PHENOTYPIC VARIATION IN CONTRASTING TEMPERATURE ENVIRONMENTS - GROWTH AND PHOTOSYNTHESIS IN PENNISETUM-SETACEUM FROM DIFFERENT ALTITUDES ON HAWAII

1. Plasticity and genetic differentiation of growth, biomass allocation, and photosynthesis in response to temperature were investigated for populations of the introduced grass Pennisetum setaceum collected from different altitudes on the island of Hawaii. 2. The temperature optimum for photosynthesis was similar (35-degrees-C) for plants collected from different altitudes and grown in growth chambers with low (25/13-degrees-C) and high (33/25-degrees-C) temperature environments, indicating that P. setaceum has limited potential for photosynthetic temperature acclimation. Plants grown in the low-temperature environment, however, had 16% greater maximum photosynthetic rates on a leaf area basis than plants grown at the high temperature (25.4 and 21.4 mumol CO2 M-2 s-1, respectively). 3. Greater photosynthetic rates for plants grown at the low temperature were accompanied by reduced specific leaf area and lower internal to ambient CO2 concentration ratios. Plants grown in the high-temperature environment, however, had greater leaf mass and leaf areas and accumulated 42% more total biomass than plants grown in the low-temperature environment. 4. Only total leaf area was significantly different among the populations. Plants collected from the high-altitude population produced the highest total leaf area, followed by plants from the mid- and low-altitude populations, respectively. Although some variation in photosynthetic capacity was found among genets, it was unpredictable and did not correspond to patterns of variation for growth and biomass traits. 5. The extensive altitudinal distribution of P. setaceum on Hawaii, therefore, is a result of the broad ecological tolerance of individuals and not of local adaptation. Phenotypic plasticity for leaf photosynthesis and biomass allocation in response to differences in temperature may contribute to the success of this grass in these different habitats.