Immunological estimation of growth regulator distribution in phototropically reacting sunflower seedlings

Abstract

The distribution of immunoassayable xanthoxin (XA), abscisic acid (ABA) and indole‐3‐acetic acid (IAA) in all parts of sunflower (Helianthus annuus L.) seedlings was determined. During the course of phototropic curvature, including the lag phase (5 min), the distribution of these growth regulators was analyzed in the illuminated and shaded side of the hypocotyl, as well as in the peripheral and central tissues. All three growth regulators showed no detectable asymmetries between the illuminated and shaded hypocotyl halves during the lag phase and early phototropic curvature. Also, no indication for an exchange of XA, ABA or IAA between the peripheral and central tissues was observed. Partial removal of the peripheral cell layers revealed that changes in the growth properties of this tissue, preferentially at the illuminated side of the hypocotyl, are required for the phototropic reaction. Complete removal of the peripheral cell layers abolishes the phototropic response. In dark‐incubated, green sunflower seedlings, the loss of sensitivity to phototropic stimulation is correlated with decreasing levels of IAA immunoreactivity, whereas no changes in the levels of ABA‐ and XA immunoreactivity were recorded. The findings are discussed with respect to the involvement of ABA, XA and IAA in phototropic reactions of green dicotyledonous shoots.