Of all the plant processes examined, leaf growth and canopy development is the most sensitive to water stress. The consequent reduction in cumulative radiation interception by the plant leads to a smaller biomass as well as reduced transpiration, usually without altering radiation-use efficiency or water-use efficiency of the canopy. Sensitivity of leaf growth to the growth medium or aerial environment of the plant will be illustrated. A way to quantify the consequent and often marked impact on productivity will be discussed. In contrast with the high sensitivity of leaf growth to water stress, root growth is more resistant. This allows at least the partial maintenance of root growth as the stress intensifies. The result is a more thorough extraction of soil water while transpiration is restricted by the smaller leaf area. The possible mechanisms for the differential sensitivity of leaf and root growth to water stress will be evaluated. Emphasis will be placed on processes underlying cell enlargement. Recent data, obtained with the pressure microprobe that measures turgor pressure in individual cells, will be presented to illustrate the contrasting responses in growth, cell wall extending ability, and solute transport to the growing cells when the plant adjusts and accommodates to changes in water status.
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