Optimizing soybean water use and nitrogen fixation to improve productivity under climate change

Abstract

Climate change effects are driving crop yield decreases globally, and yield penalties are often associated with increases in frequency and intensity of heat waves and soil moisture deficits, particularly during reproductive development. Additionally, climate change is driving increases in vapor pressure deficit (VPD) in recent decades, suggesting global atmospheric drying. In a U.S. study, VPD during reproductive stages was identified as the most important predictor of soybean yield variation. Investigations into the effect of VPD on soybean yields often consider its effect on crop transpiration rate (TR) and gas exchange, and such efforts have led to development of drought-tolerant soybean cultivars, which take advantage of particular TR responses to VPD. Despite such progress, no studies have yet examined the effect of VPD on nitrogen fixation, likely due to difficulties of taking non-invasive, real-time measurements.A common technique for monitoring nitrogen fixation rates non-destructively is the acetylene reduction assay (ARA), which has received critiques due to hazards posed to researchers and potential effects on root nodules. The first goal of this investigation was to develop an alternative method for non-invasively measuring nitrogen fixation that minimizes the above difficulties. A system was designed that enabled us to quantify N2 fixation as a function of the rate of by-product hydrogen gas (H2) production by soybean nodules. A second goal was to examine rates of nitrogen fixation in relation to concurrent changes in environmental variables, namely VPD and temperature. The system was successfully tested on two soybean genotypes in both field and controlled environment conditions. A key result is the confirmation of H2 production being associated with root nodules, seen by a lack of H2 signal from plants without nodules. Additional key observations include increases in H2 production rate from the morning to afternoon in the field, driven at least partly by increases in VPD, consistent with H2 production rate increases observed when shifting from low to high VPD in a controlled environment. Consistent genotypic differences were observed in both settings, signifying potential for genotypic diversity, which could be exploited in breeding efforts. This study invites further research into the effect of VPD and temperature on N fixation, as soybean growing environments continue to undergo increases in atmospheric drying. A second focus of this research on soybean is centered on plant water use, which is largely driven by evaporative demand as VPD increases drive increases in TR. Increased TR drives processes beneficial to carbon fixation, so in an environment where available water is not limiting, such behavior should lead to higher yield, or at least be neutral. However, in a water-deficit prone environment, genotypes exhibiting lower TR at high VPD will outperform those exhibiting a ‘profligate’ TR, by reducing the amount of water lost during the time of day when evaporative demand is the highest. Despite progress in phenotyping soybean TR responses to VPD, so far no QTL have been detected for traits directly reflecting such response curves, due to methodological and logistical challenges. The QTL mapping performed here resulted in the detection, for the first time, of two QTL for the response of TR to both low and high VPD. If confirmed, these QTL could be leveraged to design soybean varieties that are optimized for specific water availability regimes in order to maximize productivity.