Gas exchange and water relations of the root hemi-parasite Santalum album L. in association with legume and non-legume hosts

This paper examines foliar nitrogen (N) levels, photosynthesis, transpirati on, water use efficiency and tissue water relations of the xylem-tapping ro ot hemi-parasite Santalum album in pot culture with various N-2-fixing wood y hosts, a non-fixing host (a eucalypt), or in the absence of a host. Folia r N concentrations of Santalum were significantly greater than correspondin g hosts and higher when on N-2-fixing hosts than on the eucalypt, or withou t a host. Strong positive relationships were evident in Santalum between fo liar N concentration, rates of net photosynthesis and instantaneous water u se efficiencies. Photosynthesis rate and water use efficiency of Santalum w ere generally lower than in corresponding hosts, but transpiration rates we re not noticeably different between associations. delta(13)C values of tota l shoot dry matter of Santalum were poorly correlated with instantaneous wa ter use efficiency as measured by gas exchange, but associations involving the three legumes showed less negative delta(13)C values and better water u se efficiencies for hosts, than corresponding parasites. Interpretation of such differences was difficult in view of an earlier demonstration of subst antial heterotrophic gain of C from certain hosts. Diurnal profiles of gas exchange and leaf water potential of hosts and parasites indicated closely coordinated diurnal stomatal responses of the parasite water relations to i ts host, thus resulting in transpiration rates of the parasite generating l eaf water potential gradients favouring continuous abstraction of water and nutrients from a host. Tissue water relations of Santalum generally resemb led those of water-stress tolerant species. Host-specific effects on relati ve water content and osmotic adjustment were slight and rated unimportant i n regulation of water flow to the parasite, or in protecting it from tempor ary water stress in a host. (C) 1999 Annals of Botany Company.