The link between transpiration and nutritional status in wheat grown under elevated CO2

© 2016 Dr Alireza HoushmandfarAtmospheric [CO2] is expected to reach 550 µmol mol-1 by 2050, 37% higher than the current concentration of approximately 400 µmol mol-1. Elevated [CO2] directly affects crop growth and development through the effects on net assimilation rates and stomatal conductance: Increasing intercellular [CO2] results in higher net assimilation rates (in C3 crops), thus higher biomass and grain production, and lower stomatal conductance, thus lower transpiration rate with higher leaf and canopy level water use efficiency. The reduced transpiration rate, however positive for dryland agriculture where water is nearly always the most limiting factor, is suggested to play a role in the reduction of nutritional status, often observed in crops grown under elevated [CO2]. This thesis predominantly used the Australian Grains Free Air CO2 Enrichment (AGFACE) facility with wheat to test whether a reduction in transpiration-driven mass flow of nutrients contributed to nutrient decline under elevated [CO2]. The potential CO2-driven changes in (i) the xylem nutrient concentrations, (ii) the relationship between nutrient uptake and transpiration rate, and (iii) the negative relationship between nutrient uptake and leaf level transpiration efficiency among wheat genotypes were investigated. At anthesis, both transpiration rate and xylem nutrient concentrations were lower under elevated [CO2], suggesting decreased delivery rate of nutrients to the shoot. On the other hand, averaged over longer periods during the growing season, transpiration rates and nutrient uptake were correlated under both ambient and elevated [CO2], but nutrient uptake per unit water transpired was higher in plants grown under elevated than ambient [CO2]. The negative relationship between nutrient uptake and transpiration efficiency among wheat genotypes remained unchanged under e[CO2]. These results suggest that reduced nutrient delivery rate in the transpiration stream contributes to decreases in nutrient concentration under elevated [CO2], but cannot fully account for the overall, more complex relationship between plant nutrition and elevated [CO2]