Effects of temperature, light and nutrition on color and anthocyanin content of poinsettia bracts

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Virginia Polytechnic Institute

Investigations were undertaken to establish an objective procedure for measuring the color of poinsettia bracts, to develop a method for quantitatively determining the concentration of bract anthocyanins, and to determine the effect of temperature, light intensity and nutrition on the color and the anthocyanin content of poinsettia bracts.

A procedure was developed to objectively measure the color poinsettia bracts using the Hunter Color-Color Difference Meter. The Hunter Color-Color Difference Meter was effective in measuring subtle color differences not observable to the naked eye. The Hunter color values were calculated into Munsell color notations to provide standard color terms for poinsettia bracts.

A method was developed to quantitatively determine the concentration of anthocyanins in poinsettia bracts. The aglycone fractions, pelargonidin and cyanidin of poinsettia anthocyanins were used for quantitative purposes rather than the actual glycosides. Pelargonidin was purified using standard chemical and chromatographic procedures. Pure cyanidin was obtained from a commercial company. The absorption maxima, specific absorption coefficients and the response of pure pelargonidin and cyanidin to Beer’s Law in amyl alcohol saturated with 2N HCl were determined. The amounts of pelargonidin and cyanidin in the same solution were calculated using the specific absorption coefficients and the optical densities obtained at the absorption maxima of each anthocyanidin. Pelargonidin and cyanidin were accurately measured to 0.1 milligram amounts per 50 milliliters of solvent.

The effects of temperature, light intensity and nutrition on the color and anthocyanin content of Indianapolis Red poinsettias were determined. The results of these environmental studies may be summarized as follows:

  1. Bracts grown at 55 F contained more anthocyanin than bracts grown at 62 and 70 F. The higher the temperature, the less anthocyanin produced per unit bract are; hence, the less “red” colored were the bracts. Bracts grown at 70 and 62 F changed in red coloration proportionately to changes in anthocyanin concentration per unit area of bract; hence, bracts grown at high temperatures were less “red” colored. Changes in bract coloration were due to changes in cyanidin glycosides rather than pelargonidin glycosides. Bracts grown at 70 F and moved to 55 F during the final two weeks of growth were more “red” and contained more anthocyanin than the bracts kept at 70 F.

  2. Light was necessary for anthocyanin formation in poinsettia bracts. The amount of anthocyanin accumulated in bracts was dependent on the amount of sunlight incident to the bracts. Bracts grown at high intensity contained more anthocyanin and were more “red” than bracts grown at low light intensity. The amount of anthocyanin produced in bracts at a given light intensity was controlled by the prevailing night temperature.

  3. Bracts grown at combinations of each of the 3 levels of nitrogen and potassium produced slight changes in color and anthocyanin content in bracts. Potassium did not influence anthocyanin content of bracts. There was less anthocyanin produced per unit bract area at low nitrogen regimes than at medium or high nitrogen regimes. The differences in anthocyanin content per unit bract area were not great enough to cause significant changes in bract color.

  4. Results for bracts grown at varying temperatures and light intensity regimes indicate that there is a relationship for anthocyanin accumulation to bract dry weight. Bract dry weight-anthocyanin relationship was somewhat obscure for bracts grown at varying nitrogen and potassium regimes.