PARENT SESSION
Poster Session 1: Photosynthesis and Water Relations
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Leaf life span and leaf age effects on photosynthetic rates in shade- vs. gap-grown seedlings of five neotropical shade tolerant trees.

Machado, Jose-Luis^*,1, Stefanescu, Carla ², Wright, S³, Kitajima, Kaoru^{2, 3}, ¹ Swarthmore College, Swarthmore, PA² University of Florida, Gainesville, FL³ Smithsonian Tropical Research Institute, Balboa, Ancon, Panama

ABSTRACT- A leaf has to ′pay-back′ the cost of construction by its photosynthetic production through its life span. Thus, leaf life span should be longer in shaded environment where photosynthetic rates are constrained. As the rate of leaf production is slow, development of self-shading and nitrogen gradient within a plant should be slow in shade conditions. Consequently, long-lived leaves in shaded seedlings should have very slow or little change in photosynthetic capacity over time. These predictions were supported in a comparison of leaf life span for seedlings (> 2 yrs old) of five shade tolerant tree species (Aspidosperma curenta, Callophylum longifolia, Gustavia superva, Tetragastris panamensis, and Virola surinamensis), grown in common gardens in the shaded understory (0.8% of full sun) and enlarged natural treefall gaps in a moist forest in Panama. During the early rainy season in 2004, photosynthetic light responses curves were measured in-situ for marked leaves of known age, ranging from 25 to 800 days old. In all species, photosynthetic rates per unit leaf area exhibited a monotonic decline with leaf age, while dark respiration did not change. The decline in maximum photosynthetic rate with leaf age ranged from 0.0021 to 0.0048 mol m^-2 s^-1 day^-1 for shade-grown seedlings, which were the slowest reported so far in the literature for tropical species. The decline rates were much faster (0.0064-0.012) for seedlings growing in gaps across the five species without significant cross-species correlation between the two environments, as a result of large phenotypic plasticity in leaf traits and plant growth habits exhibited by each species. These results support the trend predicted by the cost-benefit theory of optimal leaf life span with regard to the productivity of the habitat. However, decline rates of photosynthetic capacity with leaf aging were not correlated with specie′s median leaf life span within either environment.

Key words: leaf longevity, photosynthetic capacity, shade tolerant, tropical trees