Growth, photosynthesis and haustorial development of semiparasitic Santalum album L. penetrating into roots of three hosts: a comparative study

Santalum album L. is an important semiparasitic tree whose roots join to host roots, through haustoria, to acquire water and nutrients. Yet little is known how different hosts affect S. album growth, photosynthetic performance and haustorial development. In this study, Dalbergia sissoo Roxb., a leguminous tree, and Lonicera japonica Thunb., a non-leguminous vine, were suitable hosts while Aquilaria sinensis (Lour.) Gilg, a nonleguminous tree, was not. The three host plants were cultivated with sandalwood seedlings for 1 year, inducing significant differences in net photosynthetic rate, stomatal conductance and transpiration rate, plant height, haustorial size and number of haustoria. S. album growth was strongly related to net photosynthetic rate, stomatal conductance and transpiration rate. Well-formed and frequently replaced haustoria in the good host D. sissoo likely enhanced metabolism and nutrient transport. Ultrastructural observations showed differences in the interfacial region and internal structure between finger parenchymal cells and host root cells, vascular tissue, the reaction of host root cells and penetration modes. Among them, the finger parenchymal cells in the haustoria that connected with hosts D. sissoo and L. japonica roots were rich in organelles, including mitochondria, endoplasmic reticulum and lysosomes. Numerous small vacuoles accumulated inside membranes of the host root cells and finger parenchymal cells. Many lysosomes were present at the interfacial region where large-scale digestion of the host root cells in D. sissoo and L. japonica occurred. In contrast, in the finger parenchymal cells of the haustoria that joined to A. sinensis roots, organelles were not that obvious and black-stained material at the interfacial regions was indistinct. The modes of invasion were also different. The finger parenchymal cells penetrated into the roots of hosts D. sissoo and L. japonica by cell membrane fusion, pressure and degrading enzymes while in A. sinensis, host root cells were surrounded by the finger parenchymal cells and were self-digested. Even though mature haustoria could penetrate into the root xylem of hosts D. sissoo and A. sinensis, the haustoria that connected with D. sissoo had a mature xylem while A. sinensis had not yet formed any vessel elements. In all S. album haustoria connected to the three host roots, the vascular system developed some xylem but no phloem was observed.