© 1994 Heron Publishing—Victoria, Canada
Ecotypic differentiation of gas exchange responses and leaf anatomy in a tropical forest understory shrub from areas of contrasting
rainfall regimes
Kevin P. Hogan (1, 2), Alan P. Smith (1), José Luís Araus (3) and Angel Saavedra (1)
1. Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Republic of Panama / 2. Landcare Research, P.O. Box 31-011, Christchurch, New Zealand / 3. Unitat de Fisiologia Vegetal, Facultat de Biologia, Universitat de Barcelona, E-08028 Barcelona, Spain / Received October 8, 1993
Summary
Many studies have documented genetic differentiation of physiological ecotypes along environmental gradients in the temperate
zone, but this topic has received little attention in tropical plants. We collected cuttings of Psychotria horizontalis (Rubiaceae) from Atlantic and Pacific coastal areas in central Panama, which differed twofold in annual rainfall, and grew
them under common conditions in a screened, open-air growing house for 14 months. Plants from the wetter (Atlantic) region
showed significantly higher stomatal conductance, but photosynthetic rates were similar in both groups, leading to higher
water use efficiency in plants from the drier (Pacific) region. Responses of stomatal conductance to atmospheric humidity
were similar in both groups. Anatomical studies show that plants from the wetter region had a higher mesophyll surface area
per unit leaf area (Ames/Aleaf) than plants from the drier region (17.2 versus 13.9), and also had a higher stomatal density (161.5 versus 98.0 mm–2) and fewer trichomes (2.0 versus 18.7 mm–2). The proportion of palisade cell surface area that was exposed to intercellular airspaces is higher in plants from the Pacific
coast than from the Atlantic coast, such that the total palisade cell surface area exposed to the intercellular airspaces
is similar in plants from the two regions (Ames/Aleaf = 1.7). Paired plants transplanted into natural forest understory conditions showed considerable variability among sites,
but plants from the drier region consistently had lower stomatal conductance than plants from the wetter region. After 20
months in the field, plant growth was similar regardless of plant origin, but plants of Pacific coast origin had longer roots
and more (but smaller) leaves than plants of Atlantic coast origin. Stomatal density in field-grown plants was higher in plants
of Atlantic (135.9 mm–2) than Pacific (90.1 mm–2) origin. An understanding of genetically based adaptations to local environmental conditions is important for predicting
the consequences of climatic change and forest fragmentation.
Keywords:
ecotypes, photosynthesis, stomatal conductance, transpiration, vapor pressure deficit, water use efficiency.
