1. Introduction

Water deficits are the major limitation in increasing crop yields in many regions of the world. Various plant traits that might result in yield increases in water-limited environments have been vigorously discussed for decades. Only recently, however, have two specific traits to achieve conservative soil water use received major attention. Both traits are based on partial stomatal closure under specific environmental conditions to limit transpiration rate. One trait is partial stomatal closure at higher soil water contents than normally occurs in most plants, and the other is partial stomatal closure under elevated atmospheric vapor pressure deficit. The resulting conservative soil water use increases the amount of water available for sustained physiological activity later in the growing season, particularly during reproductive development.