CONTRASTING PATTERNS OF PHOTOSYNTHETIC ACCLIMATION AND PHOTOINHIBITION IN 2 EVERGREEN HERBS FROM A WINTER DECIDUOUS FOREST

The relationship between the microclimate within an Oak-Hickory forest and photosynthetic characters of two resident evergreen herbs with co ntrasting leaf phenologies was investigated on a monthly basis for 1 f ull year. Heuchera americana has leaf flushes in the spring and fall, with average leaf life spans of 6-7 months. Hexastylis arifolia produc es a single cohort of leaves each spring with a leaf life span of 12-1 3 months. We predicted that among evergreen plants inhabiting a season al habitat, a species for which the frequency of leaf turnover is grea ter than the frequency of seasonal extremes would have a greater annua l range in photosynthetic capacity than a species that only produced a single flush of leaves during the year. Photosynthetic parameters, in cluding apparent quantum yield, maximum photosynthetic capacity, (P-ma x), temperature of maximum photosynthetic, photochemical efficiency of PSII and leaf nitrogen (N) and chlorophyll concentrations, were perio dically measured under laboratory conditions in leaves sampled from na tural populations of both species. Mature leaves of both species accli mated to changing understory conditions with the mean seasonal differe nces being significantly greater for Heuchera than for Hexastylis. Are a based maximum photosynthetic rates at 25 degrees C were approximatel y 250% and 100% greater in winter leaves than summer leaves for Heuche ra and Hexastylis respectively. Nitrogen concentrations were highest i n winter leaves. Chlorophyll concentrations were highest in summer lea ves. Low P-max/N values for these species suggest preferential allocat ion of leaf nitrogen into non-photosynthetic pools and/or light-harves ting function at the expense of photosynthetic enzymes and electron tr ansport components. Despite the increase in photosynthetic capacity, t here was evidence of chronic winter photoinhibition in Hexastylis, but not in Heuchera. Among these ecologically similar species, there appe ars to be a trade-off between the frequency of leaf production and the balance of photosynthetic acclimation and photoinhibition.