Nitrogen Form and Ratio Impact Swiss Chard (Beta vulgaris subsp. cicla) Shoot Tissue Carotenoid and Chlorophyll Concentrations

Swiss chard (Beta vulgaris subsp. cicla) shoots contain high amounts of carotenoid and chlorophyll pigments and can be enhanced through varying cultural management practices, such as temperature, light, and mineral nutrient levels. The purpose of this study was to determine the effects of N form and ratio on the carotenoid and chlorophylls concentrations in Swiss chard leaf tissue. Seeds of ‘Rhubarb Chard’ and ‘Oriole Orange’ Swiss chard germinated under greenhouse conditions at the University of Tennessee (UT) and Mississippi State University (MSU). Plants were grown at 25/20 °C (day/night) and grown hydroponically in different nitrogen (N) forms and ratios given as ammonium and nitrate N. Plants were analyzed for biomass production, carotenoid and chlorophyll concentrations, and xanthophyll pigment ratios in the shoot tissue. Swiss chard biomass accumulation responded significantly to N form (P ≤ 0.001), and the interaction of N form and location (P ≤ 0.01). Carotenoid and chlorophyll concentrations were also significantly affected by N form (P ≤ 0.001), and the interaction of N form and location (P ≤ 0.01). Specifically, concentrations of lutein (LUT), β-carotene (BC), neoxanthin (NEO), violaxanthin (VIO), and antheraxanthin (ANTH) increased by 42.0%, 45.1%, 49.0%, 59.3%, and 36.0%, respectively, when decreasing the treatment ratio of NH₄-N: NO3-N from 100:0 to 25:75. Similarly, chlorophyll a (Chl a) and chlorophyll b (Chl b) increased 44.9% and 40.9%, respectively. There were significant differences in xanthophyll cycle pigments between the two locations. Higher light intensities at the MSU location may have contributed to lower pigment concentrations in the shoot tissues of Swiss chard. Since N fertility is important to overall health of the plant, it is crucial to understand the influence of managing it in a production system aimed to increase yields and nutritionally important pigmentation in Swiss chard. Thus, the recommendation for increase biomass accumulation and health promoting carotenoid and chlorophyll concentrations in the leaf tissue should consider decreasing NH₄-NO₃ ratios in a fertility program to 25% NH₄-N:75% NO₃-N.