Physiological Cost-Benefit Analysis of Traits That Reduce Crop Water Use.

Various traits may allow crops to maintain soil moisture during the growing season and thus avoid drought; but changing crop water use must not be at the cost of reduced productivity. Here we develop a hydraulic model of crop water use, and use it to generate field testable hypotheses of the physiological benefits of traits that maintain soil moisture during a dry period, and express these benefits relative to the costs of the traits if drought had not occurred. Stomatal density, or traits that constitutively reduce maximum stomatal conductance, are shown to increase water use efficiency, and maintain soil moisture, but at the expense of large reductions in photosynthesis during well watered periods. Alternative traits were tested, that allow the stomata to function with a high stomatal conductance during low vapor pressure deficit (VPD) conditions, but limit transpiration to a maximum during periods of high VPD. These traits resulted in less loss of photosynthesis, but also varying amounts transpiration reduction. A reduced leaf, stem or root hydraulic conductance led to the same stomatal responses, transpiration and photosynthetic costs and benefits. However reduced stem and root conductance lead to a higher risk of stem cavitation, as these organs experience more negative water potentials than if leaf hydraulic conductance is reduced. More sensitive stomata had limited benefits, while stomata that closed while still well hydrated had considerable benefits. The effect of mild drought – more negative soil water potentials – on the nature of the transpiration response to VPD was significant, but largely not interactive. That is, transpiration is reduced by drought, but cultivars with a limited transpiration rate could be expected to maintain that type of response during drought. The latter hypothesis was tested in results from a drydown by VPD by soil experiment on six genotypes of soybean.