Mustard 'Amara' Benefits from Superelevated CO2 While Adapting to Far-red Light Over Time

Compared with the ambient Earth carbon dioxide concentration (⠁415 mu mol ⠃mol-1), the International Space Station has superelevated carbon dioxide (⠁2800 mu mol ⠃mol-1), which can be a stressor to certain crops. Far-red light can drive plant photosynthesis and increase extension growth and biomass. However, the effects of far-red light under superelevated carbon dioxide are unclear. We grew hydroponic mustard (Brassica cari-nata) 'Amara' seedlings in four growth chambers using a randomized complete block de-sign with two carbon dioxide concentrations (415 and 2800 mu mol ⠃mol-1), two lighting treatments, and two blocks at temperature and relative humidity set points of 22 ⠄C and 40%, respectively. Each growth chamber had two lighting treatments at the same total photon flux density of 200 mu mol ⠃m-2 ⠃s-1. Under the same blue and green light at 50 mu mol ⠃m-2 ⠃s-1 each, plants received either red light at 100 mu mol ⠃m-2 ⠃s-1 or red + far-red light at 50 mu mol ⠃m-2 ⠃s-1 each. At day 15 after planting, far-red light did not influence shoot fresh or dry mass at 415 mu mol ⠃mol-1 carbon dioxide, but decreased both parame-ters by 22% to 23% at 2800 mu mol ⠃mol-1 carbon dioxide. Increasing the carbon dioxide concentration increased shoot fresh and dry mass 27% to 49%, regardless of the lighting treatment. Far-red light decreased leaf area by 16% at 2800 mu mol ⠃mol-1 carbon dioxide, but had no effect at 415 mu mol ⠃mol-1 carbon dioxide. Increasing the carbon dioxide con-centration increased leaf area by 21% to 33%, regardless of far-red light. Regardless of the carbon dioxide concentration, far-red light promoted stem elongation and decreased chlorophyll concentrations by 39% to 42%. These responses indicate far-red light elicited a crop-specific shade avoidance response in mustard 'Amara', increasing extension growth but decreasing leaf area, thereby reducing light interception and biomass. In ad-dition, carbon dioxide enrichment up to 2800 mu mol ⠃mol-1 increased the biomass of mus-tard 'Amara' but decreased the biomass of other crops, indicating crop-specific tolerance to superelevated carbon dioxide. In conclusion, mustard 'Amara' seedlings benefit from superelevated carbon dioxide, but exhibit growth reduction under far-red light under superelevated carbon dioxide.