PHOTOSYNTHETIC GAS-EXCHANGE AND THE STABLE-ISOTOPE COMPOSITION OF LEAF WATER - COMPARISON OF A XYLEM-TAPPING MISTLETOE AND ITS HOST

Photosynthetic gas exchange and the stable isotopic composition of fol iage water were measured for a xylem tapping mistletoe, Phoradendron j uniperinum, and its host tree, Juniperus osteosperma, growing in south ern Utah. The observed isotopic composition of water extracted from fo liage was compared to predictions of the Craig-Gordon model of isotopi c enrichment at evaporative sites within leaves. Assimilation rates of juniper were higher and stomatal conductance was lower than the value s observed for the mistletoe. This resulted in lower intercellular/amb ient CO2 values in the juniper tree relative to its mistletoe parasite . For mistletoe, the observed foliage water hydrogen and oxygen isotop ic enrichment was less than that predicted by the model. In juniper, f oliage water hydrogen isotopic enrichment was also lower than that pre dicted by the evaporative enrichment model. In contrast, the oxygen is otopic enrichment in juniper foliage water was slightly greater than t hat predicted for the evaporative sites within leaves. Hydrogen isotop ic enrichment in mistletoe foliage shows systematic variation with ste m segment, being highest near the tips of the youngest stems and decre asing toward the base of the mistletoe, where isotopic composition is close to that of stem water in the host tree. In a correlated pattern, mid-day stomatal conductance declined abruptly in mistletoe foliage o f increasing age.