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.