Foliar Anthocyanin Content In Solenostemon Scutellarioides (L.) Codd. and Panicum Virgatum L. Varies with Irradiance, Temperature, and Cultivar

Solenostemon scutellarioides (L.) Codd. (coleus) and Panicum virgatum L. (switchgrass) are two species cultivated for ornamental leaf color.  We conducted experiments to determine how foliar anthocyanin content changes over time in response to changes in irradiance or temperature.  Five S. scutellarioides cultivars [three anthocyanic: (‘Big Red Judy’, ‘Royal Glissade’, and a red selection from ‘Twist and Twirl’) and two acyanic: (‘LifeLime’ and a green selection from ‘Twist and Twirl’)] and two P. virgatum cultivars [‘Ruby Ribbons’ (anthocyanic) and ‘Heavy Metal’ (acyanic)] were grown in a common greenhouse for three weeks, then transferred to growth chambers.  In growth chambers, plants were grown at 75, 150, 300, or 600 umol∙m^-2∙s^-1 (22 °C leaf temperature and 16 h photoperiod; irradiance experiment), and for the temperature experiment, plants were grown at 12, 18, 24, or 30 °C (300 umol∙m^-2∙s^-1 irradiance, 16 h photoperiod).  Two leaf punches (0.3 cm²) per plant were collected from a fully-expanded, non-shaded leaf every fourth day for 20 d (irradiance expt.) or every fifth day for 25 d (temperature expt.).  One sample was used to quantify relative anthocyanin content (extracted in 99:1 methanol:HCl; λ_max = 530 nm) and the other to quantify chlorophyll content (extracted in 95% ethanol).  All anthocyanic S. scutellarioides cultivars exhibited an increase in anthocyanin as irradiance increased, and regression analysis indicated a quadratic relationship between irradiance and anthocyanin content.  In P. virgatum ‘Ruby Ribbons’ leaf anthocyanin and percentage of anthocyanic leaves per plant increased (from 10% at 75 umol∙m^-2∙s^-1 to 83% at 600 umol∙m^-2∙s^-1).  At 600 umol∙m^-2∙s^-1, P. virgatum ‘Heavy Metal’ also exhibited some leaf reddening (17% of leaves).  Higher temperatures initially resulted in increased anthocyanin content in all anthocyanic S. scutellarioides cultivars; after 25 d, however, relative anthocyanin content was highest in 30 °C for ‘Royal Glissade’ and in 12 °C for ‘Big Red Judy’ and ‘Twist and Twirl’ red.  Slight leaf reddening occurred in ‘LifeLime’ after 20 d at 30 °C.  Foliar anthocyanin content in P. virgatum was less affected by temperature than S. scutellarioides.  Leaf cross-sections showed the increase in foliar anthocyanin content resulted from increased accumulation in anthocyanic epidermal cells and not in previously acyanic cells.