Does elevated CO2 cause human malnutrition? A new understanding from small and slow CO2 change across generations in rice grain quality

Crop's grain quality at elevated CO2 (e[CO2]) within-generation have well received attention, while the long-term response to e[CO2] over multiple generations, especially in gradual e[CO2], is not known. Grain quality was tested in japonica rice grown in open-top chambers after one generation (F1) under abrupt e[CO2] and after five generations (F5) under gradual e[CO2] and abrupt e[CO2]. Abrupt e[CO2] in F1 was ambient CO2 (a[CO2])+ 200 mu mol mol-1 in 2020; gradual e[CO2] in F5 was a step of a[CO2]+ 40 mu mol mol-1 per year until a[CO2]+ 200 mu mol mol-1 from 2016 to 2020; abrupt e[CO2] in F5 maintained a[CO2]+ 200 mu mol mol-1 from 2016 to 2020. Results showed that the response to abrupt e[CO2] in F1 was consistent with previous consensus with generally negative effects on grain quality, such as decreased protein, nitrogen, and amino acids as well as unchanged starch concentrations. In contrast, gradual e[CO2] in F5 did not affect these traits with only reduction in starch, while abrupt e[CO2] in F5 alleviated the deterioration of grain quality related with nitrogen. The current study suggests that multigenerational exposure to gradual e[CO2], actual case for crops in future climate, did not threaten human nutrition, but the amount of calories supplied by starch could be insufficient. This promotes a new understanding of the CO2 effect from small and slow CO2 increase across generations, closer to what happens in the natural word.