is anisohydric, which means that it tends to keep its stomata open, even when conditions are harsh. Miniat and her colleagues showed that limitations in how effectively trees move water from their roots to their leaves helped them prevent the formation of potentially fatal air pockets in their tissues.
“Our study makes connections between the multiple obstacles plants face when exchanging gases with the atmosphere,” says Miniat. “We hope this model can inform ongoing efforts to explore how trees function during drought.”
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For more information, email Chelcy Miniat at cfminiat@fs.fed.us.