Effects of exogenously applied growth regulators on shoot growth of inbred lines of Plantago major differing in relative growth rate: differential response to gibberellic acid and (2‐chloroethyl)‐trimethyl‐ammonium chloride

The possibility of modulating shoot growth charaeteristics of seedlings of two inbred lines of Plantago major L., differing in relative growth rate (RGR), by exogenously applied 6‐benzylaminopurine (BA), α‐naphthalene acetic acid (NAA), (gibberellic acid (GA₃) and (2‐chloroethyl)‐trimethyl‐ammonium chloride (CCC) was investigated. BA completely inhibited growth of the shoot at a concentration of 1 mM, while lower concentrations had no effect. NAA reduced growth of the shoot at 10 üM, while 1 mM completely inhibited growth. Addition of 10 μM GA₃ or higher stimulated shoot fresh weight up to 20% and leaf area up to 30% for the slow growing inbred line (W9), but less for the fast growing line (A4). Application of 1 mM CCC, an inhibitor of gibberellin metabolism, reduced growth of both inbred lines, but to a larger extent in the fast growing seedlings. The lower shoot growth of W9 was associated with a lower specific leaf area (SLA) and a higher dry matter percentage of the shoot, as compared with A4. NAA reduced growth by reducing SLA and increasing leaf thickness, but the percentage dry matter of the leaves was unaffected. Stimulation of the shoot growth by GA₃ application was associated with higher SLA and lower dry matter percentage. Application of CCC had opposite effects on SLA and dry matter percentage as compared with GA₃. GA seems to be involved in the regulation of at least part of the genetic difference in RGR in Plantago major.