A 50-year look-back on the efficacy of limited transpiration traits: does the evidence support the recent surge in interest?

We examine limited transpiration (LT) traits in crop species, which are claimed to conserve early season water for critical late season growth. Despite there being theoretical support for LT crops, we suggest that there is insufficient empirical evidence to support the general acceptance of this theory. Our criticism focuses on two main points: the undervaluation of early season carbon assimilation and investment over the lifetime of the plant; and the overestimation of soil water savings. We argue that forgoing early season water use, and therefore also future investment in deeper and denser roots (improved resource acquisition), will negatively impact plant performance in many soil and climate contexts. Furthermore, we challenge the assumption that conserved soil water remains available for later use without loss, noting significant losses resulting from evaporation and other sinks. We advocate for a re-evaluation of LT traits, incorporating a balance of water and carbon dynamics throughout a plant's lifetime. We caution against the adoption of LT traits where they have not been empirically evaluated in the soils and climates of interest to individual research and breeding programs. We propose a more physiologically integrated approach to crop improvement, focusing on water extraction efficiency and strategic carbon investment.