EARLY-SPRING GAS-EXCHANGE AND UPTAKE OF DEUTERIUM-LABELED WATER IN THE POIKILOHYDRIC FERN POLYPODIUM-VIRGINIANUM

This study was carried out to determine if the desiccation-tolerant fe m Polypodium virginianum L. ecologically resembles lower plants by abs orbing atmospheric water through its fronds and actively growing in ea rly spring when the soil along cliff edges is still frozen. Three time s between February and April, 1991, P. virginianum clonal mats were tr eated with deuterium-labelled water. Following each application, frond s were collected over several days and analyzed for the presence of de uterium. Two treatment groups plus a control were used: fronds were sp rayed directly while covering the soil, or the roots were watered whil e protecting the fronds. The control mats were left untreated. Soil, a ir, and frond temperatures, plus photosynthesis and frond conductance were monitored throughout the study period. At subfreezing temperature s in February, no labelled water was taken up from the soil and no pho tosynthesis took place. Small amounts of label were absorbed from the soil in March during freeze-thaw cycles when rates of photosynthesis a nd stomatal conductance were both low. Large amounts of label were tak en up from the soil in April when the soil was fully thawed and gas ex change was at normal seasonal levels. Label was not absorbed directly through the fronds when the plants were actively growing. Despite the desiccation tolerance of P. virginianum, the timing and patterns of it s water uptake and gas exchange in early spring resemble those found i n higher vascular plants, not poikilohydric lower plants.