Plants respond to natural climatic fluctuations, this much is common knowledge. However, the latest in a series of revelations about plants' daily rhythms suggests plants remember water stress and can regulate their uptake of water accordingly.

Last year, scientists at Norwich's John Innes Centre revealed plants 'do sums' to enable them to use up food (starch reserves) at a constant rate over the course of a night.

In November a team from France's state agricultural research institute INRA and the Catholic University of Louvain, improved our understanding of plants' circadian rhythms (the cycles that govern daily activity, like the sleep-wake cycle in animals).

Water for plants is often more readily available at night, and less so in the afternoon.  The cycle of water use explains why plants seem to perk up at night, apparently to do well in the morning, and can sometimes wilt in the afternoon, even when the ground isn't totally dry. According to the French scientists, organ growth follows the same rhythm: maximum growth occurs at night, minimal growth during the day.

Plants use molecular switches, or aquaporins, to govern how well their roots take up water. When plants need water most – in the morning – the roots are made more permeable, allowing more water to enter. The researchers made their discovery by growing plants under continuous light and closely observing their leaf growth.

The researchers hypothesised that, if plants are exposed to water stress (eg sunny days and dry soil), their aquaporin activity will vary wildly over the course of the day, but after cloudy days in moist soil, the changes will be more subtle. They discovered that "Water movement and leaf growth follow these same oscillations, which depend on the recent history of the plant."

Water regulation limits growth

The research showed how plants can acclimatise to drier conditions, continuing to access water but preventing over-drying of the soil that surrounds their roots, which would make accessing water harder in the future. Altering the permeability of their roots limits plants' growth, so changing roots' ability to take up water must be done as infrequently as possible.

The French team suggested that by registering water conditions experienced in previous days, plants can anticipate the number of switches needed to optimise their growth. The researchers said, "This study is the first physiological explanation of a mechanism linked to the evolutionary advantage associated with circadian rhythms."