Heteroblasty and preformation in mayapple, Podophyllum peltatum (Berberidaceae): developmental flexibility and morphological constraint

Developmental preformation can constrain growth responses of shoots to curr ent conditions, but there is potential for flexibility in development prece ding formation of the preformed organs. Mayapple (Podophyllum peltatum) is strongly heteroblastic, producing rhizome scales, bud scales, and either a single vegetative foliage leaf or two foliage leaves on a sexual shoot. To understand how and when preformation constrains growth response-, we compar e (1) how leaf homologs of the renewal shoot differ in development. (2) whe ther there are differences in shoot development that occur in advance of mo rphological determination of shoot type, and (3) whether there are points o f developmental flexibility in renewal shoot growth prior to preformation o f the foliage and floral organs. We use scanning electron microscopy and hi stology to show that the three vegetative leaves (both types of scale leave s and the vegetative foliage leaf) are similar in the initial establishment of an encircling and overarching leaf base. Differences among them are fou nd in the timing of differentiation of the leaf base and in the relative ti ming and degree of growth of the lamina and petiole. In contrast, foliage l eaves on sexual shoots show less expression of the leaf base and precocious growth of the lamina and petiole. Prior to shoot type determination, there are no morphological differences in the sequence or position of leaf homol ogs that Predict final shoot type. In this colony, leaves at positions 12 a nd 13, on average, appear to be identical in development until they are bet ween 700 and 800 mum in length, when it becomes possible to distinguish lea ves that will become vegetative foliage leaves from additional bud scale le aves on vegetative or sexual shoots. We suggest that late developmental det ermination Of leaves at positions 12 and 13 reflects ontogenetic sensitivit y to a transition to flowering. Thus, in mayapple, heteroblasty appears to facilitate developmental flexibility prior to the point where shoot growth becomes constrained by preformation of determined aerial structures.