Host Specificity and Seasonal Variation in the Colonization of Tubakia sensu lato Associated with Evergreen Oak Species in Eastern Japan

Foliar fungal endophytes are ubiquitous and hyperdiverse, and tend to be host-specific among dominant forest tree species. The fungal genus Tubakia sensu lato is comprised of foliar pathogens and endophytes that exhibit host preference for Quercus and other Fagaceae species. To clarify interspecific differences in ecological characteristics among Tubakia species, we examined the endophyte communities of seven evergreen Quercus spp. at three sites in eastern Japan during summer and winter. Host tree species was the most significant factor affecting endophyte community composition. Tubakia species found at the study sites were divided into five specialists and three generalists according to their relative abundance in each host species and their host ranges. Specialists were dominant on their own host in summer, and their abundance declined in winter. To test the hypothesis that generalists are more widely adapted to their environment than specialists, we compared their spore germination rates at different temperatures. Spores of generalist Tubakia species were more tolerant of colder temperatures than were spores of specialist Tubakia species, supporting our hypothesis. Seasonal and site variations among Tubakia species were also consistent with our hypothesis. Host identity and ecology were significantly associated with endophyte community structure.

Factors affecting stress tolerance in recalcitrant embryonic axes from seeds of four Quercus (Fagaceae) species native to the USA or China

BACKGROUND AND AIMS

Quercus species are often considered 'foundation' components of several temperate and/or subtropical forest ecosystems. However, the populations of some species are declining and there is considerable urgency to develop ex situ conservation strategies. In this study, the storage physiology of seeds within Quercus was explored in order to determine factors that affect survival during cryopreservation and to provide a quantitative assessment of seed recalcitrance to support future studies of this complex trait.

METHODS

Water relations and survival of excised axes in response to water loss and cryo-exposure were compared for four Quercus species from subtropical China (Q. franchetii, Q. schottkyana) and temperate USA (Q. gambelii, Q. rubra).

KEY RESULTS

Seed tissues initially had high water contents and water potentials. Desiccation tolerance of the embryonic axis was not correlated with the post-shedding rainfall patterns where the samples originated. Instead, higher desiccation tolerance was observed in samples growing in areas with colder winters. Survival following cryo-exposure correlated with desiccation tolerance. Among species, plumule tissues were more sensitive than radicles to excision, desiccation and cryo-exposure, and this led to a higher proportion of abnormally developing embryos during recovery following stress.

CONCLUSIONS

Quercus species adapted to arid and semi-humid climates still produce recalcitrant seeds. The ability to avoid freezing rather than drought may be a more important selection factor to increase desiccation tolerance. Cryopreservation of recalcitrant germplasm from temperate species is currently feasible, whilst additional protective treatments are needed for ex situ conservation of Quercus from tropical and subtropical areas.