Abstract
Through stimulation of root growth, increasing atmospheric CO2 concentration ([CO2]) may facilitate access of crops to sub-soil water, which could potentially prolong physiological activity in dryland environments, particularly because crops are more water use efficient under elevated [CO2] (e[CO2]). This study investigated the effect of drought in shallow soil versus sub-soil on agronomic and physiological responses of wheat to e[CO2] in a glasshouse experiment. Wheat (Triticum aestivum L. cv. Yitpi) was grown in split-columns with the top (0–30 cm) and bottom (31–60 cm; ‘sub-soil’) soil layer hydraulically separated by a wax-coated, root-penetrable layer under ambient [CO2] (a[CO2], ∼400 μmol mol-1) or e[CO2] (∼700 μmol mol-1) [CO2]. Drought was imposed from stem-elongation in either the top or bottom soil layer or both by withholding 33% of the irrigation, resulting in four water treatments (WW, WD, DW, DD; D = drought, W = well-watered, letters denote water treatment in top and bottom soil layer, respectively). Leaf gas exchange was measured weekly from stem-elongation until anthesis. Above-and belowground biomass, grain yield and yield components were evaluated at three developmental stages (stem-elongation, anthesis and maturity). Compared with a[CO2], net assimilation rate was higher and stomatal conductance was lower under e[CO2], resulting in greater intrinsic water use efficiency. Elevated [CO2] stimulated both above- and belowground biomass as well as grain yield, however, this stimulation was greater under well-watered (WW) than drought (DD) throughout the whole soil profile. Imposition of drought in either or both soil layers decreased aboveground biomass and grain yield under both [CO2] compared to the well-watered treatment. However, the greatest ‘CO2 fertilisation effect’ was observed when drought was imposed in the top soil layer only (DW), and this was associated with e[CO2]-stimulation of root growth especially in the well-watered bottom layer. We suggest that stimulation of belowground biomass under e[CO2] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems. If sufficient water is available in the sub-soil, e[CO2] may help mitigating the effect of drying surface soil.
Highlights
Atmospheric carbon dioxide concentration ([CO2]) has been increasing since the Industrial Revolution and exceeded 406 μmol mol-1 in 2017 [1]
We suggest that stimulation of belowground biomass under e[CO2] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems
Stomatal conductance was lower under e[CO2] than under a[CO2], and the magnitude of this difference was greatest for WW and subsequently decreased for was dry (WD), DW and DD (CO2 x water treatments (WT); P < 0.001)
Summary
Atmospheric carbon dioxide concentration ([CO2]) has been increasing since the Industrial Revolution and exceeded 406 μmol mol-1 in 2017 [1]. Increasing [CO2] stimulates growth and grain yield of C3 crops [4,5,6], due to the ‘CO2 fertilisation effect’. Elevated [CO2] (e[CO2]) of about 150 μmol mol-1 above ambient increases aboveground biomass by 16 to 79% for C3 crops [4,5,6]. Grain yield stimulation of C3 crops ranged from 6 to 70% in Free Air CO2 Enrichment (FACE) facilities with a target [CO2] of 550 μmol mol-1 [4,5,6], and can be even higher (31 to 166%) when grown in glasshouse facilities at higher [CO2] The magnitude of relative yield stimulation by e[CO2] is dependent on growing conditions [5, 8, 10, 11] and frequently predicted to be greater under drier than well-watered conditions [6, 12, 13]
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