Ecotypic Differentiation of Gas Exchange Responses and Leaf Anatomy in a Tropical Forest Understory Shrub from Areas of Contrasting Rainfall Regimes

Stomatal control of gas exchange is an important component of a local plant's adaptation to different rainfall regimes. Species of the forest understory shrub Psychotria horizontalis were collected from sites near the Pacific and Atlantic coasts of Panama and grown under similar conditions in a greenhouse. Stomatal responses and leaf anatomical characteristics were examined. Results indicated that plants from the drier Pacific coast had a significantly higher water-use efficiency than the wetter Atlantic coast plants. Stomatal conductance was weakly but significantly correlated with photon flux density, and stomatal conductance was higher in Atlantic than in Pacific coast plants. Overall leaf and individual tissue layers were thinner in the Pacific coast plants than in the Atlantic coast plants. Pacific coast plants also had higher trichome density and lower stomatal density than Atlantic coast plants. These physiological and anatomical differences imply genetic differentiation of ecotypes.