Plants optimize their growth by “remembering” water stress and adjusting root water uptake accordingly. These are the ground breaking findings of researchers at INRA and the Catholic University of Louvain, thanks to a system of precise measurements of growth in a large number of plants in natural conditions. Published in the November, issue of Nature Communications, their research is the first physiological explanation of a mechanism linked to circadian rhythms that contributes to a plant’s evolutionary advantage.
Plants respond to natural climatic fluctuations. Their water status ranges from favourable at night to unfavourable in the afternoon, and the drier the ground and air, the more this is the case. That is why plants seem to do well in the morning, tend to wilt in the afternoon, and perk up at night, even when the ground is only partially dry. Organ growth follows the same rhythm: maximum growth occurs at night, minimal growth during the day. Moreover, when plants are observed under continuous light, scientists have noticed that almost all their physiological functions are governed by circadian rhythms, much like in animals (in 24-hour periods). The opening of molecular switches, or aquaporins – and in turn the permeability of roots – reaches a maximum at dawn and a minimum at sunset. These aquaporins therefore foster water transport in the plant in the morning hours when its water needs are at a peak. This provokes daily oscillations in leaf growth under continuous light.
Research carried out at INRA and the Catholic University of Louvain has revealed a new phenomenon: the full range of daily oscillations of leaf growth depends on the water stress the plant experienced in the past. This finding was made possible by a system of precise measurements of the rate of leaf expansion in a large number of plants at three-minute intervals in natural conditions.
The phenomenon may be explained as follows: if plants are exposed to water stress (eg sunny days and dry soil), the gene expression of aquaporins will vary wildly over the course of the day, but only slightly after cloudy days in moist soil. Water movement and leaf growth follow these same oscillations, which depend on the recent history of the plant.
Thanks to the mathematical modelling of water transfer from ground to leaf via plant roots, scientists have demonstrated the benefits of such an ability to acclimatise. If plants experience drought, the daily transport of water to the roots is facilitated provided the roots lower their permeability in the afternoon and restore it the following morning via aquaporins. This prevents over-drying of the soil that surrounds the roots, which can become practically impenetrable when it dries out.
Conversely, in favourable climatic conditions (moist soil and humid air), strong oscillations in root permeability are detrimental to plant growth. Performance gains or losses mount to 10-15% in either case. By registering water conditions experienced in previous days, plants can anticipate the degree of oscillations that will best foster growth. This study is the first physiological explanation of a mechanism linked to the evolutionary advantage associated with circadian rhythms. By registering water conditions experienced in previous days, plants can anticipate the degree of oscillations that will most effectively promote growth.