Gas exchange characteristics and nitrogen relations of two Mediterranean root hemiparasites: Bartsia trixago and Parentucellia viscosa

Plant height, light-saturated rates of photosynthesis ($A_{\text{max}}$) and foliar nitrogen concentration (N1) were measured for Bartsia trixago under field conditions in Mallorca. All three variables were postively correlated, and were also positively related to the abundance of nitrogen-fixing legumes in the associated vegetation (putative host species).$A_{\text{max}}$for B. trixago ranged from 7.7 to 18.8 μmol m-2s-1; similar rates were measured for a second hemiparasite Parentucellia viscosa, and both species were within the range of rates measured for six putative hosts (10.6-19.2 μmol m-2s-1). Fertilization of unattached B. trixago plants with inorganic nitrogen (ammonium nitrate) elicited neither the growth nor the photosynthetic responses observed in plants considered to be parasitic on legumes and in receipt of an enriched organic nitrogen supply. Both hemiparasites had high diurnal leaf conductances (gs) (469-2291 mmol m-2s-1) and were at the upper end of the range of those measured in putative hosts (409-879 mmol m-2s-1). In contrast with the latter, high nocturnal rates of gswere also recorded for the two hemiparasites (517-1862 mmol m-2s-1). There was no clear relationship between either$A_{\text{max}}$or N1and either gs, transpiration (E) or water use efficiency ($A_{\text{max}}$/E) in B. trixago plants. The economics of water loss appear to be independent of both the supply of nitrogen from the host and autotrophic carbon fixation.