PHOTOSYNTHESIS AND CARBON ALLOCATION IN TIPULARIA DISCOLOR (ORCHIDACEAE), A WINTERGREEN UNDERSTORY HERB

Seasonal patterns of photosynthesis and carbon allocation were determi ned for Tipularia discolor, a summer-deciduous wintergreen orchid of t he southeastern United States, to assess the effects of environmental conditions and leafage on carbon acquisition and allocation patterns. There was no shift in the optimum temperature for photosynthesis (T-op t) on a seasonal basis and T-opt (approximate to 26 C) was at least 10 C higher than daily maximum air temperature during most of the growin g season. Lack of photosynthetic adjustment in Tipularia to seasonal f luctuations in temperature and light suggested that the photosynthetic characteristics of this wintergreen were more similar to those of spr ing ephemerals than to those of evergreens and summer-active herbs. Th e decline in photosynthetic capacity during the winter growing season for Tipularia, largely due to leaf age effects, gradually reduced net photosynthetic rates in the field despite more favorable light and tem perature conditions. Photosynthesis in the field was primarily limited by environmental conditions in early- and mid-season and by photosynt hetic capacity in late-season. A (CO2)-C-14 labelling experiment demon strated that patterns of carbon allocation to vegetative structures we re affected by the season of photosynthetic carbon fixation, whereas r eproductive structures received 21% of the recovered labelled carbon r egardless of the period of labelling. Carbon acquired and stored durin g all periods of the growing season was used to produce new vegetative and reproductive structures.