Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Ulf Büntgen, István Bagi, Oszkár Fekete, Virginie Molinier, Martina Peter, Richard Splivallo, Maryam Vahdatzadeh, Franck Richard, Claude Murat, Willy Tegel, Ulrich Stobbe, Fernando Martínez-Peña, Ludger Sproll, Lisa Hülsmann, Daniel Nievergelt, Barbara Meier, Simon Egli, Emidio Albertini.
Despite an increasing demand for Burgundy truffles (Tuber aestivum), gaps remain in our understanding of the fungus’ overall lifecycle and ecology. Here, we compile evidence from three independent surveys in Hungary and Switzerland. First, we measured the weight and maturity of 2,656 T. aestivum fruit bodies from a three-day harvest in August 2014 in a highly productive orchard in Hungary. All specimens ranging between 2 and 755 g were almost evenly distributed through five maturation classes. Then, we measured the weight and maturity of another 4,795 T. aestivum fruit bodies harvested on four occasions between June and October 2015 in the same truffière. Again, different maturation stages occurred at varying fruit body size and during the entire fruiting season. Finally, the predominantly unrelated weight and maturity of 81 T. aestivum fruit bodies from four fruiting seasons between 2010 and 2013 in Switzerland confirmed the Hungarian results. The spatiotemporal coexistence of 7,532 small-ripe and large-unripe T. aestivum, which accumulate to ~182 kg, differs from species-specific associations between the size and ripeness that have been reported for other mushrooms. Although size-independent truffle maturation stages may possibly relate to the perpetual belowground environment, the role of mycelial connectivity, soil property, microclimatology, as well as other abiotic factors and a combination thereof, is still unclear. Despite its massive sample size and proof of concept, this study, together with existing literature, suggests consideration of a wider ecological and biogeographical range, as well as the complex symbiotic fungus-host interaction, to further illuminate the hidden development of belowground truffle fruit bodies.
Fruit bodies of the symbiotic Burgundy truffle (T. aestivum Vittad.)  range amongst the most expensive of gourmet foods . T. aestivum can be found almost throughout the entire year and most of Europe [3–5]. Despite a long history of human consumption, there are still many open questions concerning the origin, biogeography and ecology of most truffle species [1–5], including T. aestivum. The ecological and commercial interest in this ectomycorrhizal ascomycete has rapidly increased over the past few years , primarily because of a rising demand for truffles worldwide [2, 6, 7], as well as possibly also due to some declining harvests of the Périgord truffle (T. melanosporum Vittad.) in parts of its Mediterranean distribution . The wide ecological range of T. aestivum , its prolonged harvest season in comparison to T. melanosporum, as well as a much lower price level though higher cultivation potential , further contribute to the increasing commercial importance of T. aestivum.
The majority of this study was conducted in a planted oak stand in central Hungary (Fig 1A–1C). Trees are 21-year old and cover 41.5 ha of flat terrain ~90 m asl in the county of Jász-Nagykun-Szolnok near the village Jászivány (around 47.5° N and 20.2° E). This plantation was naturally colonized by T. aestivum some years ago. The recent truffle orchard, called Jászivány, is characterized by a uniform distribution of Calcic Chernozem (Loamic) soil. Being poor in stone content, its texture corresponds to a silt-clay-loam (12% silt, 38% clay, 50% loam) with a high organic matter content and a high biological activity. The pH value of this very deep mull-like soil spatially varies from 6.9–8.2 (in water). The continental climate exhibits approximately 2000 sunshine hours per year, resulting in an annual temperature mean of 10.2 C°. Nevertheless, exceptionally cold winters are contrasted by warm summers. Average annual precipitation totals fluctuate between 560 and 580 mm, with seasonal hydroclimatic minima in summer.
Fresh weight of the 2,656 individual T. aestivum fruit bodies from the August 12–14, 2014 harvest in Hungary varied from 2–755 g with an average of 33 g (Fig 3A). A total of 1,699 fruit bodies weighted <30 g, 818 samples ranged from 30–89 g, and the remaining 139 truffles weighted >90 g. Although the giant of 755 g clearly marks an exception, a better understanding of the ecological and climatological conditions, as well as the time needed for such a fruit body to grow, is more than scientific curiosity alone . Fruit bodies of different weight were found in all five maturation classes (Fig 3B). There is a slight positive trend between mean fruit body weight and maturation class (Fig 3C). At the same time, there also persists a high level of variability since small and large truffles can be more or less mature.
This study demonstrates that 7,451 T. aestivum fruit bodies from Hungary (~180 kg), harvested by trained dogs during two years and different periods in a very productive truffle orchard, were distributed throughout five maturation classes. Our results supplement the long-term observations of local truffle hunters: Although there is a slight positive trend between mean fruit body weight and maturation class, small truffles can be ripe while large truffles can be unripe. In addition, we found no evidence for a systematic link to one (predominant) or more (interacting) external factors. An independent long-term assessment of another 81 T. aestivum specimens from four harvesting seasons between 2010 and 2013 in Switzerland (~2 kg) also disproves any statistically robust relationship between fruit body weight and maturity. The predominant co-occurrence of small-ripe and large-unripe T. aestivum fruit bodies is, indirectly, also indicative of a rather heterogeneous size-ripeness association, which generally differs from a more or less species-specific final size of some mature epigeous basidiomycetes and ascomycetes. This circumstance might be related to the subterranean mode of truffle life with specific growth conditions and dependencies on the surrounding ecology, including soil conditions and microclimatic variations, as well as effects of microbial communities and mycelial connectivity.