Q.Name the plant growth regulator that is most closely associated with

Auxin was discovered through a sequence of classic experiments: Charles and Francis Darwin observed that a canary-grass coleoptile bends toward unilateral light only if its tip is intact; Boysen-Jensen showed the tip's influence could cross a gelatin block but not an impermeable mica barrier, proving it was a diffusible chemical; and F. W. Went isolated the substance itself using excised oat coleoptile tips placed asymmetrically on agar blocks, whose diffused chemical alone reproduced the bending in a decapitated coleoptile even in complete darkness, and named it auxin. The principal natural auxin is indole-3-acetic acid (IAA), synthesised chiefly in the shoot apex and young leaves; synthetic auxins include NAA and 2,4-D. Its physiological effects include promoting cell elongation (the basis of the phototropic response with which its discovery story began), maintaining apical dominance (suppressing lateral …