Growth before photosynthesis: how trees regulate their water balance

(Press-News.org) Plants have small pores on the underside of their leaves, known as stomata. When the sun rises, these pores open and the plants absorb carbon dioxide (CO2) from the atmosphere, which they need, in addition to sunlight and water, for photosynthesis. At the same time, water evaporates through the open stomata; for a tree, this may be several hundred liters per day.

When water is scarce, plants can close their stomata and thus prevent it from evaporating too much water. The fact that plants have this protective mechanism at their disposal is nothing new. Until now, however, it has not been clear when this closure occurs and what the trigger was. Researchers at the Department of Environmental Sciences at the University of Basel have provided new findings in the scientific journal Nature Plants. Most of the measurement data comes from the University of Basel’s forest laboratory in Hölstein, in the canton of Basel-Landschaft, where a crane makes it possible to study processes in the crowns of mature trees.

A balancing act within the canopy

The evaporation of water through the stomata is a passive process during CO2 absorption. Water loss is therefore the price a plant pays for photosynthesis. By closing the stomata, it can stop evaporation, but then it cannot photosynthesize.

“When it comes to plants, researchers have traditionally focused on photosynthesis. So it was previously assumed that trees treated this process as a priority and therefore kept the stomata open for as long as possible to absorb CO2, only closing them when there was no other option,” explains study leader Professor Ansgar Kahmen.

When water evaporates through the stomata, negative pressure is created within the cells and the xylem (i.e. the woody tissue that transports water up from the roots). This suction pulls water up from the roots, via the xylem, into the growth layer of the trunk and into the tree crown. There it replaces the water that has been released into the atmosphere.

Preventing the system from collapsing

It usually takes trees all night to replace the water lost during the day. During this time, the stomata are closed and the plant cells fill up with water. This creates the turgor pressure on the cell walls that is necessary for the elongation growth of the cells. Trees therefore grow at night.

If the soil is dry, there is no water to fully replenish their water reserves. As a result, the water saturation in the cells is too low and the turgor pressure remains low. This inhibits tree growth even in intermediately dry conditions. With increasing levels of drought, the suction in the cells and vascular pathways becomes stronger and stronger until at some point the water columns in the woody tissue break. This results in air bubbles, known as embolisms. “When this happens, irreparable damage occurs, the water transport system collapses and the plant eventually dies,” says Ansgar Kahmen.

Water supply in the tree is key

It used to be assumed that, in order to maintain photosynthesis for as long as possible, trees would close their stomata only shortly before the onset of these embolisms. The new study now shows that the stomata remain closed at an earlier point in time, namely when water absorption at night has become difficult. “For the first time, we were able to show that a tree does not even open its stomata in the morning if it cannot absorb enough water overnight,” says Kahmen. This means that the tree forgoes photosynthesis in favor of growth.

According to Kahmen, this prioritization makes sense: If the plant stops growing due to a shortage of water, then, no matter how much photosynthesis it carries out, it will not be able to use the resulting products. “So the aim is not to optimize photosynthesis and maintain it for as long as possible, but to use the products of photosynthesis as efficiently as possible for growth,” says the plant physiologist.

Carbon cycle and climate models

The findings could also influence calculations relating to carbon sequestration by forests. When the stomata are open for shorter periods during drought than previously expected, the trees absorb less carbon dioxide from the atmosphere. “Climate models that assume a certain growth in carbon storage volume would therefore have to be adapted,” says lead author Richard L. Peters, a former postdoc at the University of Basel and now professor at the Technical University of Munich. Particularly in the context of climate change, which is leading to warmer and, above all, drier summers in countries including Switzerland, carbon uptake could change more dramatically than previously assumed.

“What is remarkable is that our early stomatal closure observations apply to all tree species, whether deciduous or coniferous. How well a tree species copes with drought therefore cannot be solely determined by the process of stomata closure” says Peters.

Plants have small pores on the underside of their leaves, known as stomata. When the sun rises, these pores open and the plants absorb carbon dioxide (CO2) from the atmosphere, which they need, in addition to sunlight and water, for photosynthesis. At the same time, water evaporates through the open stomata; for a tree, this may be several hundred liters per day.

When water is scarce, plants can close their stomata and thus prevent it from evaporating too much water. The fact that plants have this protective mechanism at their disposal is nothing new. Until now, however, it has not been clear when this closure occurs and what the trigger was. Researchers at the Department of Environmental Sciences at the University of Basel have provided new findings in the scientific journal Nature Plants. Most of the measurement data comes from the University of Basel’s forest laboratory in Hölstein, in the canton of Basel-Landschaft, where a crane makes it possible to study processes in the crowns of mature trees.

A balancing act within the canopy

The evaporation of water through the stomata is a passive process during CO2 absorption. Water loss is therefore the price a plant pays for photosynthesis. By closing the stomata, it can stop evaporation, but then it cannot photosynthesize.

“When it comes to plants, researchers have traditionally focused on photosynthesis. So it was previously assumed that trees treated this process as a priority and therefore kept the stomata open for as long as possible to absorb CO2, only closing them when there was no other option,” explains study leader Professor Ansgar Kahmen.

When water evaporates through the stomata, negative pressure is created within the cells and the xylem (i.e. the woody tissue that transports water up from the roots). This suction pulls water up from the roots, via the xylem, into the growth layer of the trunk and into the tree crown. There it replaces the water that has been released into the atmosphere.

Preventing the system from collapsing

It usually takes trees all night to replace the water lost during the day. During this time, the stomata are closed and the plant cells fill up with water. This creates the turgor pressure on the cell walls that is necessary for the elongation growth of the cells. Trees therefore grow at night.

If the soil is dry, there is no water to fully replenish their water reserves. As a result, the water saturation in the cells is too low and the turgor pressure remains low. This inhibits tree growth even in intermediately dry conditions. With increasing levels of drought, the suction in the cells and vascular pathways becomes stronger and stronger until at some point the water columns in the woody tissue break. This results in air bubbles, known as embolisms. “When this happens, irreparable damage occurs, the water transport system collapses and the plant eventually dies,” says Ansgar Kahmen.

Water supply in the tree is key

It used to be assumed that, in order to maintain photosynthesis for as long as possible, trees would close their stomata only shortly before the onset of these embolisms. The new study now shows that the stomata remain closed at an earlier point in time, namely when water absorption at night has become difficult. “For the first time, we were able to show that a tree does not even open its stomata in the morning if it cannot absorb enough water overnight,” says Kahmen. This means that the tree forgoes photosynthesis in favor of growth.

According to Kahmen, this prioritization makes sense: If the plant stops growing due to a shortage of water, then, no matter how much photosynthesis it carries out, it will not be able to use the resulting products. “So the aim is not to optimize photosynthesis and maintain it for as long as possible, but to use the products of photosynthesis as efficiently as possible for growth,” says the plant physiologist.

Carbon cycle and climate models

The findings could also influence calculations relating to carbon sequestration by forests. When the stomata are open for shorter periods during drought than previously expected, the trees absorb less carbon dioxide from the atmosphere. “Climate models that assume a certain growth in carbon storage volume would therefore have to be adapted,” says lead author Richard L. Peters, a former postdoc at the University of Basel and now professor at the Technical University of Munich. Particularly in the context of climate change, which is leading to warmer and, above all, drier summers in countries including Switzerland, carbon uptake could change more dramatically than previously assumed.

“What is remarkable is that our early stomatal closure observations apply to all tree species, whether deciduous or coniferous. How well a tree species copes with drought therefore cannot be solely determined by the process of stomata closure” says Peters.

END