Hormonal control of branching and flowering in Zantedeschia species : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University, Palmerston North, New Zealand
Calla lilies (Zantedeschia sp. Family: Araceae) are perennial herbaceous geophytes, gaining commercial importance as a cut flower and potted flowering plant. Stimulating branching in Zantedeschia would equate to higher floral productivity via increasing tuber size/weight and/or via triggering the sympodial flowering cascade. Bud outgrowth is however controlled by an autonomous developmental programme, executed via different degrees of para- (apical dominance) and/or endodormancy.
Based on visual clues that represent underlying changes in the shoot apical meristem, the growth cycle of Zantedeschia was demarcated into three phases, which coincide with the transition of buds from apical dominance to endodormancy. Application of 6-benzylaminopurine (BAP; an aromatic cytokinin) was successful in stimulating branching in phase 1. This equated to an increase in tuber size/weight, which in turn resulted in increased floral productivity in the next growth cycle. . Efficacy of BAP alone to stimulate branching declined from phase 1 to phase 3, and the need for a sequential application of gibberellin (GA3) increased concomitantly. GA3 alone had no effect on branching. Efficacy of GA3 alone to stimulate flowering declined from phase 1 to phase 3, and the need for a sequential application of BAP increased concomitantly. BAP alone had no effect on flowering. Stimulation of branching and enhanced flowering achieved by the reciprocal cross-talk between cytokinin and gibberellin may have major commercial implications.
When applied with unlabelled BAP, a significant decline in the uptake of [8-14C] BAP ([8-14C] BAP + BAP) was observed in phase 3, resulting in a decline in radioactivity available in the buds and upper region of the tuber. With unlabelled GA3 ([8-14C] BAP + GA3) however, increased radioactivity was available in these parts in phase 3. Meta-topolin (mT) was identified as a metabolic product of BAP. Application of [8-14C] BAP + BAP resulted in a decline in the amount of mT from phase 1 to phase 3. However, application of [8-14C] BAP + GA3 resulted in an increase in the amount of mT in phase 3. mT and 6-benzylaminopurine riboside (BAR) were also identified in natural plants.
Further studies on branching control in phase 1 involving topolins and strigolactones, elucidating the mechanisms of cross-talk between cytokinin and gibberellin in phase 3, reevaluating the relationship between branching and floral productivity, and corroborating the common mechanisms between dormancy and flowering are recommended.