Mayapple as a Study Organism

Mayapples are perennial clonal herbs of deciduous forests of the northeastern United States; they are found growing in discrete patches or colonies containing one or more genotypes (genets) (Policansky, 1983; Whisler and Snow, 1992). Mayapples are morphologically simple and have a limited number of developmental options that can directly influence their demography (their pattern of birth, death and ramet population increase) and their life history expression (whether they are sexually reproductive or not). These developmental options include annual decisions regarding rhizome branching, a developmental process that results in an increase in the number of ramets present, and whether to differentiate a vegetative or a sexual aerial shoot (Fig. 17-1) (Watson et al., 1997). Nitrogen and carbon availability influence the pattern of mayapple development and, hence, demographic and life history expression (Benner and Watson, 1989; de Kroon et al., 1991; Landa et al., 1992; Geber et al., 1997 a,b). Carbon appears to be the more limiting resource (Sohn and Policansky, 1977; Benner and Watson, 1989; de Kroon et al., 1991); it is extensively stored throughout the rhizome system (Landa et al., 1992; Jonsdottir and Watson, 1997; Watson et al., unpublished).

Mayapple rhizome systems are composed of series of physically (Sohn and Policansky, 1977) and physiologically (Landa et al., 1992; Jonsdottir and Watson, 1997; Watson et al., unpublished) interconnected rhizome segments (ramets) that may persist for more than ten years. The life history status of the rhizome system in a given year reflects the morphology and function of its current aerial shoot (Fig. 17-1). Sexual shoots tend to be produced by larger rhizome segments (Sohn and Policansky, 1977; Benner and Watson, 1989; Geber etal., 1997a).

The timing of mayapple seasonal phenology varies as a function of latitude and seasonal rainfall patterns. Northern populations emerge and senesce later than southern ones (Sohn and Policansky, 1977; Lu, pers. obs.; Geber, pers. obs.). Both late season drought (Watson and Lu, 1999) and high rainfall early in the growing season (Watson and Jones, pers. obs.) accelerate the onset of senescence. In Indiana, shoots emerge in early April. Senescence is first observed in early May. However, this early senescence usually is associated with the presence of either a noctuid larva (probably Papaipema cerina) or a rust (Puccinia podophylii) (Sohn and Policansky, 1977; Parker, 1988; Lu, pers. obs.), both of which we consider to be agents of imposed senescence. Uninfected plants generally do not senesce until later in the season, and it is this larger, later wave of senescence that we consider to be under endogenous control.

Rhizome Diagrams

Figure 17-1. Diagrams of vegetative and sexual rhizome systems of mayapple, Podophyllum peltatum, as they would appear in early July in south-central Indiana. The terminal bud of the new rhizome segment contains next year's preformed shoot, which may be either vegetative or sexual. The ultimate old rhizome segment is the one that gave rise to the current year's shoot. (Reproduced with permission from Watson and Lu, 1999.)

Figure 17-1. Diagrams of vegetative and sexual rhizome systems of mayapple, Podophyllum peltatum, as they would appear in early July in south-central Indiana. The terminal bud of the new rhizome segment contains next year's preformed shoot, which may be either vegetative or sexual. The ultimate old rhizome segment is the one that gave rise to the current year's shoot. (Reproduced with permission from Watson and Lu, 1999.)

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