Stomata and starch: the dark side of plant water loss

Leaves have tiny pores called stomata, which are surrounded by a pair of guard cells. In the vast majority of plants, stomata open during the day, facilitating the capture of atmospheric CO₂ —indispensable for photosynthesis — but leaving the door open for water vapour to escape through transpiration. At night, stomata close, enabling the plant to save water when photosynthesis is no longer empowered with sunlight. However, depending on the species, or even the variety, residual nighttime transpiration may result in inefficient water loss.

More than a century ago, observations under the microscope revealed that stomata progressively reopen throughout the night (see references). This reopening of stomata depends on the circadian clock — the plant’s internal timer that is entrained by the day/night cycles. In addition, starch in the leaves acts as a transitory metabolic clock: starch is synthesised during the day through photosynthesis, and used at night to generate sugars, making it the main source of energy at night. Could starch metabolism play a role in the nighttime reopening of stomata?

To answer this question, the authors used the model plant Arabidopsis thaliana to study the dynamics of water loss. They worked with Phenopsis high-throughput phenotyping platform, and developed a new series of computational tools, called PhenoLeaks, to analyse the dynamics of plant transpiration By screening a series of plants whose starch metabolism was differentially affected, the team discovered that severe mutations in starch metabolism prevented stomata from reopening at night and, unexpectedly, altered the rhythm of stomatal movements throughout the day. This means that the plant uses starch not only as a source of energy but also as a means of adjusting its circadian clock.

The authors also discovered that stomata did not react to the starch available in their own guard cells, but rather to the starch in the leaf as a whole. In other words, the bulk starch content keeps stomata on time, most likely through the sugars generated in the internal tissues of the leaves, which travel to the guard cells to "set the local time". A great way for the plant to keep all its