Effect of plant growth regulators and spear bud scales on growth of Asparagus officinalis spears

The effect of plant growth regulators and spear bud scales on spear elongation in asparagus was investigated. Spears of uniform size were immersed completely in indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellic acid (GA3) or N1-(2-chloro-4-pyridyl)-N3-phenylurea (CPPU) in test tubes. IAA and GA3 slightly stimulated, and ABA inhibited, elongation of specific regions of the shoot, but overall the effects on total spear elongation were small. In contrast the cytokinin-active compound, CPPU, significantly increased spear length. Increasing CPPU concentration from 0 to 10 ppm increased the elongation rate of spears. CPPU may influence spear length by stimulating the biosynthesis of native cytokinins or other plant growth promoter hormones as the naturally occurring cytokinin, zeatin riboside, was also found to stimulate spear elongation. In particular, CPPU greatly stimulated the growth of the first and second segments below the spear tip, and removal of spear bud scales inhibited this growth. CPPU did not reverse the growth suppression caused by removing spear bud scales suggesting that other factors may interact with cytokinins in promoting elongation.