Effect ofTemperature on Gibberellin (GA)Responsiveness andon Endogenous GA1Content ofTall andDwarfWheat Genotypes

Near-isogenic wheat(Triticum aestivum L.)lines differing in height-reducing (Rht) alleles wereusedtoinvestigate theeffects oftemperature onendogenous gibberellin (GA)levels andseedling growth response toapplied GA3.Sheath andlamina lengths ofthefirst leaf weremeasured inGAtreated andcontrol seedlings, grownat11,18,and250C, ofsixRhtgenotypes ineachof twovarietal backgrounds, cvMarisHuntsmanandcvApril Bearded. Endogenous GA,levels intheleafextension zoneof untreated seedlings weredetermined bygaschromatographymassspectrometry withadeuterated intemal standard inthesix Maris Huntsman Rhtlines grownat10and250C. Higher temperature increased leaf length considerably inthetall genotype, less sointheRhtlandRht2genotypes, andhadnoconsistent effect ontheRhtl+2, Rht3andRht2+3genotypes. Inallgenotypes, endogenous GA1washigher at250Cthanat100C. At10°Cthe endogenous GA1wasata similar level inallthegenotypes (except Rht2+3). At250Citincreased 1.6-fold inthetall genotype, 3-fold inRhtlandRht2, 6-fold inRht3, and9-fold inRhtl+2. Likewise, thegenotypic differences inleaflength werevery conspicuous at250C, butwereonly slight andoften unsignificant at11°C. Theresponse ofleaf length toapplied GA3intheRhtl, Rht2, andRhtl+2 genotypes increased significantly withlowering oftemperature. Theseresults suggest thepossibility thatthe temperature effect onleafelongation ismediated through its effect onthelevel ofendogenous GA,andthatleafelongation response toendogenous orapplied GAsisrestricted bytheupper limits setbythedifferent Rhtalleles. Mostmodemhigh-yielding wheat cultivars haveshort straw andarereferred toas'dwarf or'semi-dwarf.' Themajority of these cultivars owetheir short stature tothepresence ofone orbothoftheNorin10-derived, height-reducing genes, RhtJ andRht2(5). Almost nocommercial cultivars carry themore potent height-reducing 'TomThumb'allele, Rht3, andthe combination ofRht2+ Rht3alleles hasbeengenerated only recently inexperimental lines. TheRhtJ andRht2alleles are carried onchromosomes 4BS'and4DS,respectively, theRht3 isanallele attheRhtJlocus onchromosome 4BS(5).