COMPARISON OF PHOTOSYNTHETIC TRAITS BETWEEN TWO POPULATIONS OF STYRAX AMERICANUS

Styrax americanus Lam. (American snowbell) is a shrub or small tree (up to 4 meters) that is underutilized in the nursery industry. Patchy populations of this North American species are most common in the southeastern United States, where winters are mild. We are studying a disjunct population of 15 or fewer individuals indigenous to the harsher climate of northern Illinois (latitude = 41°10’N). Our overall goal is to evaluate the horticultural potential of this ecotype, particularly whether its temperature tolerances differ from those of S. americanus indigenous to Florida (latitude = 27˚10’N). We examined photosynthetic parameters (A_max, J_max, and V_cmax) of plants from the two populations. Plants were exposed to 35°C/22°C day/night for 12 days followed by 15°C/5°C for 12 days, and then re-exposed to 35°C/22°C. A/C_i curves (net CO₂ exchange rate versus increasing intercellular CO₂ concentration) were developed after each temperature regimen and used to calculate A_max, J_max (maximum rate of electron transport) and V_cmax (carboxylation capacity of RuBP [ribulose-1,5-bisphophate]) by using published models. Styrax americanus was highly resilient in response to temperature change. The two ecotypes exhibited similar A_max during the first exposure to 35°C. A_max decreased by an average of 70% at 15°C and was similar for the two ecotypes. A_max of plants of the northern ecotype fully recovered upon re-exposure to 35°C, while plants of the southern ecotype exhibited a mean rate of A_max greater than during the initial exposure to 35°C. High plasticity in response to temperature was illustrated in both ecotypes by an increase in the ratio of J_max and V_cmax at 15°C. Preliminary results suggest that the successful horticultural use of S. americanus beyond its native range will be due in part to the capacity for the species to adjust its photosynthesis in response to temperature.