Light-induced, Dark-reversible Absorbance Changes in Roots, Other Organs, and Cell-free Preparations

Abstract

Irradiation of maize (Zea mays) roots and coleoptiles with visible light causes dark-reversible absorbance changes in these organs. There is an increase in absorbance near 440 nm and smaller increases below 410 nm and about 595 nm. Decreases in absorbance are observed at about 420 nm and minor ones at 537 and 575 nm. These responses are also observed in cell-free preparations from roots and coleoptiles if dithionite, NADPH, or NADH is added prior to illumination. The dose curve for these effects has a distinct maximum at 420 nm and a minor one at 575 nm. Difference spectra and dose response curves indicate that heme compounds such as cytochromes or, more probably, peroxidase complexes are the photoreceptive and chemical reacting molecules. Siroheme-containing proteins may also be taken into consideration. The light-induced absorbance changes have half-lives of more than 200 s and 100 s in roots of maize and soybean, respectively. Two reactions, each with first order kinetics, appear to be superimposed. The respective rate constants for maize roots are about 0.004 and 0.04 s -1. The generation of the effect has a much shorter half-life dependent on light intensity and wavelength. Little deviations from first order kinetics were detected. Rate constants for corn roots range between 0.05 and 0.01 s -1. Apart from the problem in which hemoproteins are involved, there is the problem of correlating the reaction of the photoreactive and chemically reacting molecules to macroscopic responses such as phototropism.