Biomass allocation in plants: Ontogeny or optimality? A test along three resource gradients

We examined biomass allocation patterns throughout the entire vegetative gr owth phase for three species of annual plants along three separate gradient s of resource availability to determine whether observed patterns of alloca tional plasticity are consistent with optimal partitioning theory. Individu als of the annual plant species Abutilon theophrasti, Chenopodium album, an d Polygonum pensylvanicum were grown from locally field-gathered seed in co ntrolled greenhouse conditions across gradients of light, nutrients, and wa ter. Frequent harvests were used to determine the growth and allocation (ro ot vs. shoot, and leaf area vs. biomass) responses of these plants over a 5 7-d period. Growth analysis revealed that each species displayed significan t plasticity in growth rates and substantial amounts of ontogenetic drift i n root: shoot biomass ratios and ratios of leaf area to biomass across each of the three resource gradients. Ontogenetically controlled comparisons of root : shoot and leaf area ratios across light and nutrient gradients were generally consistent with predictions based on optimal partitioning theory ; allocation to roots decreased and leaf area increased under low light and high nutrient conditions. These trends were confirmed, though were less dr amatic, in allometric plots of biomass allocation throughout ontogeny. Thes e species did not alter biomass allocation (beyond ontogenetic drift) in re sponse to the broadly varying water regimes. Furthermore, many of the obser ved differences in biomass allocation were limited to a given time during g rowth and development. We conclude that, for these rapidly growing annual species, plasticity in b iomass allocation patterns is only partially consistent with optimal partit ioning theory, and that these plastic responses are ontogenetically constra ined. Further, while these species did adjust biomass allocation patterns i n response to light and nutrient availability, they did not adjust biomass allocation in response to water availability, despite dramatic plasticity i n growth rates along all three resource gradients. Our results support a de velopmentally explicit model of plasticity in biomass allocation in respons e to limiting resources.