Influence of Sunlight on Photosynthesis, Water Relations, and Leaf Structure in the Understory Species Arnica Cordifolia

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Abstract
Intraspecific variation in photosynthesis, transpiration, and leaf structure according to natural exposure to sunlight was evaluated for the understory species Arnica cordifolia (Compositae) within the Rocky Mountains of southeastern Wyoming. Plants were arbitrarily divided into sun plants (receiving >12.6 MJ°m—2°d—1) and shade plants (receiving 50% of the total leaf resistance to water loss for plants in full sun, while the mesophyll resistance accounted for >80% of the total leaf resistance to CO2 uptake for both sunlit and shaded plants. Sun plants had higher photosynthetic rates, light saturation values, and temperature optima for photosynthesis. Leaves of sun plants also had large stomata, almost twice as many trichomes, and were °1.3 times as thick as leaves from shade plants. Chlorophyll contents were 36% greater in shade plants when expressed as a mass per mass basis, but 47% less if expressed as mass per leaf area. The results are discussed in terms of adaptive mechanisms for the evolution of sun— or shade—tolerant species in understory habitats.
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