A Controlled Gravimetric Phenotyping Approach Reveals Variation in Transpiration Responses to Evaporative Demand across Maize NAM Parents.

Water-saving traits are likely to increase crop productivity in drought-prone areas. For maize, such traits are particularly desirable because this crop is grown in increasingly drought-prone areas. One trait that has been shown to result in substantial yield benefits under drought consists of the plant limiting its transpiration rate (TR) in response to increasing hourly vapor pressure deficit (VPD), as exemplified by the AQUAmax maize hybrid. Early in the crop cycle when water availability is not limiting, such a reduction integrated over several weeks results in a water-saving strategy that typically increases water availability during the critical grain filling phase. In this study we investigated the genetic variability of TR response to VPD across a group of 25 nested association mapping (NAM) maize parents provided by the maize genetics cooperation stock center, which are widely used for various genetic studies. Using a novel, mid-throughput whole-plant gravimetric phenotyping approach, we characterized for the first time TR response curves to increasing VPD for those genotypes under a strictly constant temperature (30 °C) in three independent experiments. Such treatment was needed to allow for investigating the effects of VPD per se, i.e., independently from the often confounding effects stemming from co-occurring variation in temperature and light. The experiments revealed a substantial genetic variability (100% variation) in the slopes of TR response curves to VPD, but did not reveal a bi-linear TR response to VPD, a signature for the limited-TR trait. Because of the large genetic diversity captured by this panel, our finding indicates that such water-saving trait is a rare phenotype. This suggests that assembling diversity panels for studies aiming at discovering the genetic basis of drought tolerance would benefit from maximizing the diversity of functional phenotypes of the parent genotypes.