The vegetative development of the maize shoot can be divided into juvenile and adult phases based on the types of leaves produced at different times in shoot development. Models for the regulation of phase change make explicit predictions about when the identity of these types of leaves is determined. To test these models, we examined the timing of leaf type determination in maize. Clones induced in transition leaf primordia demonstrated that the juvenile and adult regions of these leaves do not become clonally distinct until after the primordium is 700 m in length, implying that these cell fates were undetermined at this stage of leaf development. Adult shoot apices were cultured in vitro to induce rejuvenation. We found that leaf primordia as large as 3 mm in length can be at least partially rejuvenated by this treatment, and the location of rejuvenated tissue is correlated with the maturation pattern of the leaf. The amount and distribution of juvenile tissue in rejuvenated leaves suggests that rejuvenation occurs nearly simultaneously in all leaf primordia. In vitro culture rejuvenated existing leaf primordia and the P0 primordium, but did not change the identity of subsequent primordia or the total number of leaves produced by the shoot. This result suggests that leaf identity can be regulated independently of the identity of the shoot apical meristem, and it implies that vegetative phase change is not initiated by a change in the identity of the shoot apical meristem. P lants produce different types of leaves or leaf-like organs at different stages in their development (a phenomenon called heteroblasty or phase change) (1-4). Among these are cotyledons, the variety of leaf types produced during the vegetative growth of the shoot, inflorescence bracts, and the highly modified leaves found in flowers and other reproductive structures. Genetic and molecular analyses of floral morphogenesis have provided a detailed model (the ABC model) for the regulation of floral organ identity (5, 6). In contrast, the specification of vegetative organ identity remains very poorly understood (7).One of the questions that is still unresolved is when the identity of a leaf is determined. Many investigators have reported that heteroblastic variation in leaf anatomy and morphology is discernable very early in leaf development (8-13), and have therefore concluded that leaf identity is specified during the earliest stages of leaf initiation. However, this conclusion has rarely been tested experimentally. The effect of exogenous factors on leaf morphology has been studied extensively (1, 4), but there is surprisingly little information about when phase-specific aspects of leaf anatomy and morphology are determined during leaf development. Sussex and Clutter (14) found that adult leaf primordia of the fern Osmunda cinnamomea develop as juvenile leaves when cultured on a medium containing low levels of sucrose, demonstrating that leaf identity is not specified until after leaf initiation in this species. In addition, experimental an...

Recessive mutations of the early phase change ( epc ) gene in maize affect several aspects of plant development. These mutations were identified initially because of their striking effect on vegetative phase change. In certain genetic backgrounds, epc mutations reduce the duration of the juvenile vegetative phase of development and cause early flowering, but they have little or no effect on the number of adult leaves. Except for a transient delay in leaf production during germination, mutant plants initiate leaves at a normal rate both during and after embryogenesis. Thus, the early flowering phenotype of epc mutations is explained completely by their effect on the expression of the juvenile phase. The observation that epc mutations block the rejuvenation of leaf primordia in excised shoot apices supports the conclusion that epc is required for the expression of juvenile traits. This phenotype suggests that epc functions normally to promote the expression of the juvenile phase of shoot development and to suppress the expression of the adult phase and that floral induction is initiated by the transition to the adult phase. epc mutations are epistatic to the gibberellin-deficient mutation dwarf1 and interact additively with the dominant gain-of-function mutations Teopod1 , Teopod2 , and Teopod3 . Genetic backgrounds that enhance the mutant phenotype of epc demonstrate that, in addition to its role in phase change, epc is required for the maintenance of the shoot apical meristem, leaf initiation, and root initiation.

Background and Aims: Leaf morphology, anatomy, degree of lignification, and tensile strength were studied during vegetative phase change in an inbred line of Zea mays (OH43 × W23) to determine factors that influence mechanical properties during development.Methods: Tensometer, light microscopy, histochemistry. Key results: Mature leaf length increased linearly with plant development, peaked at leaves 7 and 8 (corresponding to the onset of the adult phase) and then declined. Leaf width was stable for leaves 1 through 3, increased to leaf 7, remained stable to leaf 10, and then declined through leaf 13. Lamina thickness was highest for leaf 1 and decreased throughout development. Leaf failure load to width ratio and failure load to thickness ratio increased with development suggesting that changes in leaf morphology during development do not entirely account for increases in failure load. Histochemical analyses revealed that leaf tensile strength correlates with percent lignification and the onset of anatomical adult features at various developmental stages.Conclusions: These data demonstrate that in Zea mays lignification of the midrib parenchyma and epidermis may be directly correlated with increased tensile strength associated with phase change from juvenility to adulthood. Failure load and resultant tensile strength values are primarily determined by the percent tissue lignification and the appearance of leaf architectural characters that are associated with the transition from the juvenile to the adult phase. Increased mechanical stability that occurs during the phase transition from juvenility to adulthood may signify a fundamental change in strategy for an individual plant from rapid growth (survival) to reproduction.

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