Interaction of gravi- and phototropic stimulation in the response of maize (Zea mays L.) coleoptiles

The influence of gravitropic stimulation upon blue-light-induced first positive phototropism for stimulations in the same (light source and center of gravity opposite to each other) and in opposing directions was investigated in maize cole-optiles by measuring fluence-response patterns. As a result of gravitropic counterstimulation, phototropic bending was transient with maximum curvature occurring 100 min after stimulation. On a horizontal clinostat, however, the seedlings curved for 20 h. Gravistimulation in the opposite direction acted additively upon blue-light curvature. Gravistimulation in the same direction as phototropic stimulation produced a complex behaviour deviating from simple additivity. This pattern can be explained by a gravitropically mediated sensitization of the phototropic reaction, an optimal dependence of differential growth on the sum of photo-and gravistimulation, and blue-light-induced inhibition of gravitropic curvature at high fluences. These findings indicate that several steps of photo-and gravitransduction are separate. Preirradiation with red light desensitized the system independently of applied gravity-treatment, indicating that the site of red-light interaction is common to both transduction chains.Abbreviations BL blue light - G⁺ stimulation by light and gravity in the same direction (i.e. light source and center of gravity opposite to each other) - G^- stimulation by light and gravity in opposing directions