Source–sink balance of wheat determines responsiveness of grain production to increased [CO2] and water supply

Manipulation of source (flag-leaf removal) and sink (ear trimming) was conducted in a factorial CO2-irrigation field chamber experiment with spring wheat (Triticum aestivum L.) in south-west Sweden to test the hypothesis that responsiveness of grain production and biomass partitioning to CO2 concentration ([CO2]) and water supply is dependent on the source–sink balance of the plant. Negative effects of doubled [CO2] on both individual grain mass (IGM) and harvest index (HI) were strongly related to decreasing relative sink strength (i.e. increasing source:sink ratio), probably as a result of feedbacks from sink limitation on source activity under elevated [CO2] being relatively more important as relative sink strength decreases. Substantial down-regulation of photosynthetic capacity in elevated [CO2], resulted in lack of significant stimulation of grain yield (GY) of unmanipulated shoots growing under elevated [CO2]. GY was even reduced by elevated [CO2] in sink-manipulated shoots, implying that high source:sink ratio may result in a down-regulation of photosynthetic capacity that more than offsets the direct stimulating effect of elevated [CO2]. High irrigation positively affected IGM and HI in source-manipulated shoots only, probably as a result of the timing of irrigation treatment effects on soil moisture during, but not before, grain filling. Irrigation thus probably affected source activity during grain filling rather than potential sink capacity determined before and around anthesis, an effect that should be more pronounced for shoots with low source:sink ratio. We conclude that effects of [CO2] and irrigation on grain production and biomass partitioning of wheat are strongly modified by source–sink balance of the plant, and that sink limitation is a major constraint on CO2-induced GY enhancement of spring wheat under Scandinavian climatic conditions. These findings may explain earlier observations of decreased CO2 responsiveness of GY in modern wheat cultivars, with lower whole-plant sink strength before and around anthesis compared to old cultivars, and call into question current attempts of molecular plant breeding to maximise photosynthetic activity before and around anthesis in order to enhance GY in a world with rising atmospheric [CO2].