Carotenoid Pigments In Kale Are Influenced by the Ratio of Calcium to Magnesium

Sufficiency ranges for delivering essential elements to crop plant are routinely used by producers to determine fertilizer applications.  Because many production regions in the U.S. have very high or very low amounts of one or more essential nutrients, crop consultants often use nutrient ratios as a means of ensuring sufficient root uptake of competitive ions.  Two essential nutrient cations which compete for uptake are calcium (Ca) and magnesium (Mg).  Kale (Brassica oleracea L. var. acephala D.C.) has been reported to accumulate high levels of carotenoids and these compounds serve antioxidant functions in photosynthetic processes, as well as actions of disease reduction in mammalian systems.  The objectives of our research were determine the influence of the ratio of Ca to Mg fertilization on the quality factors of kale which include 1) biomass production, 2) chlorophyll content, and 3) accumulation of the important carotenoids for human health β-carotene and lutein.  ‘Redbor’ kale was greenhouse-grown in nutrient culture using a modified Hoagland’s solution in Normal, IL and Knoxville, TN in the fall of 2008.  The ratio of Ca:Mg was varied from 9:1, 6:1, 3:1, 1:3, 1:6, and 1:9, respectively.  Kale was seeded the first week of Sept. 2008 in both locations and 15 days later plants were transferred to solution culture in a randomized block design with 4 replications.  After six weeks plants reached a marketable size and were harvested.  At harvest, fresh biomass was taken and leaf tissue samples were frozen at -20 °C.  Plant tissues were freeze-dried and carotenoid and chlorophyll compounds were extracted and separated using HPLC.  The ratio of Ca:Mg significantly affected plant biomass, total carotenoids, β-carotene, lutein, and chlorophyll b in leaf tissues.  Carotenoid concentrations were within previously reported ranges.  Total plant biomass decreased linearly (P<0.001) from 68.21 mg/plant under the 9:1 Ca:Mg treatment to 21.86 mg/plant under the 1:9 Ca:Mg treatment.  Total carotenoids also decreased linearly (P=0.04) from 20.13 mg/100 grams fresh weight (gfw) under the 9:1 Ca:Mg treatment to 13.31 mg/100 gfw under the 1:9 Ca:Mg treatment.  The highest levels of the individual carotenoids β-carotene and lutein were observed under the 3:1 Ca:Mg treatment in each environment.  These results indicate that producers wishing to maximize the carotenoid content of kale need to consider the ratio of Ca:Mg in their fertility programs.