Phase change in inbred and exotic lines of maize
--Scott Poethig and Hilli J. Passas

The development of the shoot has traditionally been divided into two discrete phases: a juvenile phase characterized by a variety of vegetative traits and the inability to produce reproductive structures, and an adult phase characterized by a different set of vegetative traits and the ability to produce reproductive structures. This interpretation implies that the reproductive and vegetative development of the shoot are linked in some way, although there is no conclusive evidence for this assumption. To determine if reproductive development in maize is correlated with the change from a juvenile to adult phase of vegetative development, we studied the expression of phase-specific vegetative traits in inbred and exotic lines that varied in total leaf number (i.e., in the "timing" of tassel initiation). Figure 1 is a graph of the last completely waxy leaf, the last leaf with any epicuticular wax (a juvenile trait), the first node with epidermal hairs (an adult trait), the ear node, and the total leaf number in 10 inbreds and exotic varieties of maize ranging in leaf number from 15 (Black Mexican Sweet) to 23 (Sc76). As expected from previous studies, the position of the ear was tightly correlated with leaf number; with the exception of Argentine popcorn, there were 5 or 6 nodes between the ear and tassel. Argentine popcorn was unusual in having only 3 nodes between the topmost ear and tassel. The loss of epicuticular wax was highly correlated with the appearance of epidermal hairs, although in a few inbreds (e.g. Sc76 and A632) hairs appeared on completely waxy leaves. This observation suggests that the juvenile and adult programs are not mutually exclusive.

With respect to relationship between vegetative and reproductive development, the most interesting feature of these data is that the vegetative maturation of the shoot is unrelated to total leaf number or ear position. Although both the ear and the tassel are only produced by meristems that are in an adult phase, the positions of the tassel and ear do not correlate with the change in the vegetative phase of the shoot. Therefore, while it is still unclear whether the adult phase is a prerequisite for reproductive development, it is likely that the
initiation of the ear and tassel is not a direct consequence of the change in the vegetative phase of the shoot.

Figure 1:  The expression of vegatative and reproductive traits in inbred and exotic lines of maize.

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