Photosynthesis, Respiration, and Phosphate Absorption by Carex Aquatilis Ecotypes along Latitudinal and Local Environmental Gradients

Abstract

The genetic and environmental determinants of photosynthesis, respiration, and phosphate absorption were studied in Carex aquatilis populations from diverse thermal and nutritional environments, using reciprocal transplants to subarctic hot spring and muskeg sites and temperate alpine and subalpine sites. Ecotypic differences in growth rate (300% variation) reported by Chapin and Chapin (1981) were more important than ecotypic differences in maximum photosynthetic rate (<20% variation) or temperature optimum (5°C variation) in explaining population differences in tiller size. Acclimatization of photosynthesis to different gardens was most pronounced in populations from thermally variable environments. Differences in photorespiration among populations and among gardens generally paralleled differences in photosynthesis. The temperature optimum for photosynthesis of each population in all gardens was higher than average air temperature in any site and may have represented a compromise between (1) effective photosynthesis and (2) high nitrogen requirement associated with a low temperature optimum in a cold environment. All processes associated with plant carbon balance (respiration rate of shoots, rhizomes, and roots, and photosynthesis) lacked marked ecotypic variation. In contrast, phosphate absorption rate differed substantially among populations grown in a common garden, being highest in populations evolving in cold, phosphate—deficient soils. Soil phosphate status was more important than soil temperature in eliciting this evolutionary response. In contrast, acclimatization of phosphate absorption by each population was more sensitive to temperature than to phosphate availability. Rates of both phosphate absorption and photosynthesis were highest in those populations that had the highest genetic potential for growth. We conclude that rates of physiological processes associated with resource gain or loss are closely correlated with growth rate and can be best understood by considering the total selective regime impinging upon the plant.