ECOTYPIC DIFFERENTIATION OF GAS-EXCHANGE RESPONSES AND LEAF ANATOMY IN A TROPICAL FOREST UNDERSTORY SHRUB FROM AREAS OF CONTRASTING RAINFALL REGIMES

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 Paci fic coastal areas in central Panama, which differed twofold in annual rainfall, and grew them under common conditions in a screened, open-ai r growing house for 14 months. Plants from the wetter (Atlantic) regio n showed significantly higher stomatal conductance, but photosynthetic rates were similar in both groups, leading to higher water use effici ency in plants from the drier (Pacific) region. Responses of stomatal conductance to atmospheric humidity were similar in both groups. Anato mical studies show that plants from the wetter region had a higher mes ophyll surface area per unit leaf area (A(mas)/A(leaf)) than plants fr om 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 surfac e area exposed to the intercellular airspaces is similar in plants fro m the two regions (A(mes)/A(leaf) = 1.7). Paired plants transplanted i nto natural forest understory conditions showed considerable variabili ty among sites, but plants from the drier region consistently had lowe r stomatal conductance than plants from the wetter region. After 20 mo nths in the field, plant growth was similar regardless of plant origin , but plants of Pacific coast origin had longer roots and more (but sm aller) leaves than plants of Atlantic coast origin. Stomatal density i n field-grown plants was higher in plants of Atlantic (135.9 mm-2) tha n Pacific (90.1 mm-2) origin. An understanding of genetically based ad aptations to local environmental conditions is important for predictin g the consequences of climatic change and forest fragmentation.