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

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 <4.2 MJ°m—2°d—1). Significant differences in sunlight patterns and windspeeds at locations with sun or shade plants led to variations in leaf temperature, stomatal resistance, xylem water potentials, and photosynthesis. Very small differences in soil water potentials and air temperatures occurred between sun and shade plant locations. Leaf size, thickness, and trichome density along with chlorophyll contents also varied between leaves from sun and shade plants. Sunlit periods for sun plants contained nearly twice as much energy and quantum flux and lasted nearly twice as long as sunlit periods for shade plants. Windspeeds and soil temperatures were also considerably greater at sun plant locations. Stomatal opening occurred for both sun and shade plants during sunlit periods which led to large increases in pohotosynthesis and transpiration. Aerodynamic leaf resistances accounted for >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.