Far-red Light and Nitrogen Concentration Elicit Crop-specific Responses in Baby Greens under Superelevated CO2 and Continuous Light

Abstract

Baby greens are becoming increasingly popular in the consumer market because of their desired flavor and leaf size. The short life cycles and fast response times to environmental stimuli make baby greens ideal for testing environmental conditions for space crop production. Additionally, far-red (FR) light has been used for microgreen and baby green research to enhance stem elongation, leaf expansion, and biomass; however, how it interacts with nutrient solution nitrogen (N) concentrations remains unclear. During this ground-based study, we characterized how FR light and N concentrations influenced the growth and morphology of Chinese cabbage (Brassica rapa var. chinensis cv. Tokyo Bekana) and kale (Brassica oleracea var. sabellica cv. Red Russian) baby greens under similar superelevated CO2 and low relative humidity to levels observed in spaceflight. Plants were subject to combinations of four sole-source light spectra and three N concentrations (75, 125, and 175 mg⋅L−1). At the same total photon flux density (PFD) of 200 μmol⋅m−2⋅s−1, we maintained the same blue and green PFDs at 25 μmol⋅m−2⋅s−1 each; the remaining 150 μmol⋅m−2⋅s−1 comprised four red (R) and FR PFD combinations (FR: 0, 25, 50, and 75 μmol⋅m−2⋅s−1). Increasing the FR PFD enhanced the typical shade-avoidance morphology of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’, exhibiting leaf length increases of 20% to 26% and 31% to 61%, respectively. Edible biomass did not increase with increasing FR PFDs for either species, regardless of the N concentration. Increasing the N concentration increased the Chinese cabbage ‘Tokyo Bekana’ fresh mass and dry mass by 32% to 59% and 37% to 74%, respectively, except under 25 μmol⋅m−2⋅s−1 of FR light, with which shoot fresh mass increased by 55% with an increasing N concentration from 75 to 125 mg⋅L−1; however, the shoot dry mass was unaffected. Increasing the N concentration did not affect kale ‘Red Russian’ growth under various FR PFDs. We conclude that partially substituting incremental FR light for R light elicits the shade-avoidance response, with little influence on the growth, of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ baby greens under superelevated CO2 and continuous light, and that the former, but not the latter, crop can benefit from increased N fertilization.