Plant Water Relations: A Whirlwind of Change

Abstract

Water is increasingly recognized as a limiting resource consumed by plants in copious amounts. Its large role in agriculture has awakened interest in how plants use it and how they conserve it. Fortunately the study of plant water relations underwent major changes starting about 50 years ago with thermodynamically based methods for measuring plant water status. The methods allowed conditions to be repeated, biochemical events to be repeatedly detected, and responses to water deficits understood more fully. This was followed by the realization that enzymes were not responding directly to water deficits nor was photosynthesis responding only by closing stomata (which conserved water and limited CO₂ entry) but also by diminishing CO₂ demand. The decrease in demand suggested that photosynthesis might acclimate to water deficits, and tests showed not only that this occurred but also that cellular Mg²⁺ concentrations were a central controller of the acclimation. Osmotic adjustment was discovered in a form that used photosynthetic products to maintain turgor and allowed growth where otherwise none would occur. It was found that water potentials were induced by the growth process itself and were important controllers of growth rates in land plants. In an alga surrounded by water and unaffected by these potentials, the chemistry of pectin determined cell enlargement and is now being explored in land plants most of which contain pectin in their cell walls. Ultimately, it became possible to reverse reproductive failure during a drought by feeding photosynthetic products to the plant, thus identifying biochemical origins of failure and gene targets to enhance tolerance to those environments. In fact, commercial agriculture increasingly sees drought tolerance as an important plant character and it is gratifying that reproductive reversal may be contributing to this effort. Although plant water relations are inherently multigenic and complex, these findings demonstrate benefits of understanding plant water relations that were scarcely imagined 50 years ago.

Communicated by Rainer Matyssek