The extent to which rising atmospheric CO₂ concentration has already influenced food production and quality is uncertain. Here, we analyzed annual field trials of fall-planted common wheat in California from 1985 to 2019, a period during which global atmospheric CO₂ concentration increased 19%. Even after accounting for other major factors (cultivar, location, degree-days, soil temperature, total water applied, nitrogen fertilization, and pathogen infestation), wheat grain yield and protein yield declined 13% over this period, but grain protein content did not change. These results suggest that wheat, when exposed to gradual CO₂ enrichment, sacrifices grain yield and protein yield for stable grain protein content.

The dataset of fall-planted, common wheat cultivars from field trials conducted at multiple locations throughout central California (Lundy and Dubcovsky, 2021) has 6,508 records with values for (a) year; (b) cultivar name; (c) location; (d) grain protein content (%); (e) grain yield (kg); (f) total water supplied (mm); (g) air degrees days; (h) total N fertilizer applied (kg/ha); (i) years from introduction of the cultivar into field trials; (j) quantitative scores for stripe rust, leaf rust, Septoria leaf blotch, and Yellow dwarf virus (area of flag-1 leaf affected at soft dough stage: 1 = 0 – 3%, 2 = 4 – 14%, 3 = 15 – 29%, 4 = 30 – 49%, 5 = 50 – 69%, 6 = 70 – 84%, 7 = 85 – 95%, 8 = 96 – 100%); (k) soil temperature (°C); and (l) Vapor Pressure Deficit (VPD, Pa). Soil temperatures, air degree-days, and Vapor Pressure Deficit (VPD) were derived from hourly data collected at the closest California Irrigation Management Information System (CIMIS) station (California Department of Water Resources, 2021); degree-days were calculated based on temperatures measured during January, February, and March via the single sine method with a horizontal upper cutoff with a maximum temperature for common wheat of 30°C and a minimum temperature of 7°C (Statewide Integrated Pest Management Program, 2021); and VPD was calculated from average air temperatures (Huang, 2018) and vapor pressures during January, February, and March at the closest California Irrigation Management Information System (CIMIS) station. Protein yield is the product of grain protein percentage and yield, and total water addition is the sum of rainfall and irrigation.

California fall-planted wheat field trials provided at least 16 years of data for each of five locations: Sacramento Delta (“Delta”), Imperial County (“Imperial”), Kern County (“Kern”), Kings County (“Kings”), and Yolo County at the University of California Davis (“UCD”). We merged the data for Butte County and Colusa County, two adjacent counties that had very little temporal overlap, to form a sixth location (“North”) with more than 16 years of data. The six locations extend between latitudes 32.8°N and 39.8°N, from deserts near the Mexican border (Imperial and Kern) where plants received nearly all of their water from irrigation to the Sacramento River flood plain (Delta and North) where plants received much of their water from precipitation and ground water. The trials also provided at least 18 years of data for each of six check cultivars (Anza, Blanca Grande, Express, Klasic, Serra, and Yecora Rojo). Data for the cultivars Blanca Grande, Patwin, and Summit were merged with data for derivatives in which stripe rust resistance genes Yr5 and Yr15 were introduced by four backcross generations into the susceptible parent cultivar (Jackson, 2011).