PHYSIOLOGICAL-RESPONSE TO COMPLEX ENVIRONMENTS IN ANNUAL POLYGONUM SPECIES OF CONTRASTING ECOLOGICAL BREADTH

Individual physiological response to complex environments is a major f actor in the ecological breadth of species. This study compared indivi dual patterns of both long-term and short-term response to controlled, multifactorial environments in four annual Polygonum species that dif fer in field distribution (P. cespitosum, P. hydropiper, P. lapathifol ium, and P. persicaria). To test long-term response, instantaneous net photosynthetic rate and stomatal conductance were measured in situ on one full-sib replicate from five inbred lineages from each of five fi eld populations per species, raised in all possible combinations of lo w or high light; dry, moist, or flooded soil; and poor or rich nutrien t status. Short-term plastic adjustment to changes in light level was examined by switching individual plants of the four species from one o f six multifactorial growth environments to the contrasting light envi ronment, and measuring assimilation rates 1 h after transfer. The Poly gonum species differed significantly in their patterns of long-term ph otosynthetic response to particular resources and resource combination s. The species known to have relatively broad ecological distributions (P. persicaria and P. lapathifolium) maintained high photosynthetic p erformance in a variety of moisture and nutrient environments when gro wn in high light, while the more narrowly distributed P. hydropiper ma intained such functional levels only if given both high light and ampl e macronutrients. P. cespitosum, a species limited to shaded habitats, maintained low photosynthetic rates across the environmental range. C omplex differences among the species in instantaneous water use effici ency (WUE) reflected their highly specific and to some extent independ ent patterns of photosynthetic and stomatal response to the multifacto rial environments. The species also differed significantly in short-te rm physiological adjustment to changes in light level. Plants of P. pe rsicaria and P. cespitosum reached 78% and 98%, respectively, of their maximum photosynthetic rates 1 h after transfer from low to high ligh t, but P. hydropiper and P. lapathifolium plants reached only c. 60% o f their maximum rates. When switched from high to low light, P. persic aria and P. cespitosum plants maintained 64-76% of their maximum rates , while P. hydropiper and P. lapathifolium plants decreased photosynth etic rates sharply to less than 50% of their maximum rates. These resu lts indicate that the latter two species will be less able to maintain effective functional levels in variable light environments, a result consistent with their distributions in the field.