Factors determining petal colour of baccara roses II: The effect of pigment concentration

'Blueing' in young and senescing petals was compared in the red rose cv. Baccara. The 'blueing' of senescing flowers is accompanied by a bathochromic shift in the light reflectance curve, a rise in the pH value and a decrease in the malic acid concentration of the petal tissue. These factors indicate that a complex with a co-pigment is produced. Similar changes were not found in the 'blueing' of young flowers, where a decrease was found in pigment concentration per unit weight as well as per unit area of petal. A similar 'blueing' was achieved by diluting a solution of crystalline cyanin. The phenomenon of 'blueing' by dilution is discussed in the light of Bougeur's law.