Juvenility in three composite genera with ornamental potential: Rudbeckia, Gaillardia and Solidago

Abstract

Rudbeckia hirta I Marmalade' plants at different leaf numbers were exposed to a (long day) LD photo period in order to quantify the end of juvenility. A quadratic relationship existed between flowering and both the number of leaves the plant possessed and the number of days from onset of LD. The loss of juvenility occurred in the shortest time, 42 days, plants had 15-24 true expanded leaves. The greatest stem height also corresponded to this same leaf number. The number of LD received did not affect ray floret number per inflorescence, scape length and capitulum diameter excluding ray florets. Gaillardia pulchella flowered most rapidly in 49 days, with a minimum of 14-20 expanded leaves as described in a quadratic relationship. Histological studies during evocation indicated an increase in apical meristem widths (120 pm to 224 pm) and heights (21 ~m to 48 pm), an increase of 2 to 5 tunica cell layers, greater corpus organization, and a lower point of primordia attachment with respect to increased doming of the apex all occurring with maturity of the apex. Solidago flexicaulis exhibited a complex (combination) photoperiodic response, requiring an extended LO exposure prior to SO initiation. The last treatment, consisting of 12-week-old-plants transferred from LO to SO, flowered most rapidly in 62 days. Other measured characteristics such as stem length, dry weight of all tissue above and below the soil line, number of basal shoots and main axis leaf number were not different depending on amount of SO induction. There was a difference in the above measured variables, except emerged basal shoots and leaves per main axis, between control LO plants and all averaged SO treated plants.