Wesleyan
University
Contrasting ecological breadth in co-occurring Polygonum species
Sultan S E, Wilczek A M, Hann S D, and Brosi, B J
SUMMARY
1. Individual physiological plasticity is considered an important element in the ecological breadth of species. Yet little is known about how response to particular environmental factors is influenced by other interacting factors, or about how species of contrasting ecological breadth differ in response to complex environments. This study compared the physiological responses of four annual Polygonum species (P. cespitosum, P. hydropiper, P. lapathifolium, and P. persicaria) to controlled, multifactorial combinations of light, moisture, and nutrients. The results are discussed in the context of companion studies on the species' field distributions for these resources.
2. The paper describes two experiments respectively designed to examine long-term and short-term plastic response to complex environments. The first experiment tested in situ responses of plants raised in all possible combinations of Low vs. High light; Dry, Moist, or Flooded soil; and Poor vs. Rich NPK. Instantaneous net photosynthetic rate and stomatal conductance were measured on one full-sib replicate from 5 inbred lineages from each of 5 field populations per species raised in each of the 12 factorial environments (= 25 plants per species per treatment x 12 treatments x 4 species = 1200 plants total ).
3. In addition to significant effects of environmental treatment on plants of all species, the species differed significantly in their patterns of photosynthetic response to particular resources and resource combinations. Many of these differences in photosynthetic response to environment are consistent with the contrasting field distributions of the four species.
4. The equability of photosynthetic performance across combinations of low and high resource states was generally consistent with the known ecological breadth of the Polygonum species (i.e. P. persicaria > P. lapathifolium ³ P. hydropiper); P. cespitosum, a species limited to shaded habitats, maintained equable but very low photosynthetic rates across the environmental range. Both P. persicaria and P. lapathifolium maintained effective functional levels (> 50% of the species' photosynthetic maximum) in a variety of moisture and nutrient environments when grown in High light. In contrast, P. cespitosum andP. hydropiper maintained such functional levels only if given both High light and ample macronutrients.
5. Although all species decreased stomatal conductance in Dry conditions, the species differed in their patterns of stomatal response to specific environmental factors and factor combinations. Complex differences in instantaneous Water Use Efficiency (WUE) among species reflected their highly specific and to some extent independent patterns of photosynthetic and stomatal response to the mutifactorial environments. WUE increased substantially in the Dry treatment in all four species.
6. The second experiment examined short-term plasticity of plants switched from one of six multifactorial growth environments to a contrasting light environment, for a sample of 1 replicate per treatment from 1inbred family per population from four populations of each species (N=96 plants). Instantaneous net photosynthetic rate was measured on each plant in its original growth environment and one hour after transfer from either Low to High light, or High to Low light. Photosynthetic measurements taken in the original growth environments confirmed the response patterns found in the initial experiment, including the low response of P. cespitosum to both Low and High light.
7. The species differed significantly in short-term plastic adjustment to changes in light level. Plants of P. persicaria and P. cespitosum reached 78% and 98% respectively of their maximum photosynthetic rates after switching from Low to High light, but P. hydropiper and P. lapathifolium plants reached only c. 60% of their maximum rates after the transfer. When switched from High to Low light, P. persicaria and P. cespitosum plants maintained 64-76% of their maximum rates, while P. hydropiper and P. lapathifolium plants decreased photosynthetic rates sharply to less than 50% of maximum rates. These results indicate that the latter two species will be less able to maintain effective functional levels in variable light environments; this is consistent with the known field distribution of the species.