Does a freely tillering wheat cultivar benefit more from elevated CO2 than a restricted tillering cultivar in a water-limited environment?

•The CO2 response of a freely and restricted tillering wheat cultivar was compared.•CO2 enrichment stimulated grain yield in both cultivars to a similar extent (23–24%).•Yield increases in the freely tillering cultivar were linked to greater head numbers.•The restricted tillering cultivar also had increased kernel weight.•We conclude that not only high tillering ensures greater benefits from high CO2.

This study addresses whether a freely tillering wheat cultivar with greater vegetative sink strength (cv. “Silverstar”) can benefit more from increasing atmospheric CO2 concentration [CO2] than a restricted tillering cultivar with greater reproductive sink strength (cv. H45) in a water-limited cropping system. Growth, yield, yield components and nitrogen at three developmental stages (stem elongation, anthesis, maturity) and water soluble carbohydrates (WSC, anthesis) were evaluated at two CO2 concentrations (ambient [CO2], ∼395 ppm, elevated e[CO2], ∼550 ppm) across six environments using the Australian Grains Free Air CO2 Enrichment (AGFACE) facility. Cv. “Silverstar” had more tillers than cv. “H45” throughout development; whereas, cv. “H45” had greater WSC storage and more and heavier kernels per spike. CO2 enrichment stimulated grain yield in both cultivars similarly, but this stimulation was caused differently: For cv. “Silverstar”, grain yield increase was exclusively linked to an increased number of fertile tillers; whereas, in cv. “H45”, yield stimulation was additionally associated with increased kernel weight and kernel numbers per spike. We conclude that in a Mediterranean-type, water-limited environment high tillering capacity alone does not ensure greater benefits from CO2 fertilization but that both pre and post-anthesis source-sink relationships play a significant role in this environment as well.