Physical Basis forAltered StemElongation Ratesin Internode Length Mutants ofPisum

Biophysical parameters related togibberellin (GA)-dependent stemelongation wereexamined indark-grown stem-length genotypes ofPisumsativum L.Therateofinternode expansion in thesegenotypes isaltered duetorecessive mutations which affect either theendogenous levels of,orresponse to,GA.The GAdeficient dwarfL181(Is), twoGAinsensitive semierectoides dwarfsNGB5865andNGB5862(IkaandIkb,respectively) and the'slender' line L197(lacrc), whichistall regardless ofGA content, werecompared tothewild-type tall cultivar, Torsdag. Osmotic pressure, estimated byvaporpressure osmometry, and turgor pressure, measured directly withapressure probe, didnot correlate withthedifferences ingrowth rateamongthegenotypes. Mechanical wallproperties offrozen-thawed tissue were measured using aconstant force assay. GAdeficiency resulted inincreased wallstiffness judged bothonthebasisofplastic compliance andplastic extensibility normalized forequalstem circumference. Plastic compliance wasnotreduced intheGA insensitive dwarfs, though Ikareduced circumference-normalized plasticity. Incontrast, invivowallrelaxation, determined bythe pressure-block technique, differed amonggenotypes inamanner whichdidcorrelate withextension rates. Thewall yield threshold wasIbarorlessinthetall lines, butranged from3to6barsin thedwarf genotypes. Theresults withtheIsmutantindicate that GAenhances stemelongation bybothdecreasing thewallyield threshold andincreasing thewallyield coefficient. IntheGAinsensitive mutants, IkaandIkb, thewallyield threshold issubstantially elevated. Plants possessing Ikamayalsopossess a reduced wallyield coefficient.