absorbance symbol: A; a measure of the ability of a substance or a solution to absorb electromagnetic radiation incident upon it. It equals the logarithm of the ratio of the radiant power of the incident radiation, Φ0, to the radiant power of the transmitted radiation, Φ. For a solution, absorbance is expressed as the logarithm
abscisic acid or (formerly) abscisin II or dormin or ABA; 5-(1- hydroxy-2,6,6-trimethyl-4-oxocyclohex-2-en-1-yl)-3-methylpenta- 2,4-dienoic acid; a chiral sesquiterpene. The naturally occurring form, the 2Z,4E,S isomer, also designated (S)-abscisic acid, is a phytohormone formed by the degradation of carotenoids. It controls abscission in flowers and fruit but probably not in leaves, and is also implicated in geotropism,