The Impact of Meristem Allocation on Reproductive Allocation

Meristem allocation models suggest that plant allocation to different types of meristems may affect reproductive allocation. According to the models, early allocation to reproductive meristems instead of growth meristems may reduce the expansion of the shoot system and in this way later allocation to reproduction. The result is common for many allocation models, which predict a trade-off between early and late allocation to reproduction (Roff, 1992). In meristem allocation models, the availability of meristems limits reproduction. In other allocation models, carbon, mineral nutrients, or other resources may limit reproduction. However, the role of meristems as a limiting factor has some peculiarities. The limiting factor is the production of buds, small plant parts seemingly inexpensive to produce in terms of resource expenditure.

Several empirical studies have shown that meristem availability may affect reproductive allocation. Watson (1984) studied the meristem allocation of two experimental populations of water hyacinth Eichhornia crassipes (Pontederiaceae), an aquatic clonal herb. The flowering population had a much lower clonal growth rate and lower equilibrium density in a density-dependent population growth than the vegetative population. Resource limitation did not explain the result, but plants could supply new ramets with the necessary resources. Clonal expansion was meristem limited because flowering consumed apical meristems and decreased the production rate of new ramets.

Geber (1990) showed that meristem allocation affects reproductive allocation in Polygonum arenastrum (Polygonaceae), an annual herb. There was genotypic variation in the early allocation to flowering. Genotypes that had a high early allocation to flowering suffered from a cost in terms of low growth rate and fecundity later in life. The experimental plants did not produce any dormant meristems. All genotypes produced equal numbers of seeds per flowering metamer, which suggests that plants with high early reproductive allocation did not suffer from higher resource limitation than other plants. Plants were given ample water and fertilizer, which also suggests that it was unlikely that resource limitation would explain the result.

Some studies have not found any correlation among allocation to different types of meristems. Lehtilâ et al. (1994) studied the bud dynamics of mountain birch Betula pubescens ssp. czerepanovii (Betulaceae) with population matrix models. They did not find any significant cost of flowering on production of buds. The rate of canopy expansion was similar for trees with and without inflorescences. There was, however, a 2-10% decrease of bud production rate in reproductive shoots, but statistical power was not sufficient to test for small costs. Morphological studies suggest that there may be meristematic costs of flowering in birches. Female inflorescences are located in short shoots. Short shoots are usually monopodial, and flowering terminates the shoot growth. However, short shoot growth may often continue from an axillary bud after flowering (Macdonald and Mothersill, 1983). Long shoots produce male inflorescences. Reproductive long shoot buds have fewer leaf primordia than vegetative long shoot buds (Macdonald et al, 1984). Flowering should thus imply a meristem loss. Karlsson et al. (1996) found a significant cost of producing generative long shoots in mountain birch. Long shoots that had catkins produced fewer new long shoots from axillary buds, and the new long shoots had a reduced number of buds. Trees studied by Karlsson et al. (1996) had more inflorescences than those studied by Lehtilä et al. (1994), which may affect the expression of costs. Gross (1972) showed that intensive flowering may lead to a deterioration of vegetative growth in birches. These results indicate that reproductive allocation may affect later meristem allocation, but it is possible that underlying resource limitation may have been a causal factor.

Huber and During (2001) also did not find any cost of allocation to flowering meristems in a study of two Trifolium species. Although flowering from buds of the primary shoot was negatively correlated with branching from the primary shoot, in the whole-plant level the correlation between flowering and branching was not significant. Duffy et al. (1999) also did not observe any cost of flowering in terms of allocation to growth meristems. Instead, they observed a positive association between allocation to growth and reproductive meristems in Arabidopsis thaliana. In both studies, allocation to growth and flowering was negatively associated with allocation to inactive meristems.

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