Shoot growth and histological response of dwarf sweetgum to gibberellin

In addition to supplying valuable cultivars for ornamental uses, dwarf muta nt tree forms may present opportunities for studying the genetic control of physiological factors affecting shoot growth. Few studies have addressed t he histogenetic role of GA during shoot development, and its effect on cell pattern in mature internodes. Hence, the growth responses of two genetic d warf sweetgum trees (Liquidambar styraiflua L. cv. Gumball and an unnamed w ild dwarf tree) were examined following foliar application of gibberellic a cid (GA) to young preformed leaves. Both were GA-responsive mutants and exh ibited marked increases in shoot growth following treatment. GA was effecti ve in 1) stimulating subapical meristem activity in preformed internodes wh ich were initiated the previous season, 2) inducing cell divisions in the a pical meristem proper to promote the neoformation of leaves and sylleptic l ateral branches, and 3) stimulating subapical mitotic activity in neoformed internodes. Histological evaluations of cell patterns in the pith of matur e internodes of GA-treated and central shoots showed that enhanced cell div ision accounted for 89% of the increase in final internode length; final ce ll length accounted for only 11% of the increase.