The Parasitism Between Hemiparasite Santalum Album And Its Hosts

It is its high value and supply shortage that have triggered much investment into sandalwood plantations in southern China, with a strong government policy incentive. Santalum album is a xylem-tapping obligate root hemi-parasite, which is dependent on the haustorial xylem connection with hosts'root system for water, nutrients and some organic solutes. S. album has been well known for its highly-valued heart wood, also called sandalwood, that contains aromatic compounds. The mix plantation of S. album with Dalbergia odorifera, a N₂-fixing plant, is popular in southern China. It is expected that D. odorifera could promote the growth of S. album through nitrogen fixation, resulting an accelerating formation of its own precious heartwood as a result of reduced growth due to the absorption of some water and nutrients by S. album. However, the parasitic relationship between any indigenous hosts and the hemiparasite S. album has not been studied in China.In a pot experiment, plant growth,δ(15)^N signature, shoot water potential, transpiration rate, net photosythesis rate, amino acids composition of xylem sap were analysed to examine the effects of plant physiological variables on the formation of parasitism between S. album and four indigenious species from southern China. These four species were A. confusa, D. odorifera, B. polycarpa and D. duperreranu. Nitrogen transfer between S. album and D. odorifera and the effect of nitrogen fixation on N transfer were studied by external (15)^N labelling. This research provides some physilogical evidence to S. album plantation in China with the following results:(1) Biomass, height, diameter and N concentration in 18-month old S. album were greater when S. album growing with two N₂-fixing hosts (A. confusa and D. odorifera) than with two non-N₂-fixing hosts (B. polycarpa and D. duperreranu) or with no host. Among the four hosts, D. odorifera > A. confusa.> B. polycarpa > D. duperreranu, based on their contribution to the growth of S. album. Growth of S. album was greater when growing without a host than growing with non-N₂-fixing host, indicating that the later non-N₂-fixing host might suppress the growth of S. album. (2) Compared to the non-N₂-fixing hosts (3.14⁴.32‰),δ(15)^N values in the N₂-fixing hosts (0.43².29‰) were closer to those in atmospheric N₂ (0‰).δ(15)^N values were similar between S. album (-0.08¹.64‰) and the corresponding N₂-fixing hosts, andδ(15)^N is greater in the S. album if grew alone (2.97‰) or grew with non -N₂-fixing hosts (2.21³.24‰), suggesting S. album had directly obtained N from its N₂-fixing hosts.(3) Both concentration and composition of xylem sap amino acids were similar between S. album and its N₂-fixing hosts, but they were lower in the non-N₂-fixing hosts. This result indicated a substantial N obtained by S. album from its N₂-fixing hosts, but not from its non-N₂-fixing hosts, had rapidly been assimilated in the xylem of S. album.(4) Leaf ABA content were significantly higher in the S. album (133.18²14.26 ng/g·FW) than in the hosts (52.64¹70.27 ng/g·FW). This was also true in the parasitzied hosts (52.64¹70.27 ng/g·FW) than in the unparasitzied controls (38.83¹39.10 ng/g·FW).(5) Pn values was highest when S. album growing with D. odorifera in the 190 days after transplanting (8.07μmol CO₂·m^-2·s^-1), followed by with A. confusa (6.18μmol CO₂·m^-2·s^-1) > D. duperreranu (4.30μmol CO₂·m^-2·s^-1) > B. polycarpa (3.80μmol CO₂·m^-2·s^-1) > no hosts (3.05μmol CO₂·m^-2·s^-1), indicating an improved leaf photosynthesis in the S. album was dependent on its parasitic plant species. In addition, there is no significant pattern for photosynthesis rate between Santalum and its hosts in this study, it is possible to depend on their own biological nature. Stomatal conductance and transpiration rates were greater in S. album than in their hosts, except for B. polycarpa. In contrast, water use efficiency in S. album was lower values than in any host.(6) Greater shoot water potential in the S. album ranked in the order when growing with: A. confusa (-4.38 MPa) < B. polycarpa (-1.60 MPa) < D. odorifera (-1.25 MPa) < no host (-0.65 MPa) < D. duperreranu (-0.55 MPa),indicating that water use of S. album was also dependent on its parasitic plant species. Shoot water potential were significantly lower in S. album grown with N₂-fixing hosts than their associated hosts.(7) Two-way N transfers were found between S. album and Dalbergia. Biomass (2.29-6.02 g/plant), N concentration (1.32-1.80%), N content (41.70-89.40 mg/plant), and (15)^N contents (0.150-0.160 mg/plant) were maximized in both plants when D. odorifera was nodulated. N transfer to S. album were 2.3 or 1.6 times greater than to D. odorifera in the nodulated or nonnodulated pairs, respectively, indicating a significant net transfer from the host D. odorifera to hemiparasite S. album, and N transfer was enhanced by N₂ fixation.